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Eur. Phys. J. Plus (2014) 129: 83

DOI 10.1140/epjp/i2014-14083-5

Structural, magnetic and magnetocaloric properties of La0.7Sr0.3MnO3 manganite oxide prepared by the ball milling method R. Cherif, S. Zouari, M. Ellouze, E.K. Hlil and F. Elhalouani

Eur. Phys. J. Plus (2014) 129: 83 DOI 10.1140/epjp/i2014-14083-5

THE EUROPEAN PHYSICAL JOURNAL PLUS

Regular Article

Structural, magnetic and magnetocaloric properties of La0.7Sr0.3MnO3 manganite oxide prepared by the ball milling method R. Cherif1,a , S. Zouari1 , M. Ellouze1 , E.K. Hlil2 , and F. Elhalouani3 1 2 3

Faculty of Sciences, Sfax University, B. P. 1171, 3000 Sfax, Tunisia Institut Néel, CNRS et Université Joseph Fourrier, BP 166, F -38042 Grenoble Cedex 9, France National School of Engineers, LASEM, Sfax University, B.P.W - 3038 Sfax, Tunisia Received: 6 January 2014 / Revised: 4 March 2014 c Societ` Published online: 19 May 2014 – a Italiana di Fisica / Springer-Verlag 2014 Abstract. Structural, magnetic and magnetocaloric properties of La0.7 Sr0.3 MnO3 sample have been investigated. Powder sample has been elaborated by the ball milling method. The Rietveld analysis of the powder X-ray diffraction shows that the sample crystallizes in the orthorhombic structure with Pnma space group. Magnetic measurements showed that the sample exhibits a ferromagnetic-to-paramagnetic transition at a Curie temperature close to 370 K. The magnetic entropy change (∆SM ) has been deduced max by the Maxwell relation method. The maximum value of the magnetic entropy change |∆SM | is found to be 1.1 J/kgK for an applied magnetic field of 2 T. At this value of magnetic field the relative cooling power (RCP) is 49 J/kg. At high temperature, large change in magnetic entropy has been observed in the sample. Our result on magnetocaloric properties suggests that La0.7 Sr0.3 MnO3 nanopowder is attractive as a possible refrigerant for high temperature magnetic refrigeration.

1 Introduction The mixed-valence manganites of general formula Ln1−x Ax MnO3 where Ln is a rare earth (Ln = La, Pr, Sm. . . ) and A is a divalent element (A=Sr, Ca, Ba. . . ) has been extensively studied because of potential applications extremely varied such as magnetocaloric refrigeration [1–3]. In fact, these materials have been studies from many searches such as the work of Jonker [4], Van Santen [5], Wollan and Koehler [6]. The parent compound LaMnO3 is an antiferromagnetic insulator. The substitution of trivalent lanthanum with divalent ion (A = Sr2+ , Ca2+ or Ba2+ ) shows that these properties are located between a paramagnetic semiconductor and a ferromagnetic metallic state. Our powder sample has been elaborated by the ball milling technique. This method, included in the powder metallurgy technique is very useful for preparing nanoscale powders grains, because of very high rates of deformation that can be introduced into the structure [7, 8]. It is an efficient technique to synthesize many unique materials, such as nanostructured crystalline, amorphous alloys and nanoparticules from powder oxides [9–11]. In this paper, we investigated structural, magnetic and magnetocaloric properties of nanopowders of La0.7 Sr0.3 MnO3 sample elaborated by ball-milling method.

2 Experimental Nanopowder sample was synthesized using the ball milling method using SPEX ball mill, by mixing La2 O3 , SrO2 and MnO2 up to 99.9% purity in the desired proportion according to the following reaction: 0.35 La2 O3 + 0.3 SrO2 + MnO2 −→ La0.7 Sr0.3 MnO3 + 0.325 O2 . This mixture of high-purity powders was carried out at room temperature and under normal atmosphere. A vibrating ball milling with stainless-steel bowl and balls chromed steel was used for this purpose. The rotation speed was chosen to be (320 rpm) and the ball to powders ratio was 10:1. The mixture of powders was milled for 25 h with a grinding period of 15 min and a break of 30 min between the grinding periods in order to avoid raising the temperature in the bowl during the grinding process. The obtained sample was annealed at 1000 ◦ C for 3 h. a

e-mail: [email protected]

Page 2 of 8

Eur. Phys. J. Plus (2014) 129: 83

Intensity (a.u.)

Yobs Ycal Yobs-Ycal Posr

20

30

40

50

60

70

80

2θ (˜)

Fig. 1. Powder X-ray diffraction pattern and refinement of nanopowders of the La0.7 Sr0.3 MnO3 sample. Table 1. Cell parameters and refined structural parameters of nanopowders of La0.7 Sr0.3 MnO3 , n is the occupancy and the numbers in subscript represent the error bars. Room temperature a (˚ A)

b (˚ A)

Space Group: Pnma ˚) c (A

5.45594

7.73686

5.50904

232.546

Site

x

y

z

n (%)

La/Sr

4c

−0.00817

0.25

−0.00049

70/30

Mn

4b

0

0

0.5

100

O (1)

4c

0.50093

0.25

0.02163

100

O (2)

8d

0.27306

0.02911

0.73916

100

Unit cell parameters Atoms

˚3 ) V (A

χ2 = 4.31

Table 2. Refined structural parameters from the Rietveld refinement for La0.7 Sr0.3 MnO3 samples [6] and [9] at room temperature. ˚) Ref. a (A c (˚ A) V (˚ A3 ) System Crystalline Space group [6]

5.4997 ± 0.002

13.3567 ± 0.002

–

Pseudo-Cubic

R 3c

[9]

5.50233

13.35697

350.213

Hexagonal

R 3c

The crystalline structure of the sample was examined by powder X-ray diffraction (XRD) at room temperature using a SIEMENS D500 diffractometer with Cu Kα radiation. The data was analyzed by the Rietveld method [12] using the FULLPROF program [13]. Magnetization measurements versus temperature in the range 205–450 K and versus magnetic applied field up to 5 T were carried out using BS1 magnetometer. The magnetocaloric effects (MCE) were deduced from the magnetization measurements versus magnetic applied field up to 5 T at several temperatures.

3 Results and discussion 3.1 X-ray diffraction The XRD data was refined using the Rietveld technique shows that the sample is single phase and can be indexed in the orthorhombic structure with Pnma space group. A good fit between the observed and the calculated profiles was obtained, as shown in fig. 1 for La0.7 Sr0.3 MnO3 sample. Refined cell parameters, unit cell volume, atomic positions and the goodness of fit χ2 are summarized in table 1. Our crystallographic parameters are completely different to those obtained by Z. F. Zi et al. [14] for La0.7 Sr0.3 MnO3 powder prepared by a simple chemical coprecipitation route, and to those obtained by N. Kallel et al. [15] for La0.7 Sr0.3 Mn1−x Snx O3 samples (0 ≤ x ≤ 0.2) synthesized by a solid-state reaction method in air. For comparison, we listed in table 2 the data of some results of Rietveld refinements [6, 9].

Eur. Phys. J. Plus (2014) 129: 83

Page 3 of 8 1 H = 0.05 T

Magnetization (emu/g)

0.8

0.6

0.4

0.2

0 200

250

300 350 Temperature (K)

400

450

Fig. 2. Magnetization as function of temperature for the La0.7 Sr0.3 MnO3 sample under 0.05 T.

The average crystallite size was calculated using the classical Scherrer formula [16]: D=

kλ , β cos θ

where D is the crystallite size derived from the (110) peak of the XRD profiles, k is the sphere shape fractor (0.89), θ is the angle of the diffraction, β the difference of the full-width at half-maximum (FWHM) of the peak between the sample and the standard SiO2 used to calibrate the intrinsic width associated with the instruments and λ is the wave length of the X-ray (1.54056 ˚ A). The obtained average crystallite size of La0.7 Sr0.3 MnO3 nanopowder is about 22.55 nm. 3.2 Magnetic properties We plot in fig. 2, the magnetization evolution versus temperature for La0.7 Sr0.3 MnO3 sample at an applied magnetic field of 0.05 T. The M (T ) curve shows that the sample exhibits a ferromagnetic to paramagnetic transition with increasing temperature occurring at the Curie temperature TC . The Curie temperature, was determined as the temperature corresponding to the inflection point for the M (T ) curve, is found to be 370 K, which is slightly higher than that reported by Z. F. Zi et al. [14], N. Kallel et al. [15, 17–19], N. Abdelmoula et al. [20, 21], and W. CheikhRouhouKoubaa et al. [22], where the sample has been prepared by chemical coprecipitation method and solid state reaction. This difference is probably due to the fabrication methods. Figure 3 presents the evolution of magnetization versus the applied magnetic field obtained at different temperatures (isothermal magnetization), i.e. by measuring the magnetization under an applied magnetic field (B = µ0 H) from 0 to 5 T in the temperature range of 250 to 391 K with a step of 3 K. Below the Curie temperature, the magnetization increases sharply with applied magnetic field where B < 0.5 T and then saturates above 1 T. The saturation magnetization shifts to higher values of magnetic field with decreasing temperature. This result confirms the ferromagnetic behavior of our sample at low temperatures. 3.3 Arrott curves In order to determine the Curie temperature with precision, we plot in fig. 4 the Arrott curves (M 2 versus H/M ) for the La0.7 Sr0.3 MnO3 compound. TC deduced from these curves is found to be 361 K. The temperature dependence of the spontaneous magnetization (µsp ) and the inverse of the susceptibility (χ−1 ) of the La0.7 Sr0.3 MnO3 sample are shown in fig. 5. The µsp (T ) curve decreases with increasing temperature and then drops rapidly near 361 K. The value of the spontaneous magnetization at 250 K is about 2.32 µB /mol. The theoretical value for all alignment of the Mn spin is 3.7 µB . The theoretical value is higher than the experimental one, which may

Page 4 of 8

Eur. Phys. J. Plus (2014) 129: 83 70 250K

Magnetization M (emu/g)

60 50 40

δT=3K

30 20 391K 10 0 0

1

2

3

4

5

6

Applied magnetic field µ0 H (T)

Fig. 3. Magnetization as function of applied magnetic field for the La0.7 Sr0.3 MnO3 sample measured at different temperatures around TC .

4000 250 K 3500

2 2 M (emu/g)

3000 2500 2000

δT = 3K

1500 1000 500 391 K 0 0,00

0,05

0,10

0,15

0,20

0,25

0,30

H/M (T.g/emu)

Fig. 4. Arrott curves for the La0.7 Sr0.3 MnO3 sample.

be due to a canted spin state at low temperature. In the paramagnetic state, the inverse of the susceptibility versus temperature exhibits the Curie-Weiss law χ = C/(T − θp ). From the linearity of the χ−1 curve, the paramagnetic Curie temperature θp is found to be 363 K. The obtained Curie constant C is 0.54 µB K/kOe. The evolution of magnetization obtained at different temperatures reveals a strong variation of magnetization around the Curie temperature. It indicates that there is a possible large magnetic entropy change associated with the ferromagnetic-paramagnetic transition temperature, occurring at TC .

3.4 Magnetocaloric effect In order to investigate this idea, we calculate the magnetic entropy change (∆SM ), which result from spin ordering (i.e., ferromagnetic ordering). Hence, (∆SM ) can be measured through the adiabatic change of temperature by the application of a magnetic field.

Eur. Phys. J. Plus (2014) 129: 83

Page 5 of 8 60

2.5 µsp

χ-1 50

2

µsp (µB )

30 1

χ-1 (kOe/ µ B)

40 1.5

20 0.5

10

0 270

300

330

360

0 390

T (K)

Fig. 5. Spontaneous magnetization and reciprocal susceptibility data versus temperature for La0.7 Sr0.3 MnO3 .

Magnetic entropy change (-∆S M(J/Kg.K))

2.5 ∆H=5T ∆H=4T ∆H=3T ∆H=2T ∆H=1T ∆H=0.5T ∆H=0.25T

2

1.5

1

0.5

0 240

260

280

300 320 340 Temperature (K)

360

380

400

Fig. 6. The temperature dependence of the magnetic entropy under different applied magnetic fields for the La0.7 Sr0.3 MnO3 sample.

According to a thermodynamic Maxwell’s relationship: (∂M/∂H)T = (∂M/∂T )H , the magnetic entropy change ∆SM produced by the variation of a magnetic fields is expressed as: µ0 H2 δM (T, µ0 H) (1) ∆SM (T, ∆(µ0 H)) = S(T, µ0 H2 ) − S(T, µ0 H1 ) = d(µ0 H). δT µ0 H1 In this works ∆SM calculated according to the classical thermodynamic theory using the following equation [23]: Mi − Mi+1 | − ∆SM | = (2) ∆Hi Ti+1 − Ti i Where Mi and Mi+1 are the experimental values of magnetization at temperatures Ti and Ti+1 respectively, under magnetic applied field Hi . The magnetic entropy change (∆SM ), was determined numerically and the M (T, µ0 H) curves, are shown in fig. 6. max It can be seen that ∆SM depends on µ0 H and that the sample exhibits a maximum entropy change (∆SM ) around the Curie temperature TC .

Page 6 of 8

Eur. Phys. J. Plus (2014) 129: 83

Table 3. Summary of magnetocaloric properties of La0.7 Sr0.3 MnO3 compared with the same LSMO magnetic materials. Sample

TC (K)

max |∆SM | (JKg−1 K−1 )

δTF W HM (K)

RCP(JKg−1 )

∆H(T )

Ref.

La0.7 Sr0.3 MnO3

370

1.10

44.5

49

2

This work

La0.7 Sr0.3 MnO3

370

1.27

22.8

29

2

[11]

La0.7 Sr0.3 MnO3

365

2.66

26

69

2

[18]

La0.7 Sr0.3 MnO3

374

1.78

43

77

2

[19]

La0.67 Sr0.33 MnO3

370

2.02

40

80

2

[20]

Table 4. Critical parameters β and γ calculated from n and δ. Composition

Ref.

TC (K)

n

δ

β

γ

La0.7 Sr0.3 MnO3

This work

370

0.815(9)

3.491(7)

0.608(7)

1.516(6)

Mean-field model

[25]

–

0.66

3

0.5

1

1

5.5 ln (RCP)

ln (-∆SMmax)

5

0.5

4.5

4 -0.5 3.5

ln (RCP)

ln (- ∆SMmax )

0

-1 3 -1.5

-2 -1.5

2.5

2 -1

-0.5

0

0.5 ln (µ0H)

1

1.5

2

max Fig. 7. ln(−∆SM ) and ln(RCP) vs. ln(H) plot for the studied compound.

To evaluate the applicability of (LSMO) composition as a magnetic refrigerant, the obtained values of magnetic entropy change in our study are compared with the same LSMO magnetic materials in table 3 [24–26]. In order to demonstrate the influence of critical exponent on magnetocaloric effect, the field dependence of entropy change is analyzed. According to Oesterreicher et al. [27], the field dependence of the magnetic entropy change of materials with a second order phase transition can be expressed as ∆SM ∝ H n ,

(3)

where the exponent n depends on the magnetic state of the compound. It can be locally calculated as follows: n=

d ln ∆SM . d ln H

(4)

In the particular case of T = TC or at the temperature of the peak entropy change, the exponent n becomes field independent [28]. In the case, β−1 n(Tc ) = 1 + , (5) β+γ where β and γ are the critical exponents. With, βδ = (β + γ) the relation (5) can be written as 1 1 n(Tc ) = 1 + 1− . (6) δ β

Eur. Phys. J. Plus (2014) 129: 83

Page 7 of 8 3.4

3.3

3.2

ln (M)

3.1

3

2.9

2.8

2.7 0

0.36

0.72

1.08

1.44

1.8

ln (µ0H)

Fig. 8. ln(M ) vs. ln(H) plot for La0.7 Sr0.3 MnO3 at T = TC .

To determine the exponent n, a linear plot of ∆SM vs. H is constructed at the transition temperature of the peak of the magnetic entropy change, i.e., at 358 K which is shown in fig. 7. The value of n obtained from the slope is 0.815(9). This value is higher than the mean field predictions n = 2/3 [23]. The deviation from the mean field behavior is due to the presence of local inhomogeneities in the vicinity of transition temperature [29]. Since the modified Arrott method is unsuccessful in obtaining the critical exponents for the compounds, the value of n is used to calculate the order parameters. From the values of n and δ (which are obtained from the slope of high-field region of ln(M ) vs. ln(H) plot at the respective transition temperatures showing maximum entropy change of each compound (fig. 8)). Table 4 shows the values of β and γ calculated from eqs. (5) and (6). The obtained critical exponents β and γ are related to the theoretical value of mean field model (β = 0.5, γ = 1 and δ = 3) [30]. The deviation from the mean field model is due to inhomogeneous magnetic state [31]. Generally, the important index for selecting magnetic refrigerants is based on the cooling power per unit volume, namely, the relative cooling power (RCP) [32]. The RCP has been defined as max RCP(S) = |∆SM | × δTF W HM ,

(7)

max | is the maximum magnetic entropy change and δTF W HM is the full width at half maximum of the where |∆SM magnetic entropy change curve, has been calculated also for our sample. The field dependence of RCP can be expressed as a power law [33]:

RCP ∝ H 1+1/δ

(8)

The RCP of the sample should scale with field as a power law, is shown in fig. 7. A non-linear fit of RCP vs. field for the sample has been performed to extract the value of the exponent in eq. (8) where, the critical exponent of the magnetic transition is δ = 3.491(7).

4 Conclusion We have successfully elaborated and studied the effect of the substitution of the trivalent ion La3+ by a divalent ion Sr2+ on the structural, magnetic and magnetocaloric properties of the La0.7 Sr0.3 MnO3 sample using the ball-milling method. X-ray diffraction shows that our sample is single phased and crystallizes in the orthorhombic structure with Pnma space group. The magnetic measurements show a paramagnetic-ferromagnetic transition at TC = 370 K. A high Curie temperature is observed in the La0.7 Sr0.3 MnO3 sample higher than room temperature, suggesting that these materials can be used as magnetic refrigerants. This study has been supported by the Tunisian Ministry of Scientific Research and Technology and the Neel Institute.

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Eur. Phys. J. Plus (2014) 129: 83

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A. Tozri, E. Dhahri, E.K. Hlil, Mat. Lett. 64, 2138 (2010). A. Tozri, E. Dhahri, E.K. Hlil, Phys. Lett. A 375, 1528 (2011). S. Othmani, M. Bejar, E. Dhahri, E.K. Hlil, J. Alloys Comp. 475, 46 (2009). G.h. Jonker, J.H. Van Santen, Physica 16, 337 (1950). J.H. Van Santen, G.h. Jonker, Physica 16, 599 (1950). E.O. Wollan, W.C. Koehler, Phys. Rev. 100, 545 (1955). Abir Nasri, S. Zouari, M. Ellouze, J.L. Rehspringer, A.-F. Lehlooh, F. Elhalouani, J. Supercond. Nov. Magn. 27, 443 (2014). S. Zouari, M. Ellouze, A. Nasri, W. Cherif, E.K. Hlil, F. Elhalouani, J. Supercond. Nov. Magn. 6, 2435 (2013). C. Suryanarayana, Prog. Mater. Sci. 46, 1 (2001). J.C. de Lima, V.H.F. dos Santos, T.A. Grandi, Nanostruct. Mater. 11, 51 (1999). K.D. Machado, J.C. de Lima, C.E.M. de Campos, T.A. Grandi, A.A.M. Gasperini, Solid State Commun. 127, 477 (2003). H.M. Rietveld, J. Appl. Cryst. 2, 65 (1969). J. Rodrigez-Carjaval, XVth Congess of the International Union of Crystallography, Proceedings of the Satellite Meeting on Powder Diffraction, Toulouse (1990) 127. Z.F. Zi, Y.P. Sun, X.B. Zhu, Z.R. Yang, J.M. Dai, W.H. Song, J. Magn. Magn. Mat. 321, 2378 (2009). N. Kallel, K. Fr¨ ohlich, S. Pignard, M. Oumezzine, H. Vincent, J. Alloys. Comp. 399, 20 (2005). M.I. Mendelson, J. Am. Ceram. Soc. 52, 443 (1969). N. Kallel, S. Kallel, A. Hagaza, M. Oumezzine, Physica B 404, 285 (2009). N. Kallel, M. Oumezzine, H. Vincent, J. Magn. Magn. Matter 320, 1810 (2008). N. Kallel, G. Dezanneau, J. Dhahri, M. Oumezzine, H. Vincent, J. Magn. Magn. Matter 261, 56 (2003). N. Abdelmoula, K. Guidara, A. Cheikh-Rouhou, E. Dhahri, J. Solid State Chem. 151, 139 (2000). N. Abdelmoula, L. Reversat, A. Cheikhrouhou, J. Phys.: Condens. Matter 13, 449 (2001). W. Cheikh-RouhouKoubaa, M. Koubaa, A. Cheikhrouhou, J. Alloys Comp. 453, 42 (2008). M. Foldeaki, R. Chahine, B.R. Gopal, T.K. Bose, J. Magn. Magn. Matter 150, 421 (1995). N.V. Dai, D.V. Son, S.C. Yu, L.V. Bau, L.V. Hong, N.X. Phuc, D.N.H. Nam, Phys. Status Solidi B 244, 4570 (2007). M. Bejar, N. Sdiri, M. Hussein, M. Mazen, E. Dhahri, J. Magn. Magn. Matter 316, e566 (2007). Y. Xu, M. Meier, P. Das, M.R. Koblischka, U. Hartmann, Cryst. Eng. 5383, (2002). H. Oesterreicher, F.T. Parker, J. Appl. Phys. 55, 4336 (1984). V. Franco, A. Conde, M.D. Kuz’min, J.M. Romero-Enrique, J. Appl. Phys. 105, 07A917 (2009). Q.Y. Dong, H.W. Zhang, J.R. Sun, B.G. Shen, V. Franco, J. Appl. Phys. 103, 1161 (2008). N. Moutis, I. Panagiotopoulos, M. Pissas, D. Niarchos, Phys. Rev. B 59, 1129 (1999). S. Xu, W. Tong, J. Fan, J. Gao, C. Zha, Zhang, J. Magn. Magn. Matter 288, 92 (2005). J.S. Lee, Phys. Status Solidi B 241, 1765 (2004). V. Franco, A. Conde, Int. J. Refrig. 33, 465 (2010).

Journal of Solid State Chemistry 215 (2014) 271–276

Contents lists available at ScienceDirect

Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc

Magnetic and magnetocaloric properties of La0.6Pr0.1Sr0.3Mn1 xFexO3 (0 rx r0.3) manganites R. Cherif a,n, E.K. Hlil b, M. Ellouze a, F. Elhalouani c, S. Obbade d a

Sfax University, Faculty of Sciences of Sfax, B. P. 1171, 3000 Sfax, Tunisia Institut Néel, CNRS et Université Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France c Sfax University, National Engineering School of Sfax, B. P. W, 3038 Sfax, Tunisia d LEPMI UMR 5279, CNRS – Grenoble INP – Université de Savoie – Université Joseph Fourier, 1130 rue de la Piscine, BP 75, 38402 Saint-Martin d'Hères Cedex, France b

art ic l e i nf o

a b s t r a c t

Article history: Received 4 February 2014 Received in revised form 4 April 2014 Accepted 6 April 2014 Available online 16 April 2014

The La0.6Pr0.1Sr0.3Mn1 xFexO3 (x ¼0, 0.1, 0.2 and 0.3) samples have been elaborated by the solid-state reaction method. X-ray powder diffraction shows that all the samples crystallize in a rhombohedric phase with R3c space group. The variation of magnetization as a function of temperature and applied magnetic field was carried out. The samples for x ¼ 0 and 0.1 exhibit a FM–PM transition at the Curie temperature TC, however, for x ¼0.2 and 0.3 exhibit an AFM–PM one at the Neel temperature TN, when the temperature increases. A magneto-caloric effect has been calculated in terms of isothermal magnetic entropy change. A large magneto-caloric effect has been observed, the maximum entropy change, jΔSmax M j, reaches the highest value of 3.28 J/kgK under a magnetic field change of 5 T with an RCP value of 220 J/kg for La0.6Pr0.1Sr0.3MnO3 composition, which will be an interesting compound for application materials working as magnetic refrigerants near room temperature. & 2014 Elsevier Inc. All rights reserved.

Keywords: Perovskite Solid-state reaction Magneto-caloric (MC) effect Relative cooling power (RCP)

1. Introduction Magnetic refrigeration at room temperature is of particular interest due to its expected great potential impact on energy savings and environmental friendliness compared to the conventional gas refrigeration [1,2]. Over the past years, several works on the magneto-caloric (MC) effect involving perovskite-type rare-earth manganites such as Ln1 xAxMnO3 (where Ln is the trivalent rareearth ions and A: divalent ions such as alkaline elements, which are located at A-sites in the perovskite ABO3) have attracted considerable attention of scientific communities due to their potential applications [3–5]. The MC effect has been studied widely for magnetic refrigeration technology, for which the key is to seek the proper material that can produce a large entropy variation using the magnetization– demagnetization process [6,7]. The MC effect is intrinsic to the magnetic materials and can be quantified by the isothermal magnetic entropy change, jΔSM j. The main requirements for a magnetic material to possess a largejΔSM j, are the large spontaneous magnetization as well as the sharp drop in the magnetization associated with the ferromagnetic to paramagnetic transition at the Curie temperature TC [8,9]. Over the past few years, the effects of Mn site

n

Corresponding author. Fax: þ 216 74 274 437. E-mail address: [email protected] (R. Cherif).

http://dx.doi.org/10.1016/j.jssc.2014.04.004 0022-4596/& 2014 Elsevier Inc. All rights reserved.

substitution [10–13] by foreign trivalent elements (A0 ) AMn1 xAx0 O3 compounds have received renewed interest. Indeed, extensive studies for the perovskite manganites ferrite AMn1 xFexO3 have been carried out [14–16]. These studies discuss the effect of Fe doping on the structural and magnetic properties. Such manganites have a mixed structure containing trivalent Mn3 þ and trivalent Fe3 þ ions occupying both octahedral (B) sites [17,18]. The substitution of Mn ions by Fe ions in AMn1 xFexO3 manganites, introduce lattice distortions but also reduce the number of Mn3 þ /Mn4 þ ratio. The aim of this work is to study the effect of the substitution of the Mn ions by Fe ions on the structural and magnetic properties in La0.6Pr0.1Sr0.3Mn1 xFexO3 (x¼ 0, 0.1, 0.2 and 0.3) compounds and the magnetocaloric effect for the La0.6Pr0.1Sr0.3MnO3 and La0.6Pr0.1Sr0.3Mn0.9Fe0.1O3 samples.

2. Experimental Powder samples La0.6Pr0.1Sr0.3Mn1 xFexO3 (0 r xr0.3) were prepared using the conventional solid-state reaction by mixing La2O3, Pr6O11, SrCO3, MnCO3 and (Fe2O3,H2O) up to 99.9% purity in the desired proportion according to the following reaction: 3La2O3 þ0.1/6Pr6O11 þ0.3SrCO3 þ (1 x)MnCO3 þx/2Fe2O3,H2O -La0.6Pr0.1Sr0.3Mn1 xFexO3 þδCO2.

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R. Cherif et al. / Journal of Solid State Chemistry 215 (2014) 271–276

The starting materials were intimately mixed in an agate mortar, and then heated in air at 900 1C for 10 h, also was sintered at 1000 1C for 12 h. The obtained mixture was pressed in the form of pellets and sintered at 1200 1C for 50 h. Finally, these pellets were grinded and sintered at 1400 1C, thrice for 24 h, followed by grinding in an agate mortar. For each sample, phase purity, homogeneity and crystal properties were determined by powder X-ray diffraction (XRD) data, recorded at room temperature on a PANalytical X'PERT Pro MPD diffractometer, using θ/2θ Bragg-Brentano geometry with diffracted beam monochromatized CuKα radiation. The diffraction patterns were collected by steps of 0.0171 over the angle range 10–801, with a counting time of 75 s. per step. The powder X-ray diffraction diagrams were refined by the Rietveld method, using the Fullprof program [19,20]. Magnetizations (M) versus temperature (T) and magnetic applied field (H) were measured using a SQUID magnetometer. M (T) was obtained under 0.05 T. Isothermal M (H) data were measured up to 10 T. The magnetocaloric (MC) effect results were deduced from the magnetization measurements versus magnetic applied field up to 5 T at several temperatures.

3. Results and discussion 3.1. X-ray diffraction For each sample, the unit cell parameters were refined in rhombohedric structure with R3c space group. The refinement

was carried out using the “pattern matching” option of Fullprof program, where only the profile parameters (cell dimensions, peak shapes, zero point correction and symmetry) have been refined. The peak shape was described by a Pseudo-Voigt function with an asymmetry correction at low angles. At the end of each refinement, the good fitting between observed and calculated patterns (Fig. 1) was indicated by the significant values of the profile reliability factors (Rp, Rwp), reported in Table 1. The refined lattice parameters are also summarized in Table 1. 3.2. Magnetic properties Magnetization versus temperature has been undertaken under an applied magnetic field of 0.05 T for all samples and shown in Fig. 2. When the temperature increases, the samples (x ¼0 and x¼ 0.1) show a ferromagnetic–paramagnetic (FM–PM) transition at Curie temperature (TC) as shown in Fig. 2a. Samples with x ¼0.2

Table 1 Refined structural parameters of La0.6Pr0.1Sr0.3Mn1 xFexO3 (0rxr0.3) compounds. The numbers in subscript represent the error bars. x

0

0.1

0.2

0.3

a ¼b (Å) c (Å) V (Å3) Rp (%) Rwp (%)

5.49531 13.33673 348.791 3.09 4.18

5.49831 13.34103 349.281 3.59 4.91

5.50161 13.35044 349.951 3.27 4.63

5.50561 13.35763 350.631 3.16 4.39

Fig. 1. Observed (solid symbols) and calculated (solid lines) X-ray diffraction pattern for La0.6Pr0.1Sr0.3Mn1 xFexO3 (0r x r0.3) samples. Positions for the Bragg reflections are marked by vertical bars. Differences between the observed and the calculated intensities are shown.


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Fig. 2. Magnetization measurements as a function of temperature for La0.6Pr0.1Sr0.3Mn1 xFexO3 samples under 0.05 T: (a) x¼ 0 and 0.1, (b) x¼ 0.2 and 0.3.

Table 2 Physical parameters of La0.6Pr0.1Sr0.3Mn1 xFexO3 (0 r xr 0.3) compounds. x

TC (K)

TN (K)

θP (K)

C (mB K/kOe)

0 0.1 0.2 0.3

329 205 – –

– – 40 45

336 212 – –

0.76 1.13 – –

Fig. 4. The spontaneous magnetization experimental (Mspexp) and calculated (Mspcal) as a function of Fe content for the La0.6Pr0.1Sr0.3Mn1 xFexO3 (0 r x r 0.3) compounds at T ¼5 K.

Fig. 3. Magnetization versus magnetic applied field at 5 K for La0.6Pr0.1Sr0.3Mn1 xFexO3 (0rxr0.3) samples.

x¼ 0.3. This confirms the presence of the AFM interactions, which are more important for higher Fe-contents (x ¼0.2 and 0.3) [21]. The values of the spontaneous magnetizations at T¼ 5 K were calculated considering the total spins of Mn3 þ , Mn4 þ ,Fe3 þ , þ þ and Pr3 þ ions (m3Mn ¼ 4μB, m4Mn ¼ 3μB, m3Feþ ¼ 5μB, and m3Prþ ¼2μB). The spontaneous magnetizations of the La30:6þ Pr30:1þ Sr20:3þ ðMn1 x Fex Þ30:7þ Mn40:3þ O3 (x¼ 0 0.3) compounds are expressed as follows: M sp ¼ 4 ½0:7 ð1 xÞ–5 ð0:7 xÞ þ 3 0:3 þ 2 0:1μB ¼ ½3:9 6:3 xμB

and x¼ 0.3 show an antiferromagnetic–paramagnetic (AFM–PM) transition at Neel temperature (TN) (see Fig. 2b). The TC and TN temperatures values, determined from dM/dT curves are listed in Table 2. From this figure we can see that if x-value increases, TN increases and TC decreases. Fig. 3 shows the magnetization versus magnetic applied field at 5 K curves for La0.6Pr0.1Sr0.3Mn1 xFexO3 (0 r xr 0.3) samples. It can be seen that the magnetization measurements versus magnetic applied field up to 10 T at low temperatures (T oTC) confirmed the ferromagnetic behavior for x ¼0 and x¼ 0.1 samples. The magnetization increases sharply with magnetic applied field for m0H o 1 T and then saturates (x ¼0 and x¼ 0.1). However, the variation of M (μ0H) does not reach the saturation for x ¼0.2 and

where x is the iron content and μB is the Bohr magneton. The measured spontaneous magnetizations at T ¼5 K for x ¼0, 0.1, 0.2, and 0.3 compounds are found to be about 3.54μB, 3.05μB, 0.45μB and 0.26μB, respectively, while the calculated values for full spin alignment are 3.9μB, 3.27μB, 2.64μB and 2.01μB, respectively. The spontaneous magnetization decreases with increasing of Fe content. The difference between measured and calculated values especially for x¼ 0.2 and 0.3 should be explained by spin canted state at low temperature (see Fig. 4). Fig. 5a and b shows isothermal magnetization curves for La0.6Pr0.1Sr0.3MnO3 and La0.6Pr0.1Sr0.3Mn0.9Fe0.1O3 samples, respectively, measured under applied magnetic field ranging at 0–5 T and at temperature ranging at 250–382 K and 100–280 K, respectively.

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3.3. Arrott curves Banerjee [22] has given an experimental criterion which allows the determination of the nature of the magnetic transition (first or second order). It consists in the observation of the slope of the isotherm plots μ0H/M versus M2. Applying a regular approach, the straight line was constructed simply extrapolating the high magnetization parts of the curves for each studied temperature. A positive or negative slope indicates a second order or first order transition, respectively. Fig. 6a and b shows the isotherm plots M2 versus μ0H/M above and below TC for La0.6Pr0.1Sr0.3MnO3 and La0.6Pr0.1Sr0.3Mn0.9Fe0.1O3 samples, respectively. These samples show positive slopes in the complete M2 ranges, indicating that the system exhibits a second-order ferromagnetic to paramagnetic phase transition. Fig. 7 shows the temperature dependence of the spontaneous magnetization (Msp) and the inverse of the magnetic susceptibility evolution versus temperature for La0.6Pr0.1Sr0.3MnO3 sample. The Msp (T) curve decreases with increasing temperature and then drops rapidly near 340 K. In the paramagnetic state, the inverse of the susceptibility versus temperature exhibits the Curie–Weiss law [23] χ ¼ C=ðT θp Þ. From the linearity of the χ 1 curve, the paramagnetic Curie temperature θp is found to be 336 K. The obtained Curie constant C is 0.76mB K/kOe (see Table 2).

The positive value of θp reveals the existence of a ferromagnetic exchange interaction between the nearest neighbors. The obtained value is slightly higher than TC. We can deduce that this difference depends on the substance and is related to the presence of short-range

Fig. 7. The spontaneous magnetization (Msp) and the inverse of the magnetic susceptibility χ 1 versus temperature for La0.6Pr0.1Sr0.3MnO3 sample.

Fig. 5. Isothermal magnetization M (H) for La0.6Pr0.1Sr0.3Mn1 xFexO3 samples at several temperatures: (a) x ¼0 and (b) x ¼0.1.

Fig. 6. Arrot curves M2 versus μ0H/M for La0.6Pr0.1Sr0.3Mn1 xFexO3 samples: (a) x ¼ 0 and (b) x¼ 0.1.


ordered slightly above the Curie temperature, which is related to the presence of a magnetic inhomogeneity [24]. 3.4. Magneto-caloric effect In the isothermal process of magnetization, the MC effect of the materials can be derived from Maxwell's thermodynamic relationship [25]: ∂M ∂S ¼ ð1Þ ∂T H ∂H T The magnetic-entropy change ΔSM , which results from the spin ordering (i.e. ferromagnetic ordering) and is induced by the variation of the magnetic applied field from 0 to H max is given by Z Hmax ∂M dH ð2Þ ΔSM ðT; H max Þ ¼ SM ðT; H max Þ SM ðT; 0Þ ¼ ∂T H 0 For magnetization measured at discrete field and temperature intervals, the magnetic entropy change defined in Eq. (2) can be approximated by the following equation [26]: jΔSM j ¼ ∑ i

Mi Mi þ 1 ΔH i Tiþ1 Ti

ð3Þ

As shown in Fig. 8a and b, the magnetic entropy changes of La0.6Pr0.1Sr0.3Mn1 xFexO3 (x¼ 0 and x ¼0.1, respectively) samples associated with the magnetic field variations (1, 2, 3, 4 and 5 T) were calculated using Eq. (3). From Eq. (2), it is obvious to find that the magnetic entropy changes depend on the value ofð∂M=∂TÞH . Therefore, the large magnetic entropy changes usually occur near the TC where the magnetization changes rapidly when the temperature varies. Furthermore, the effect maybe further maximized as the variation in magnetization with respect to temperature appears in a narrow temperature interval [27,28]. Similar to the above analysis, Fig. 8a and b shows that the maximum of magnetic entropy changes under the different magnetic fields always concentrate on the TC region. With the increase of magnetic field, the peak of magnetic entropy changes slightly and moves to a higher temperature due to the shift of effective TC by the applied magnetic field. For comparison, we have listed in Table 3 the data of several magnetic materials that could be used as magnetic refrigerants [29–33]. We noted that the maximum entropy change, jΔSMax M j, corresponding to a magnetic field variation of 2 T is found to be 1.83 J/kg K and 1.15 J/kg K for x ¼0 and 0.1, respectively. Similar results were also observed by Bejar et al. [30] for La0.7Sr0.3MnO3 and Wang et al. [31] for La0.7Sr0.3Mn0.9Cr0.1O3 compounds under the applied magnetic field of 2 T and 1 T,

275

respectively. Elsewhere, the jΔSMax M j result (2.4 J/Kg K) for La0.6Pr0.1 Sr0.3Mn0.9Fe0.1O3 at 5 T is slightly lower to this reported by Kawanaka et al. [34] (3.2 J/Kg K) by studying La0.67Ba0.33Mn1 xFexO3 samples prepared by the solid-state reaction method. These values are lower than that of pure Gd (3.25 and 10.2 J/kg K) in a magnetic field change of 1 T and 5 T respectively [32,33]; and Gd5Si2Ge2 system [33] which have been considered as a good magnetic refrigerants. When comparing different magneto-caloric Table 3 Maximum entropy change jΔSMax M j and relative cooling power (RCP), for La0.6Pr0.1 Sr0.3MnO3 and La0.6Pr0.1Sr0.3Mn0.9Fe0.1O3 compounds, occurring at the Curie temperature (TC) and under magnetic field variations, compared to several materials considered for magnetic refrigeration. Composition

TC (K)

m0ΔH (T)

jΔSMax M j (J/ Kg K)

RCP (J/Kg)

Reference

La0.6Pr0.1Sr0.3MnO3 La0.6Pr0.1Sr0.3Mn0.9Fe0.1O3 La0.6Pr0.1Sr0.3MnO3 La0.7Sr0.3MnO3 La0.7Sr0.3Mn0.9Cr0.1O3 Gd Gd Gd5Si2Ge2

329 205 345 374 315 293 294 276

2 2 1.5 2 1 1 5 5

1.83 1.15 2.52 1.78 1.16 3.25 10.2 18.4

95.17 43.12 48.41 – 43.3 – 410 535

Our work Our work [29] [30] [31] [32] [33] [33]

Fig. 9. Variation of the relative cooling power as function of the applied magnetic field for La0.6Pr0.1Sr0.3Mn1 xFexO3 (x¼ 0 and x ¼0.1) compounds.

Fig. 8. Magnetic entropy change versus temperature for La0.6Pr0.1Sr0.3Mn1 xFexO3 samples at several magnetic applied field changes: (a) x¼ 0 and (b) x ¼0.1.

276


materials it is useful to calculate their relative cooling power based on the magnetic entropy change. The relative cooling power evaluated by considering the magnitude of ΔSM and its full width at half-maximum δTFWHM was expressed as follows [35]: δT FWHM : RCP ¼ ΔSmax M

Acknowledgments This study has been supported by the Tunisian Ministry of Scientific Research and Technology, School Phelma Grenoble and the Neel Institute. References

The results of this estimation are shown in Fig. 9. The RCP values exhibit a linear increase with increasing field for all compounds. For our samples, the RCP values are 95.17 J/kg and 43.12 J/kg at 2 T for x ¼0 and 0.1, respectively. The RCP of La0.6Pr0.1Sr0.3MnO3 compound is higher than that found by Zhang et al. [29], and similar for La0.6Pr0.1Sr0.3Mn0.9Fe0.1O3 compound reported by Wang and Jiang. [31]. However, the pure Gd, considered a good material, exhibits an RCP value of 250 J/kg at 2 T [35] while Gd5Si2Ge2 considered as the conspicuous magnetocaloric material at room temperature, exhibits an RCP value of 130 J/kg at 2 T [36]. In other hand, the RCP under a magnetic field change of 5 T for La0.6Pr0.1Sr0.3Mn0.9Fe0.1O3 sample is 198 J/Kg at 217 K which is higher than those obtained by Tka et al. [37] for La0.57Nd0.1Sr0.33Mn0.9Sn0.1O3 compound prepared by the solid state reaction method ( 56 J/Kg at 5 T) and similar than those obtained by Sun et al. [38] for La0.67Sr0.33Mn0.9Cr0.1O3 compound ( 200 J/Kg at 5 T). Furthermore, the RCP of La0.6Pr0.1Sr0.3MnO3 sample is found to be 220 J/Kg at 343 K for 5 T is higher than that obtained by Morelli et al. [39] for La0.67Sr0.33MnO3 compound (211 J/Kg at 5 T). The RCP values show a difference between the two samples. This difference is due to the substitution of the manganese by the iron ions. The values of RCP are extended over a wide range of temperature around the Curie temperature in both the samples and hence these materials are useful for near room temperature magnetic refrigeration applications.

4. Conclusion The structural magnetic and magneto-caloric properties of polycrystalline La0.6Pr0.1Sr0.3Mn1 xFexO3, have been investigated. The samples were prepared by the standard ceramic process. They show single-phase and crystallize in the rhombohedric structure with R3c space group. Some compounds exhibit a ferromagnetic– paramagnetic transition with increasing temperature and others an antiferromagnetic–paramagnetic one. The magnetic entropy change of La0.6Pr0.1Sr0.3Mn1 xFexO3 has been estimated. The maximum of the magnetic entropy change under a magnetic field change of 5 T is found to be 3.28 J kg 1 K 1 and 2.33 J kg 1 K 1 for x ¼0 and 0.1 respectively. Our samples exhibit high RCP values of 220 J/kg and 198 J/kg at 5 T for x ¼0 and 0.1 respectively. Our investigations indicate that La0.6Pr0.1Sr0.3MnO3 and La0.6Pr0.1Sr0.3 Mn0.9Fe0.1O3 compounds have appropriate properties for being considered as suitable candidates for producing magnetic refrigeration near room temperature.

[1] J.S. Amaral, M.S. Reis, V.S. Amaral, T.M. Mendonca, J.P. Araujo, M.A. Sa’, P.B. Tavares, J.M. Vieira, J. Magn. Magn. Mater. 290 (2005) 686. [2] M.d. MotinSeikh, L. Sudheendra, C.N.R. Rao, J. Solid State Chem. 177 (2004) 3633. [3] S. Othmani, R. Blel, M. Bejar, M. Sajieddine, E. Dhahri, E.K. Hlil, Solid State Commun. 149 (2009) 969. [4] K.F. Wang, F. Yuan, S. Dong, D. Li, Z.D. Zhang, Z.F. Ren, J.-M. Liu, Appl. Phys. Lett. 89 (2006) 222505. [5] L. Li, K. Nishimura, M. Fujii, K. Mori, Solid State Commun. 144 (2007) 10. [6] Soma Das, T.K. Dey, J. Alloys Compd. 440 (2007) 30. [7] Soma Das, T.K. Dey, J. Mater. Chem. Phys. 108 (2008) 220. [8] S.Y. Dan’kov, A.M. Tishin, V.K. Pecharsky, K.A. Gschneidner, Phys. Rev. B 57 (1998) 3478. [9] M.H. Phan, S.C. Yu, N.H. Hur, Appl. Phys. Lett. 86 (2005) 072504. [10] A. Barnabe, A. Maignan, M. Hervieu, F. Damay, C. Martin, B. Raveau, Appl. Phys. Lett. 71 (1997) 3907. [11] B. Raveau, A. Maignan, C. Martin, J. Solid State Chem. 130 (1997) 162. [12] A. Maignan, C. Martin, F. Damay, M. Herieu, B. Raveau, J. Magn. Magn. Mater. 188 (1998) 185. [13] A. Barnabe, A. Maignan, M. Herieu, B. Raveau, J. Eur. Phys. B 1 (1998) 145. [14] Abir Nasri, S. Zouari, M. Ellouze, J.L. Rehspringer, A.-F. Lehlooh, F. Elhalouani, J. Supercond. Novel. Magn. 27 (2014) 443. [15] S. Zouari, M. Ellouze, E.K. Hlil, F. Elhalouani, M. Sajieddine, Solid State Commun. 180 (2014) 16. [16] S. Zouari, A. Nasri, M. Ellouze, E.K. Hlil, F. Elhalouani, J. Supercond. Novel Magn. 6 (2013) 2435. [17] G.H. Rao, J.R. Sun, A. Kattwinkel, L. Haupt, K. Baerner, E. Schmitt, E. Gmelin, Physica B 269 (1999) 379. [18] M. Sugantha, R.S. Singh, A. Guha, A.K. Raychaudhuri, C.N.R. Rao, Mater. Res. Bull. 33 (1998) 1129. [19] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [20] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [21] F. Issaoui, M.T. Tlili, M. Bejar, E. Dhahri, E.K. Hlil, J. Supercond. Novel Magn. 25 (4) (2012) 1169. [22] S.K. Banerjee, Phys. Lett. 12 (1964) 16. [23] A.H. Morrish, The Physical Principles of MagnetismIEEE Press, New York, 2001. [24] A. Tozri, E. Dhahri, E.K. Hlil, J. Magn. Magn. Mater. 322 (2010) 2516. [25] A.H. Morrish, The Physical Principles of MagnetismWiley, New York, 1965 (Chapter 3). [26] M. Foldeaki, R. Chahine, T.K. Bose, J. Appl. Phys. 77 (1995) 3528. [27] E. Bruck, J. Phys. D 38 (2005) R381. [28] J. Fan, L. Ling, B. Hong, L. Pi, Y. Zhang, J. Magn. Magn. Mater. 321 (2009) 2838. [29] YingDe. Zhang, Paula J. Lampen, The-Long Phan, Yu Seong-Cho., Hariharan Srikanth, Manh-Huong Phan, J. Appl. Phys. 111 (2012) 063918. [30] M. Bejar, N. Sdiri, M. Hussein, S. Mazen, E. Dhahri, J. Magn. Magn. Mater. 316 (2007) e566. [31] Z. Wang, J. Jiang, Solid State Sci. 18 (2013) 36. [32] J.B. Goodenough, Phys. Rev. B 100 (1955) 564. [33] V.K. Pecharsky, K.A. Gschneidner, A.O. Tsokol, Rep. Prog. Phys. 68 (2005) 1479. [34] Hirofumi Kawanaka, Hiroshi Bando, Yoshikazu Nishihara, J. Korean Phys. Soc. 63 (3) (2013) 529. [35] K.A. Gschneidner, V.K. Pecharsky, Annu. Rev. Mater. Sci. 30 (2000) 387. [36] V.K. Pecharsky, K.A. Gschneidner, Phys. Rev. Lett. 78 (1997) 4494. [37] E. Tka, K. Cherif, J. Dhahri, J. Appl. Phys. A (2013), http://dx.doi.org/10.1007/ s00339-013-8202-5. [38] Y. Sun, W. Tong, Y.H. Zhang, J. Magn. Magn. Mater. 232 (2001) 205. [39] D.T. Morelli, A.M. Mance, J.V. Mantese, A.L. Micheli, J. Appl. Phys. 79 (1996) 373.

J Mater Sci DOI 10.1007/s10853-014-8533-4

Study of magnetic and magnetocaloric properties of La0.6Pr0.1Ba0.3MnO3 and La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 perovskite-type manganese oxides R. Cherif • E. K. Hlil • M. Ellouze F. Elhalouani • S. Obbade

•

Received: 5 May 2014 / Accepted: 4 August 2014 Ó Springer Science+Business Media New York 2014

Abstract The compositional dependence of the magnetic and magnetocaloric properties of La0.6Pr0.1Ba0.3MnO3 (LPBMO) and La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 (LPBMFO) were investigated. Polycrystalline samples were prepared by the standard solid-state reaction method. Temperaturedependent magnetization measurements and Arrott analysis reveal second-order ferromagnetic transitions in both samples with Curie temperature increasing with doping iron from 94 K for LPBMO to 277 K for LPBMFO. Magnetic entropy change jDSM j was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field-dependent magnetization curves. However, the analysis of the magnetocaloric effect (MCE) using Landau theory of phase transition shows that the contributions to the free energy from the presence of ferromagnetic clusters are strongly influencing the MCE by coupling with the order parameter around the Curie temperature.

R. Cherif (&) M. Ellouze Faculty of Sciences of Sfax, Sfax University, BP 1171, 3000 Sfax, Tunisia e-mail: [email protected] E. K. Hlil Institut Neél, CNRS et Universite´ Joseph Fourrier, BP 166, 38042 Grenoble Cedex 9, France F. Elhalouani National Engineering School of Sfax, LASEM, Sfax University, BPW 3038, Sfax, Tunisia S. Obbade LEPMI UMR 5279, CNRS, Grenoble INP, Universite´ de Savoie, Universite´ Joseph Fourier, 1130 Rue de la Piscine, BP 75, 38402 Saint-Martin d’He`res Cedex, France

Introduction Recently, the refrigeration technology has been focused on the magnetic refrigeration based on the magnetocaloric effect (MCE) of a magnetic material because of their potential advantages over gas compression refrigeration [1–3]. The MCE has been studied widely for magnetic refrigeration technology with the aim of suppressing the emission of pollution components, which appear in conventional refrigeration systems. The key in using magnetic refrigeration at room temperature is to seek the proper material whose Curie temperature is near room temperature and which can produce a large entropy variation when it goes through a magnetization– demagnetization process [4, 5]. In recent years, extensive attention has been paid to the possibility of room temperature magnetic refrigeration. Pecharsky and Gschneidner [6] discovered a giant magnetic entropy change associated with the transition temperature (TC) in Gd metal and then in Gd5Si2Ge2 alloy. The last compound exhibits a MCE about twice as large as that exhibited by Gd, the best known magnetic refrigerant material for near room temperature applications. However, the purpose in searching a proper material with the large magnetic entropy change and its possibility of various temperature ranges application is always required. The MCE is defined as the heating or cooling of a magnetic material due to the application of a magnetic field. The magnetic entropy change jDSM j that results from the spin ordering and is induced by the variation of the applied magnetic field from 0 to l0Hmax, is given by [7]: Z l0 Hmax oM 1 jDSM j ¼ dH ¼ oT T T1 2 0 Z l0 Hmax Z l0 Hmax MðT2 ; HÞl0 dH MðT1 ; HÞl0 dH ; ð1Þ 0

0

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J Mater Sci

where l0Hmax is the maximum external field. For magnetization measured at discrete field and temperature intervals, the magnetic entropy change defined in Eq. (1) can be approximated by Eq. (2) [8] X Mi Miþ1 jDSM j ¼ DHi ð2Þ Tiþ1 Ti i where Mi and Mi?1 are the experimental values of the magnetization at Ti and Ti?1, respectively, under the applied magnetic field Hi. Besides alloys, they exist rare-earth perovskite manganites of the general formula La1-xMxMnO3 (M = Ca, Sr, Ba, etc.) which have attracted considerable attention because of their higher potential for magnetic sensor applications based on the magnetoresistance effect [9, 10]. In addition, these materials are very convenient for the preparation routes, and their Curie temperature can be justified under the various doping conditions. Therefore, the new trends have been focusing on studying the MCE of perovskite manganites [11–15]. Due to the substantial magnetic entropy change, they can be used for magnetic refrigeration applications in different temperature ranges. In the last few years, extensive studies have been carried out by doping at Mn site. They have suggested that the substitution of Mn by other transition metal elements having dissimilar electronic configuration, should lead to an important effects associated with the electronic configuration mismatch between Mn and the other substitute magnetic ions. Most of the studies have been focused on the Fe doping in manganites such as La1-xSrxMnO3 [16– 20] and La1-xCaxMnO3 [21–23] systems. Conversely, the Fe-doped La1-xBaxMnO3 system is much less investigated though it was the material of the initial discovery of the colossal magnetoresistance (CMR) effect in the form of thin films [24]. That is the reason for which we aim to study the Fe doping of La0.6Pr0.1Ba0.3MnO3 (LPBMO) managanites in order to shed light on their magnetic and magnetocaloric properties.

The starting materials were intimately mixed in an agate mortar, and then heated in air at 900 °C for 10 h, also was sintered at 1000 °C for 12 h. The obtained mixture was pressed in the form of pellets and sintered at 1200 °C for 50 h. Finally, these pellets were grinding and sintered at 1400 °C, for three times for 24 h, followed by grinding in an agate mortar. Phase purity and homogeneity were checked by X-ray diffraction (XRD) (Cu Ka radiation) at room temperature. The magnetization dependence on the temperature and applied external field were carried out by using a SQUID magnetometer (Quantum Design). Magnetizations of the samples were measured in an isothermal regime under an applied magnetic field varying from 0 to 5 T. The range of temperature for isotherm M vs. l0H measurements was fixed around the transition temperature of the sample (TC). This temperature was previously determined from M(T) curves, and coincides with the temperature at which a minimum of dM/dT is observed. Isothermal M vs. l0H curves were collected by steps of 3 K. The magnetic entropy changes, jDSM j, were estimated from the magnetization data using the Maxwell relation.

Results and discussions Structural properties The results of XRD indicate that both LPBMO and LPBMFO samples are single-phase perovskite manganites. Figure 1 shows a typical X-ray pattern of both samples displaying single structure without any other secondary or impurity phase.

Experiments Powder samples with the nominal composition of LPBMO and La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 (LPBMFO) were prepared using the conventional solid-state reaction by mixing La2O3, Pr6O11, BaCO3, MnCO3 and (Fe2O3, H2O) up to 99.9 % purity in the desired proportion according to the following reaction: 0:3La2 O3 þ 0:1=6Pr6 O11 þ 0:3SrCO3 þ ð1 xÞMnCO3 þ x=2Fe2 O3 ; H2 O ! La0:6 Pr0:1 Sr0:3 Mn1x Fex O3 þ dCO2 þ d0 H2

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Fig. 1 X-ray diffraction patterns for LPBMO and LPBMFO samples

J Mater Sci Table 1 Refined structural parameters of LPBMO and LPBMFO compounds at room temperature Parameter ˚) a = b (A ˚ c (A) ˚ 3) V (A

LPBMO 5.53232 13.50406 357.941

LPBMFO 5.52827 13.50128 357.342

The numbers in subscript represent the error bars

Magnetic properties

Fig. 2 Observed (solid line) and calculated (solid circles) XRD patterns of LPBMO and LPBMFO samples at room temperature. The difference between these spectra is plotted at the bottom. Bragg reflections are indicated by ticks

In Fig. 2, we displayed XRD refinement at room temperature, including the calculated and observed profiles as well as the difference profile, for both LPBMO and LPBMFO samples. The data were refined by the Rietveld technique using the Fullprof program [25, 26]. XRD patterns can be indexed in the rhombohedric system with R3c space group for both samples. With iron doping no structural changes were identified. The unit cell volume and the lattice parameters obtained by patterns refinements are shown in Table 1. We note that the lattice parameters and volume are not overly influenced by the insertion of iron. This is due to the fact that Mn3? and Fe3? have almost the same ionic ˚ and r(Mn3?) = 0.65 A ˚ [27]. These radius r(Fe3?) = 0.64 A results confirm those obtained by Liu et al. [28] and by Rao et al. [29]. Average crystallite sizes were determined from the broadening of XRD peaks using the Scherrer equation [30]. The average crystallites size is found to be 48 nm for both LPBMO and LPBMFO samples, and hence no noticeable size effect was introduced by Fe doping.

Figure 3a and b shows the temperature dependence of the magnetization in a magnetic applied field of 0.05 T. When increasing temperature, LPBMO and LPBMFO samples show a ferromagnetic–paramagnetic (FM–PM) transition. The Curie temperature TC, which is determined from the curve of M(T), as the inflection point [obtained using the numerical derivative dM/dT (see Fig. 3a, b)], increases with doping iron from 94 to 277 K for LPBMO and LPBMFO, respectively. This result is not in accordance with results from our previous work on La0.6Pr0.1Sr0.3 Mn1-xFexO3 (0 B x B 0.3) manganites [13]. The authors Boujelben et al. [31] and Ammar et al. [32] have also reported opposite results in their study carried out on Pr0.67Sr0.33Mn1-xFexO3 and Pr0.5Sr0.5Mn1-xFexO3 where they reported that Fe doping leads to the reduction of ferromagnetism and namely the decrease of the Curie temperature. It is worthy noticed that such compounds do not include Ba atoms. The observed increase of the Curie temperature from 94 to 277 K in our LPBMO and LPBMFO samples should be due to the presence of the Ba2? ion that has a greater ionic radius than the Sr2? ion. As known, the magnetic transition temperature depends on the magnetic coupling which, its turn strongly depends on inter-distances of magnetic atoms. In fact, the Ba doping leads to the distance increase between the magnetic atoms inducing a change in the magnetic coupling. This magnetic coupling could differ entirely from that existing in manganites lacking Ba atoms. Consequently, and as observed, the Fe doping in manganites including Ba atoms induces a TC augmentation. We are aware that we only provide a possible conclusion which is not an absolute explanation and this discordance will constitute an open question for forthcoming works. Figure 4a and b shows the inverse susceptibility v-1(T) (defined as (M/H)-1) vs. temperature for both samples at 0.05 T. It is well known that in the PM region, the relation between v and the temperature T should follow the Curie–Weiss law, i.e., v = C/(T - hW), where C and hW are the Curie constant and the Weiss temperature. The solid line in Fig. 4a and b is the fitting curves deduced from the Curie–Weiss equation. The best-fit parameters are

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Fig. 3 Magnetization vs. the temperature under low magnetic field of 0.05 T. The derivative of magnetization with respect to the temperature for a LPBMO and b LPBMFO

Table 2 Magnetic data TC (K)

hW (K)

TG (K)

LPBMO

94

173

272

LPBMFO

277

311

–

TC is the Curie temperature, hW refers to the Weiss temperature, and TG is the Griffiths temperature

Fig. 4 The inverse susceptibility obtained from magnetization measurements in a field of 0.05 T of a LPBMO and b LPBMFO. The red solid lines are the fits using Curie–Weiss law

summarized in Table 2. The hW value obtained is higher than TC. In general, the difference between hW and TC (hW [ TC) depends on the substance and is associated with

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the presence of short-range ordered slightly above TC. It should be related to the presence of a magnetic inhomogeneity. Based on the original paper of Griffiths [33], the v-1(T) shows a large deviation from the shape of Curie– Weiss law when the FM clusters have larger spins and the Griffiths temperature TG appears. Therefore, the low field susceptibility would give a direct support for the existence of the Griffiths phase (GP). In our present work, it should be noticed that the LPBMFO sample (Fig. 4b) shows no deviation from the Curie–Weiss law in the PM region. For the LPBMO sample a significant downturn in the v-1–T curve at TG well above TC (Fig. 4a). This suggests that more high spin FM clusters have embedded into the PM matrix before the PM–FM transition in the LPBMO sample pointing out to the existence Griffiths-like phase. As additional information, the existence Griffiths-like phase in LPBMO sample in the form of FM cluster system within a PM matrix is also reported in other closer manganites [34–37]. The M(H) isotherms have been recorded for both investigated samples at various temperatures in a narrow temperature interval around the respective TC, under magnetic field up to 5 T for a temperature interval of 3 K. Figure 5a and b shows isothermal M(H) curves of LPBMO and LPBMFO perovskite, respectively. Both samples show a ferromagnetic behavior. The magnetization for LPBMO

J Mater Sci

Fig. 5 Magnetization curves at different temperatures for a LPBMO and b LPBMFO. Arrott plots (H/M vs. M2) isotherms for c LPBMO and d LPBMFO. The temperatures of the isotherms are indicated

sample is not saturated when the magnetic field reaches 5 T. Therefore, a higher magnetic field ([5 T) is needed to reach saturation. By against, saturation is reached for magnetic field of 5 T for LPBMFO compound below TC. Above TC, the substance is paramagnetic and the magnetization curves as a function of applied magnetic field for different temperatures become increasingly linear. Arrott curves We plot in Fig. 5c and d the Arrott curves l0H/M vs. M2 for LPBMO and LPBMFO deduced from isothermal magnetization curves recorded around TC for both samples in a magnetic field up to 5 T. The curves show clearly a positive slope for the complete M2 range, which means that a second-order ferromagnetic to paramagnetic phase transition occurs, according to the criterion proposed by Banerjee [38]. Magnetocaloric effect The temperature dependence of jDSM j has been determined from the M(H) isotherms. Figure 6a and b shows the

magnetic entropy change jDSM j for both samples as a function of temperature under different magnetic fields. The jDSM j increases with an increasing of applied magnetic field. We note that the maximum entropy change, jDSmax M j, of only *0.18 and 0.49 J/kg K upon a filed variation of * 1 T, and *1.48 and 3.24 J/kg K upon a filed variation of *5 T was obtained for LPBMO and LPBMFO, respectively, indicating relatively small magnetic order fluctuations around the PM–FM transitions. For 5 T, it is seen that this field is not enough to saturate LPBMO sample. In general, the materials with second-order transition exhibit very smaller MCE than the materials with first-order transition. The small value of magnetic entropy change is reflective of the fact that the transition is of second order in the present case. The jDSmax M j obtained at 1 T for LPBMO is smaller than this reported by Chen et al. [39] (1.85 J/ kg K) by studying La0.6Nd0.1Ba0.3MnO3 sample prepared by a sol–gel method. On the other side, the jDSmax M j obtained at 1 T for LPBMFO is close to that reported by Wang et al. [15] (0.42 J/kg K) by studying La0.65Nd0.05 Ca0.3Mn0.9Fe0.1O3 sample prepared by a sol–gel method. Compared with Gd (2.8 J/kg K at 1 T field [40]), jDSmax M j value is smaller for both the present materials.

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Fig. 6 Magnetic entropy change (-DSM) as a function of temperature in various magnetic fields up to 5 T for a LPBMO and b LPBMFO

The evaluation of the refrigeration capacity passes through the so-called relative cooling power RCP based on the magnetic entropy change. The RCP evaluated by considering the magnitude of jDSM j and its full width at half maximum dTFWHM [41]. This parameter is defined as RCP ¼ DSmax M dTFWHM . For LPBMFO sample studied here, the RCP increases with the magnetic applied field. The RCP values are found to be 25, 60, 114.15, 126, and 155 J/kg under a magnetic applied field of 1, 2, 3, 4, and 5 T, respectively. These values are slightly smaller than those evidenced in the other oxides type perovskite, but high enough for technical interest. The obtained relative cooling power value at 1 T of our LPBMFO sample is comparable with this obtained by Wang et al. [42] on La0.7Ba0.3Mn0.9Cr0.1O3 sample prepared by sol–gel technique (RCP = 29.7 J/kg). Our result on magnetocaloric properties suggests that the LPBMFO sample is attractive as a possible refrigerant near room temperature magnetic refrigeration. For LPBMO sample, it is very difficult to calculate their RCP values because the non-uniform distribution of jDSM j curves. Another interesting feature in the MCE plot is that it is asymmetric, especially under high field. Similar behavior is observed in Si-substituted lanthanum calcium manganite [43]. For LPBMO sample, the value of RCP is estimated for 1 T and found about 15.2 J/kg which is lower than that obtained by Phan et al. [4] for La0.7Ba0.3MnO3 sample (36 J/kg at 1 T). Landau theory Based on Landau’s theory of phase transition, Amaral et al. [44, 45] have suggested a successful model with a contribution from magnetoelastic and electron interaction in manganites based on the Gibbs free energy which are expressed as GðM; TÞ ¼

123

AðTÞ 2 BðTÞ 4 CðTÞ 6 M þ M þ M þ . . . l0 MH 2 4 6 ð3Þ

Fig. 7 Experimental and theoretical magnetic entropy change (-DSM) for a LPBMO and b LPBMFO at magnetic field variation 0–1 T. The insets show the temperature dependence of Landau coefficient of parameter (B)

where A, B, and C are the Landau coefficients. From energy minimization, the magnetic equation of state is derived within this theory:

J Mater Sci

l0 H ¼ AðTÞ M þ BðTÞ M 3 þ CðTÞ M 5

ð4Þ

The values of the Landau coefficients and their dependence on temperature can be obtained from experimental isothermal magnetization measurements using polynomial fitting of l0H vs. M. The parameter B(TC) may be negative, zero or positive. If B(TC) is negative, the magnetic transition is the first order, otherwise it is second order. The inset of Fig. 7a and b shows the dependence of the B coefficient on temperature for both compounds LPBMO and LPBMFO, respectively. B (TC) is positive for both samples indicating a second-order magnetic transition. The corresponding magnetic entropy is obtained from differentiation of the magnetic part of the free energy with respect to the temperature oG SM ðT; HÞ ¼ oT H 1 0 1 1 ¼ A ðTÞM 2 þ B0 ðTÞM 4 þ C 0 ðTÞM 6 ð5Þ 2 4 6 where A0 (T), B0 (T), and C0 (T) are the temperature derivatives of the Landau coefficients. Figure 7a and b shows jDSM j curves, for l0H = 1 T, for LPBMO and LPBMFO, respectively. The close circles represent experimental data, while open circles show the results obtained by the Landau theory. These two curves show a discrepancy between the measured magnetic entropy change and those calculated using the Landau theory. The analysis clearly shows that the magnetoelastic coupling and electron interaction do not contribute directly to the magnetic entropy and it is temperature dependence for these samples. This suggests that an additional effect together with the magnetoelastic and electron interaction contributions is necessary for the observed magnetocaloric data.

Conclusions In summary, we have studied the MCE of a manganite LPBMO and LPBMFO polycrystalline samples. The nature of the phase transition for both samples is of second order. LPBMFO exhibits the highest value of 60 J/kg for RCP at 282.5 K, upon 2 T applied field variation. Our result on the magnetocaloric properties suggests that the LPBMFO sample is attractive as a possible refrigerant near room temperature magnetic refrigeration. Both samples show a discrepancy between the measured magnetic entropy change and that calculated using the Landau theory. Results indicate that the contributions to the free energy from the presence of FM clusters are strongly influencing the MCE by coupling with the order parameter around the Curie temperature.

Acknowledgements This study has been supported by the Tunisian Ministry of Scientific Research and Technology, School Phelma Grenoble and the Neel Institute.

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Journal of Solid State Chemistry 229 (2015) 26–31

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Critical phenomena in La0.6Pr0.1Sr0.3MnO3 perovskite manganese oxide R. Cherif a,n, E.K. Hlil b, M. Ellouze a, F. Elhalouani c, S. Obbade d a

Sfax University, Faculty of Sciences of Sfax, B.P. 1171-3000 Sfax, Tunisia Institut Neél, CNRS et Université Joseph Fourrier, BP 166, F-38042 Grenoble Cedex 9, France Sfax University, National Engineering School of Sfax, LASEM, B.P.W-3038 Sfax, Tunisia d LEPMI UMR 5279, CNRS – Grenoble INP – Université de Savoie – Université Joseph Fourier, 1130 rue de la Piscine, BP 75, 38402 Saint-Martin d'Hères Cedex, France b c

art ic l e i nf o

a b s t r a c t

Article history: Received 26 February 2015 Received in revised form 25 April 2015 Accepted 27 April 2015 Available online 5 May 2015

We report a study of the critical phenomena of perovskite-manganite compound La0.6Pr0.1Sr0.3MnO3 around the Curie temperature. Experimental results based on magnetic measurements using Banerjee criterion reveals that the sample exhibits a second-order paramagnetic–ferromagnetic transition. The critical behavior analysis and the Kouvel–Fisher method suggests that the critical phenomena around the critical point can be correctly described by the 3D-Heisenberg model. Critical exponents were estimated and found β ¼0.35470.009 and γ ¼ 1.264 7 0.035 at TC ¼325.5 70.443 K. The critical exponent δ is determined separately from the isothermal magnetization at TC and evaluated to δ ¼4.934 7 0.0004. These critical exponents obey the Widom scaling relation δ ¼ 1 þ γ/β. Based on the critical exponents, the magnetization–field–temperature (M–H–T) data around TC collapses into two curves obeying the single scaling equationM ðH; εÞ ¼ jεjβ f 7 ðH=jεjβ þ γ Þ where ε ¼(T TC)/TC is the reduced temperature. & 2015 Elsevier Inc. All rights reserved.

Keywords: Manganite Magnetic phase transition Critical exponents

1. Introduction Perovskite manganites R1 xAxMnO3 (R¼ trivalent rare earth, A¼ divalent alkaline earth) show a great variety of fascinating properties such as colossal magnetoresistance (CMR) effect, electronic phase separation [1–4] and magnetocaloric [5–7] behaviors observed in these materials. Such behaviors can be exploited as potential technological applications. To understand the mechanism of the CMR effect, extensive studies of magnetic, structural, and transport properties have been carried out on these perovskite materials [8]. Presently, it is believed that the metal–insulator transition and CMR in these compounds are induced by a combination of the double-exchange (DE) interaction between ferromagnetically coupled Mn3 þ and Mn4 þ ions and the Jahn–Teller effect [9– 11]. Therefore, to better understand the correlation between the insulator–metal transition and CMR effect, two important questions about paramagnetic–ferromagnetic (PM–FM) transition should be clarified: one is the order of phase transition, and the second is the common universality class. To make these issues clear, a detailed investigation of the critical exponents in the region of the PM–FM transition is needed. It is still not clear how the interaction is renormalized near the critical point and which universality

n

Corresponding author. Fax: þ216 74 274 437. E-mail address: [email protected] (R. Cherif).

http://dx.doi.org/10.1016/j.jssc.2015.04.039 0022-4596/& 2015 Elsevier Inc. All rights reserved.

class governs the magnetic transition. However, the theoretical calculations based on simplified DE models reveal that the FM–PM transition in CMR manganites should belong to the Heisenberg universality class [12]. By contrast, the experimental estimates for critical exponents are still controversial including those for shortrange Heisenberg interaction [13,14], the mean field values [15], and those cannot be classified into any universality known class [16]. On experimental side, studies of critical behavior of manganites near the PM–FM phase transition using a variety of techniques have yielded a wide range of values for the magnetization critical exponent β [17– 19]. The values range from about 0.3–0.5, which correspond to meanfield (β ¼0.5), three-dimensional (3D) isotropic nearest-neighbor Heisenberg (β ¼ 0.36570.003), and 3D Ising (β ¼0.32570.002). Indeed, the critical exponent β equal to 0.3770.04 for La0.7Sr0.3MnO3 (β ¼0.36570.003 in Heisenberg model) is reported by Ghosh et al. [20]. This is in good agreement with later results from Nair et al. [21], who yield β ¼ 0.3770.02 in La0.875Sr0.125MnO3, pointing out to a Heisenberg model-like behavior at the magnetic phase transition. In a recent study, Caballero-Flores et al. [22] have shown that the critical exponent β is equal to 0.394 for Pr0.5Sr0.5MnO3. In these previous studies, the Sr is substituted by La atom or the magnetic Pr atom which permits to modify the magnetism and Mn–Mn distances in the cell. Here, our propose keeps both non magnetic atoms La and Sr that allow uniquely an easier tunabity of the distances between magnetic atoms; and enhances the magnetism by imbedding the Pr rare earth on the A site. To shed light on the magnetic interaction


type of this system, the estimate of the critical exponents proves to be necessary. For this reason, we report the critical phenomena in the La0.6Pr0.1Sr0.3MnO3 manganite by analyzing the critical exponents through various techniques, such as the modified Arrott plot, the Kouvel–Fisher method and critical isotherm analysis. In addition, we demonstrate that the critical phenomena near the PM–FM transition in the sample is described better by the 3DHeisenberg model.

2. Materials and methods In this study, the La0.6Pr0.1Sr0.3MnO3 compound was prepared by the conventional solid state reaction described in our previous work [23]. In such work, the room temperature X-ray diffraction patterns show that the sample is crystallized in the rhombohedral structure with R3c space group, and the lattice parameters obtained from Rietveld refinement were estimated to a¼ b¼ 5.4953 Å and c ¼13.3367 Å [23]. Magnetic measurements were performed using a SQUID magnetometer developed at Neel Institute. This magnetometer uses extraction technique and can produce a field of 10 T. To extract accurately the critical exponents, the magnetic isotherms have been measured in the magnetic field range of 0–5 T. The temperature interval is fixed to 3 K in the vicinity of the Curie temperature (TC). In fact, the internal field used for the scaling analysis has been corrected for demagnetization, H¼ Happl DaM, where Da is the demagnetization factor obtained from M vs. H measurements in the low-field linearresponse regime at a low temperature.

3. Scaling analysis According to the scaling hypothesis, the critical behavior of a magnetic system showing a second-order magnetic phase transition near the Curie point, is characterized by a set of critical exponents, β (the spontaneous magnetization exponent), γ (the isothermal magnetic susceptibility exponent) and δ (the critical isotherm exponent). Such critical exponents are not valid for the first-order transition since the magnetic field can shift the transition, leading to a field-dependent phase boundary TC [17]. The mathematical definitions of the critical exponents from magnetization measurements are given by the following relations [24]: M s ðTÞ ¼ M 0 ð εÞβ ;

χ 0 1 ðTÞ ¼ ðh0 =M0 Þεγ ; M ¼ DH 1=δ ;

ε o 0 for T o T C ε 40 for T 4T C

ε ¼ 0 for T ¼ T C

27

4. Results In our previous work La0.6Pr0.1Sr0.3MnO3 compound [23], a transition from the paramagnetic to ferromagnetic state is reported when the temperature decreases. The transition temperature TC determined as the temperature corresponding to the minimum of dM/dT vs. T curve was estimated to 329 K. 4.1. Arrott–Noakes plot The isothermal magnetization M (H), measured from 0 to 5 T, is shown in Fig. 1. As seen in this figure, the magnetization increases steadily with increasing magnetic field in the low field range and shows no saturation, even at 5 T. In order to determine the type of magnetic phase transition in La0.6Pr0.1Sr0.3MnO3, we have analyzed H/M versus M2 curves extracted from the isothermal M vs. H data (Fig. 1) using Banerjee criterion [25]. According to this criterion, the magnetic transition is the first order (second order) if the slope of the plot H/M and M2 is negative (positive). For our sample, a second order phase transition has been confirmed from the positive slope of H/M versus M2 curves. The mean-field approximation can be generalized to the so-called modified Arrott-plot expression, based on the Arrott–Noakes equation of state ðH=MÞ1=γ ¼ ðT T C Þ=T 1 þ ðM=M 1 Þ1=β [26]. β γ The (M)1/ vs. (H/M)1/ Arrott–Noakes plots (also called modified Arrott plots (MAP)) are constructed for La0.6Pr0.1Sr0.3MnO3 compound using four different kinds of trial exponents. The curves are displayed in Fig. 2(a–d) using mean-field model (β ¼0.5, γ ¼ 1), 3D Heisenberg model (β ¼ 0.36570.003, γ ¼1.336 70.004), Tricritical Mean-Field model (β ¼ 0.25, γ ¼1), and 3D-Ising model (β ¼ 0.3257 0.002, γ ¼1.241 70.002), respectively. These figures give evidence that the four considered models yield quasi straight lines in the high field region. As consequence, it is difficult to conclude which one of them is the best for critical exponents determination. To distinguish which model better describes this system, we calculated the so called relative slope (RS) defined at the critical point: RS ¼S(T)/S(TC). The S(T) and S(TC) are the slope for a given T close to TC and the slope at T ¼TC, successively. The considered TC ¼325 K is determined from the Arrott Nokaes curves, where at β γ T¼ 325 K, the curves M1/ vs. (H/M)1/ for our models is a linear curve crossing the origin. If the modified Arrott plots show a series of absolute parallel lines, the relative slope of the most satisfactory model should be kept to 1 irrespective of temperatures [27].

ð1Þ ð2Þ ð3Þ

where M0, h0/M0, and D are the critical amplitudes and ε ¼(T TC)/ TC is the reduced temperature. M s ðTÞ, χ 0 1 ðTÞ and H are the spontaneous magnetization, the inverse initial susceptibility and the demagnetization adjusted applied magnetic field, respectively. Another independent way to determine the exponent β and γ is available as well. It uses the scaling theory, which predicts the existence of a reduced equation of state given by MðH; εÞjεj β ¼ f 7ðH jεj ðβ þ γ Þ Þ

ð4Þ

where f þ and f are regular analytical functions for ε 40 and ε o0, respectively. Eq. (4) implies that plots of MðH; εÞjεj β vs. H jεj ðβ þ γ Þ would lead to two universal curves, one for temperatures T 4TC (ε 40) and the other for T oTC (ε o0).

Fig. 1. Isothermal magnetization around TC for La0.6Pr0.1Sr0.3MnO3 compound at different temperatures with 3 K steps.

28


Fig. 2. (a) Standard Arrott plot (isotherms M2 vs. H/M). Modified Arrott plots: isotherms of M1/β vs. (H/M)1/γ with (b) the 3D-Heisenberg model, (c) the tricritical mean-field model and (d) the 3D-Ising model for La0.6Pr0.1Sr0.3MnO3 compound.

the La0.6Pr0.1Sr0.3MnO3 compound and appropriately able to determine the critical exponent. Based on the modified Arrott plots displayed in Fig. 2(b), the spontaneous magnetization MS(T) as well as the inverse of magnetic susceptibility χ 1 ðTÞ were determined from the intersections β γ of the linear extrapolation line with the (M)1/ and the (H/M)1/ axis, respectively. The Ms(T) and χ 1 ðTÞ vs. T are shown in Fig. 4. The curves in Fig. 4 denote the power low fitting of Ms (T) and χ 1 ðTÞ according to Eqs. (1) and (2), respectively. Thus, new critical exponent values were determined and reported in Table 1. In addition, the Curie temperatures associated with the fitting of Ms(T) and χ 1 ðTÞ to Eqs. (1) and (2) respectively, are also determined. 4.2. Kouvel–Fisher plot

Fig. 3. Relative slope (RS) of La0.6Pr0.1Sr0.3MnO3 sample as a function of temperature defined as RS ¼ S(T)/S(TC), using several methods.

The RS vs. T curves for the four models, mean-field, 3DHeisenberg, 3D-Ising and tricritical mean-field are reported in Fig. 3. The most adequate model should be the first that possesses an RS value very close to the unit. As conclusion, the 3DHeisenberg model is the best model that can correctly describe

Such critical exponents and TC can be obtained more accurately by analyzing the Ms(T) and χ 1 ðTÞ with the Kouvel–Fisher (KF) method [28] 1 ¼ ðT T C Þ=β ð5Þ M s ðTÞ dM s ðTÞ=dT

χ 0 1 ðTÞ dχ 0 1 ðTÞ=dT

1

¼ ðT T C Þ=γ

ð6Þ

According to these equations obtained Ms(T) and and using the 1 and χ 0 1 ðTÞ dχ 0 1 ðTÞ= dT 1 versus temperature are reported in Fig. 5. These curves

χ 1 ðTÞ curves, a plots of M s ðTÞ dMs ðTÞ=dT


should yield straight lines with slopes 1/β and 1/γ, respectively, and the intercepts on the T axes are equal to TC. The estimated critical exponents and TC are also listed in Table 1. It is clear that the values of critical exponents as well as TC calculated using the MAP and KF plots match reasonably well.

29

the critical region. Using the β and γ values obtained from KF method, the log–log scale of M=jεjβ vs. H=jεjβ þ γ curves were calculated and reported in Fig. 7. The data fall on two curves, one

4.3. Critical isotherm exponent According to the Widom scaling relation, the critical exponent

δ can be determined from β and γ using δ ¼1 þ γ/β expression [29]. The used β and γ are determined from the modified Arrott plot and KF model, respectively. Both models lead to δ ¼4.392 and 4.570 for our La0.6Pr0.1Sr0.3MnO3 sample. Also, the exponents δ can also be

calculated directly from the fitting of the high field region of critical isotherm M(TC,H) on log–log scale as reported in Fig. 6. According to Eq. (3), log(M) vs. log(H) plot would give a straight line with slope 1/δ. The obtained value is δ ¼4.934 70.0004 for La0.6Pr0.1Sr0.3MnO3 compound. Obviously, this value is slightly larger to those estimated from the Widom scaling relation. This difference is probably due to the experimental errors. Fig. 5. Kouvel–Fisher plots for the spontaneous magnetization and the inverse initial susceptibility vs. T for determination of β and γ in La0.6Pr0.1Sr0.3MnO3 sample.

4.4. Scaling law As a further check of these critical exponents values and whether our data obey the magnetic equation of state Eq. (4) in

Fig. 4. Temperature dependence of the spontaneous magnetization MS(T, 0) and the inverse initial susceptibility χ 0 1 ðTÞ, with the fitting curves based on the power laws.

Fig. 6. Critical isotherms La0.6Pr0.1Sr0.3MnO3 sample.

of

M

vs.

H

corresponding

to

TC ¼ 325 K

for

Table 1 Comparison of critical exponents of La0.6Pr0.1Sr0.3MnO3 compound with earlier reports, and with the various theoretical models. Abbreviations: CI, critical isotherm; exp, experimental; cal, calculated; MAP, modified Arrott plots and KF, Kouvel–Fisher. Composition

Technique

TC(K)

β

γ

La0.6Pr0.1Sr0.3MnO3

MAP KF C.I. (exp) C.I. (cal) MAP KF C.I. (exp) C.I. (cal) – – – – – Theory Theory Theory Theory

326.1097 0.170 325.4687 0.443

0.364 70.004 0.3547 0.009

1.2357 0.017 1.2647 0.035

350.18 350.48

0.371 0.386

1.380 1.306

334.54 326 3547 0.2 – 247 – – – –

0.344 0.36 7 0.01 0.37 7 0.04 0.37 70.02 0.394 0.5 0.365 7 0.003 0.325 7 0.002 0.25

1.296 1.22 7 0.01 1.22 7 0.03 1.38 7 0.03 1.44 1 1.3367 0.004 1.2417 0.002 1

La0.57Nd0.1Pb0.33MnO3

La0.7Pb0.05Na0.25MnO3 La0.7Ca0.1Sr0.2MnO3 La0.7Sr0.3MnO3 La0.875Sr0.125MnO3 Pr0.5Sr0.5MnO3 Mean-field model 3D-Heisenberg model 3D-Ising model Tricritical mean-field model

δ

Ref. This work

4.934 7 0.0004 4.570 [30] 4.270 4.383 4.80 4.4 7 0.2 4.25 7 0.2 4.727 0.04 4.651 3 4.80 7 0.04 4.82 7 0.02 5

[31] [32] [20] [21] [22] [24] [24] [24] [24]

30


underlines that the substitution of La by Pr do not induce a change in the universality class of such parent compounds. The critical exponents in our material are governed by the lattice dimension (3D), the dimension of the order parameter (n ¼3, magnetization), and the range of interaction (short range, long range, or infinite). Moreover, Fisher et al. [35] performed a normalization group analysis of systems with an exchange interσ action of the form J(r)¼1/rd þ (d is the dimension of the system and σ is the range of the interaction). If σ is less than 3/2, the mean-field exponents are valid. The Heisenberg exponents are valid for σ greater than 2. For intermediate range, i.e., for J(r) σ Er 3 with 3/2 r σ r2, the exponents belong to a different universality class which depends upon σ. In general, the critical exponent values of this sample are close to those expected for the nearest-neighbor 3D-Heisenberg ferromagnets with short-range interactions. Fig. 7. Scaling plots indicating two universal curves below and above TC for La0.6Pr0.1Sr0.3MnO3 sample. Inset shows the same plots on a log–log scale.

6. Conclusion above TC and the other below TC, in accordance with the scaling theory. Therefore, the FM behavior around the Curie temperature was properly renormalized following the scaling equation of state, indicating that the present exponents are reasonably accurate and unambiguous.

5. Discussion From this analysis, we can see that the scaling is well obeyed. Indeed, all data fall on two universal curves, one for temperature below TC and the other for temperature above TC. This confirms that the obtained values of the critical exponents and Curie temperature are reliable and in agreement with the scaling hypothesis, suggesting that the estimated values TC, β, γ and δ are reasonably accurate. The values of critical exponents of our La0.6Pr0.1Sr0.3MnO3 compound and some other manganites [20– 22,30–32] as well as the theoretical values based on various models [24] are summarized in Table 1. Table 1 gives evidence that, typically, the critical exponent β has a value in the range of 0.3–0.5, similar to those of 3DHeisenberg ferromagnets. Nevertheless, the reported value of γ is not close to the theoretical value of the 3D-Heisenberg model. In our case, analysis of critical exponents for La0.6Pr0.1Sr0.3MnO3 shows that the β value is very close to the Heisenberg model value and the γ value lies between 3D-Ising and 3D-Heisenberg model-like. The difference originates from β being calculated from fittings below TC, whereas γ is fitted above TC. Furthermore, the critical isotherm exponent δ ¼4.57 is slightly smaller than the value predicted by Heisenberg model. Generally, the presence of Griffiths-type phase is characterized by a large δ values [33,34]. As conclusion, our high δ value points out to the absence of a Griffiths-type phase in our sample. For our experiment, the value of the critical exponent β (β ¼0.36470.004 (MAP), 0.35470.009 (KF)) for La0.6Pr0.1Sr0.3MnO3 sample match well with this derived from the Heisenberg model (β ¼0.36570.003). This is in agreement with results predicted by Ghosh et al. [20] and Nair et al. [21], in their investigations on the ferromagnetic phase transition in La0.7Sr0.3MnO3 and La0.875Sr0.125MnO3 compounds, respectively. They reported β ¼ 0.3770.04;70.02 for both samples. Nevertheless, we underline that our predicted values are not in accordance with those reported (β ¼0.394, γ ¼1.44, δ ¼4.651 at TC ¼247 K) for Pr0.5Sr0.5MnO3 [22]. If we consider the previously mentioned La0.7Sr0.3MnO3 and La0.875Sr0.125MnO3 compounds [20–21] as parent compounds, it could be reasonably concluded that our work

In summary, we have used magnetization measurements to investigate the critical phenomena at temperatures close to the PM–FM phase transition temperature in the La0.6Pr0.1Sr0.3MnO3 manganite. This transition is identified to be second order nature. The 3D-Heisenberg model is revealed to be the best model to describe the critical properties. The reliable critical exponents (TC, β, γ and δ) are obtained based on various research techniques including modified Arrott plot, the Kouvel–Fisher method, and critical isotherm analysis. With these obtained critical exponents, the magnetization field temperature (M–H–T) data below and above TC collapse into two different curves obeying the single scaling equation.

Acknowledgments This study has been supported by the Tunisian Ministry of Scientific Research and Technology, School Phelma Grenoble and the Neel Institute. References [1] R. Von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [3] S. Yunoki, J. Hu, A.L. Malvezzi, A. Moreo, N. Furukawa, E. Dagotto, Phys. Rev. Lett. 80 (1998) 845. [4] A. Moreo, S. Yunoki, E. Dagotto, Science 283 (1999) 2034. [5] S. Othmani, R. Blel, M. Bejar, M. Sajieddine, E. Dhahri, E.K. Hlil, Solid State Commun. 149 (2009) 969. [6] A. Tozri, E. Dhahri, E.K. Hlil, M.A. Valente, Solid State Commun. 151 (2011) 315. [7] M. Khlifi, M. Bejar, O.E.L. Sadek, E. Dhahri, M.A. Ahmed, E.K. Hlil, J. Alloy. Compd. 509 (2011) 7410. [8] Y. Motome, N. Furukawa, Phys. Rev. B71 (2005) 014446. [9] C. Zener, Phys. Rev. 82 (1951) 403. [10] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [11] A.J. Millis, Phys. Rev. B53 (1996) 8434. [12] Y. Motome, N. Furukawa, J. Phys. Soc. Jpn. 70 (2001) 1487. [13] B. Padmanabhan, H.L. Bhat, S. Elizabeth, S. Röbler, U.K. Röbler, K. Dörr, K. H. Müller, Phys. Rev. B 75 (2007) 024419. [14] A. Tozri, E. Dhahri, E.K. Hlil, Phys. Lett. A 375 (2011) 1528. [15] S. Taran, B.K. Chaudhuri, S. Chatterjee, H.D. Yang, S. Veeleshwar, Y.Y. Chen, J. App. Phys. 98 (2005) 103903. [16] D. Kim, B.L. Zink, F. Hellman, J.M.D. Coey, Phys. Rev. B 65 (2002) 214424. [17] D. Kim, B. Revaz, B.L. Zink, F. Hellman, J.J. Rhyne, J.F. Mitchell, Phys. Rev. Lett. 89 (2002) 227202. [18] J. Mira, J. Rivsa, F. Rivadulla, C.V. Vazquez, M.A.L. Quintela, Phys. Rev. B 60 (1999) 2998. [19] H.S. Shin, J.E. Lee, Y.S. Nam, H.L. Ju, C.W. Park, Solid State Commun. 118 (2001) 377. [20] K. Ghosh, C.J. Lobb, R.L. Greene, S.G. Karabashev, D.A. Shulyatev, A.A. Arsenov, Y. Mukovskii, Phys. Rev. Lett. 81 (1998) 4740.


[21] S. Nair, A. Banerjee, A.V. Narlikar, D. Prabhakaran, A.T. Boothroyd, Phys. Rev. B 68 (2003) 132404. [22] R. Caballero-Flores, N.S. Bingham, H. Phan, M.A. Torija, C. Leighton, V. Franco, A. Conde, T.L. Phan, S.C. Yu, H. Srikanth, J. Phys.: Condens. Matter 26 (2014) 286001. [23] R. Cherif, E.K. Hlil, M. Ellouze, F. Elhalouani, S. Obbade, J. Solid State Chem. 215 (2014) 271. [24] H.E. Stanley, Oxford University Press, London, 1971. [25] B.K. Banerjee, Phys. Lett. 12 (1964) 16. [26] L.M. Rodriguez-Martinez, J.P. Attfield, Phys. Rev. B 54 (R1) (1996) 5622. [27] J. Fan, L. Ling, B. Hong, L. Zhang, L. Pi, Y. Zhang, Phys. Rev. B 81 (2010) 144426.

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[28] J.S. Kouvel, M.E. Fisher, Phys. Rev. 136 (1964) A1626. [29] B. Widom, J. Chem. Phys. 43 (1965) 3898. [30] A. Dhahri, J. Dhahri, E.K. Hlil, E. Dhahri, J. Supercond. Nov. Magn. 25 (2012) 1475. [31] Y. Motome, N. Furulawa, J. Phys. Soc. Jpn. 69 (2000) 3785. [32] M. Dudka, R. Folk, Y.u. Holovatch, J. Magn. Magn. Mater. 294 (2005) 305. [33] M.B. Salamon, P. Lin, S.H. Chun, Phys. Rev. Lett. 88 (2002) 197203. [34] W. Jiang, X.Z. Zhou, G. Williams, Y. Mukovskii, K. Glazyrin, Phys. Rev. Lett. 99 (2007) 177203. [35] M.E. Fisher, S.-K. Ma, B.G. Nickel, Phys. Rev. Lett. 29 (1972) 917.

Journal of Materials Science Critical behavior near the ferromagnetic-paramagnetic phase transition in La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound --Manuscript Draft-Manuscript Number:

JMSC-D-16-04105

Full Title:

Critical behavior near the ferromagnetic-paramagnetic phase transition in La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound

Article Type:

Manuscript (Regular Article)

Keywords:

Magnetic materials; Solide state reaction; Critical behavior

Corresponding Author:

cherif rim, Ph.D Universite de Sfax Sfax, TUNISIA

Corresponding Author Secondary Information: Corresponding Author's Institution:

Universite de Sfax

Corresponding Author's Secondary Institution: First Author:

cherif rim, Ph.D

First Author Secondary Information: Order of Authors:

cherif rim, Ph.D El Kebir Hlil Mohamed Ellouze Foued Elhalouani Said Obbade

Order of Authors Secondary Information: Abstract:

We have investigated the critical behavior of the ferromagnetic La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound by the data of magnetization measurements. To investigate the nature of the magnetic phase transition, a detailed critical exponent study has been performed. The critical exponents' β, γ, and δ determined using the modified Arrott plot, the Kouvel-Fisher method, as well as the critical isotherm analysis. The deduced critical exponents (β, γ) are between those predicted for a 3D-Heisenberg model and those predicted by 3D-Ising model. The exponent δ deduced separately from isotherm analysis at TC = 275 K was found to obey to the Widom scaling relation δ = 1 + (γ/β). The reliability of obtained exponents was confirmed by using the universal scaling hypothesis.

Funding Information:

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Title: Critical

behavior near the ferromagnetic-paramagnetic phase transition in La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound

Authors: R. Cherif, E. K. Hlil, M. Ellouze, F. Elhalouani, S. Obbade Corresponding author: R. Cherif University of Sfax, Faculty of Sciences of Sfax, B.P. 1171, Sfax 3000, Tunisia Phone: +216 (0) 27888885, +216(0) 95759130, Email: [email protected] Manuscript type: Article Designation of the Journal: Our intent is to publish in the Journal of Materials Science. Also, we state the manuscript is original has not been published elsewhere. Explanation of the manuscript’s significance: In this work, we present a complete investigation on the critical phenomena of La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 from experimental point of view. Our work regards the critical exponents which allows to define model describing the interaction magnetism this material. To our knowledge, it is the first time that such investigation on the critical phenomena of La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 is reported. List of recommended reviewers: 1) S. H. Mahmood, e-mail: [email protected] The University of Jordan, Amman, Jordan Research field: He is working on similar experimental work; see article below 

Critical behavior in Fe-doped manganites La0.67Ba0.22Sr0.11Mn1-xFexO3 (0 ≤ x ≤ 0.2) Journal of Materials Science, Volume 49, June 2014, Pages 6883-6891

2) H. Lassri, E-mail address: [email protected] University of Hassan II Casablanca, Faculty of Sciences, Department of Physics, LPMMAT, B.P. 5366 Maậrif, Morocco Research field: He is working on similar experimental work; see article below 

Structural, Magnetic, Magnetocaloric, and Critical Exponent Properties of La0.67Sr0.33MnO3 Powders Synthesized by Solid-State Reaction Journal of Superconductivity and Novel Magnetism 29(1), November 2015 DOI: 10.1007/s10948-015-3212-5

3) S.K. Oh, E-mail address: [email protected] Department of Physics, Chungbuk National University, 361-763 Cheongju, Republic of Korea Research field: He is working on similar experimental work; see article below

 Critical exponents of La0.9Pb0.1MnO3 perovskite Journal of Magnetism and Magnetic Materials, Volume 304, Issue 2, September 2006, Pages e778– e780

4) M.H. Phan, E-mail address: [email protected] Department of Physics, University of South Florida, Tampa, FL 33620, USA Research field: He is working on similar experimental work; see article below

 Influence of magnetic field on critical behavior near a first order transition in optimally doped manganites: The case of La1−xCaxMnO3 (0.2 ≤ x ≤ 0.4) Journal of Magnetism and Magnetic Materials, Volume 348, December 2013, Pages 146–153

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Critical behavior near the ferromagnetic-paramagnetic phase transition in La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound R. Cherif1,*, E. K. Hlil2, M. Ellouze1, F. Elhalouani3 and S. Obbade4 1

2

Sfax University, Faculty of Sciences of Sfax, B. P. 1171 – 3000 Sfax, TUNISIA

Institut Neél, CNRS et Université Joseph Fourrier, BP 166, F -38042 Grenoble Cedex 9, FRANCE

3

Sfax University, National Engineering School of Sfax, LASEM, B. P. W – 3038Sfax, TUNISIA 4

LEPMI UMR 5279, CNRS - Grenoble INP - Université de Savoie - Université Joseph

Fourier, 1130 rue de la Piscine, BP 75, 38402 Saint-Martin d’Hères Cedex, FRANCE

Abstract We have investigated the critical behavior of the ferromagnetic La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound by the data of magnetization measurements. To investigate the nature of the magnetic phase transition, a detailed critical exponent study has been performed. The critical exponents’ β, γ, and δ determined using the modified Arrott plot, the Kouvel–Fisher method, as well as the critical isotherm analysis. The deduced critical exponents (β, γ) are between those predicted for a 3D-Heisenberg model and those predicted by 3D-Ising model. The exponent δ deduced separately from isotherm analysis at TC = 275 K was found to obey to the Widom scaling relation δ = 1 + (γ/β). The reliability of obtained exponents was confirmed by using the universal scaling hypothesis.

Keywords: Magnetic materials; Solide state reaction; Critical behavior. *Corresponding author: [email protected]

Introduction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

A great fundamental and experimental interest in hole-doped manganites R1-xAxMnO3 was devoted, due to their interesting electrical and magnetic properties leading to the discovery of the so-called colossal magnetoresistance (CMR) [1–3] and magnetocaloric effects (MCE) [4–6]. These properties in manganite compounds were widely interpreted by means of the double-exchange (DE) interaction between ferromagnetically coupled Mn3+ and Mn4+ ions [7], the strong electron-phonon interaction known as the Jahn-Teller effect [8] and phase separation [9]. However, the origin of the observed properties is still not fully understood. Particularly, it is unclear how the magnetic interactions are renormalized near the paramagnetic–ferromagnetic (PM–FM) transition range and what universality class governs the PM–FM transitions in these systems. Previous studies on the critical behaviors around the Curie temperature have indicated that the critical exponents play important roles in the elucidation of interaction mechanisms near TC [10, 11]. The critical behavior in the DE model was first described with long-range mean-field theory [12, 13]. Sequentially, Motome and Furulawa suggested that the critical behavior should be attributed to short-range Heisenberg model [14, 15]. The experimental estimates are still controversial concerning the critical exponents and even the order of the magnetic transitions including three-dimensional 3D-Heisenberg interaction [16, 17], 3DIsing values [18, 19], mean field values [20] and those that cannot be classified into any universality class ever known [21]. On experimental side, studies of critical behavior of manganites near the PM–FM phase transition using a variety of techniques have yielded a wide range of values for the magnetization critical exponent β [22– 24]. The values of critical exponent β obtained from a variety of techniques range from 0.3 to 0.5 [25, 26]. For example the β value reported for the La0.7Ba0.3MnO3 single crystal [27] is close to that of the 3D-

Heisenberg model. However, Moutis et al. [28] reported that the calculated values of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

critical exponents for La0.67Ba0.33MnO3 polycrystalline are between those predicted for threedimensional (3D) Heisenberg model and those predicted by mean-field theory. The values deduced for the critical exponents in La0.67Ba0.33Mn0.98Ti0.02O3 [29] are close to the theoretical prediction of the mean-field model; while those concluded from Oumezzine et al. for La0.67Ba0.33Mn0.9Cr0.1O3 [30] sample belongs to the three-dimensional Heisenberg class with short-range interaction. Moreover, recent theoretical calculations have predicted that the critical exponents in La0.8Ba0.2Mn0.85Fe0.15O3 manganites [31] are in agreement with the 3DHeisenberg model. Furthermore, in the samples Pr0.67Ba0.33Mn1-xFexO3 (x = 0 and 0.05), Baazaoui et al. showed that for the sample without doping (x = 0) the critical exponents are close to the parameters predicted by the tricritical mean-field theory, whereas, for x = 0.05 sample, these exponents are close to the mean-field model [32]. In the present work, we focus on the critical behavior of La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound which presents both rare earth La and Pr. We demonstrate that the critical phenomena near PM-FM Curie temperature transition in this perovskite is described by both the 3D-Heisenberg model and the 3D-Ising model with short-range interactions. Experimental Polycrystalline La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 sample has been elaborated using the conventional solid-state reaction at high temperature. Stoichiometric quantities of La2O3, Pr6O11, BaCO3, MnCO3 and (Fe2O3, H2O) were mixed and heated in air at 900 °C for 10 h, also were sintered at 1000 °C for 12 h. After that, the sample was pressed into pellets and sintered for 50 h at 1200 °C. Finally, these pellets were grinding and sintered at 1400 °C, for three times for 24 h, followed by grinding in an agate mortar. Room-temperature X-ray diffraction patterns show that the sample is crystallized in the rohmboedric structure with R3c space group [4]. The magnetic measurements were carried out using a SQUID (Quantum

Design) developed in Louis Neel Laboratory of Grenoble. To extract accurately the critical 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

exponents of the sample, magnetization isotherms were measured in the range of 0-5 T and with a temperature interval of 3 K in the vicinity of Curie temperature (TC). These isothermals are corrected by a demagnetization factor D that has been determined by a standard procedure from M (H) measurements in the low-field linear response regime at low temperature (Hint = Happ - DM). Scaling analysis According to the scaling hypothesis [33], the second order magnetic transition near the Curie point TC is characterized by a set of interrelated critical exponents β, γ and δ which are associated with the spontaneous magnetization MS, magnetic susceptibility  0-1 and critical magnetization isotherm at TC, respectively. Those exponents from magnetization measurements can be described as [34, 35]: M s (T )  M 0 (- )  , ε < 0 for T < TC

(1)

 0-1 (T )  (h0 / M 0 )  , ε > 0 for T > TC

(2)

M  DH 1/  , ε = 0 for T = TC

(3)

Where M0, h0, and D are the critical amplitudes and ε = (T-TC)/TC is the reduced temperature. According to the prediction of the scaling equation in the asymptotic critical region, the magnetic equation of state can be written as: M (H ,  ) 

-

 f  (H 

-(   )

)

(4)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Where f  and f - are regular analytical functions for ε > 0 and ε < 0 respectively. Eq. 4 implies that plots of M (H ,  ) 

-

vs. H 

-(   )

would lead to universal curves, one for

temperatures T > TC (ε > 0) and the other for T < TC (ε < 0). Results and discussion In Fig. 1, the magnetic field dependence of magnetization is shown for La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 with a maximum magnetic field of 5 T. Usually, the curves M (H) of manganites exhibit a very high increase in M at low fields and then a gradual saturation at high fields. The present compound also shows a steep rise in the low field range, the magnetization still increases steadily with increasing magnetic field and does not show any sign of saturation, even at 5 T. In order to check the nature of the magnetic phase transition, we used Arrott plots in the vicinity of the phase transition temperature. The order of magnetic transition can be determined from the slope of isotherm plot according to the criterion proposed by Banerjee [36]. Indeed a negative slope corresponds to first-order transition while a positive slope indicates a second-order transition. These plots (see Fig. 2. a) indicate a positive slope in the complete M2 range for our sample, which confirms that the sample exhibits a second-order ferromagnetic-paramagnetic (FM–PM) phase transition. The greater knowledge of the magnetic phase transition may be checked by analyzing the critical phenomena. Generally, the critical exponents and the Curie temperature TC can be facilely definite from the Arrott– Noakes plots (also called modified Arrott plots (MAP)). In this technique, the M = f(H) data are translated into a series of isothermal (M1/β = f((H/M))1/γ) depending on the following relationship [37]: (H / M )1/   (T -TC ) /T1  (M / M 1 )1/ 

(5)

The MAP isotherms are plotted at different temperatures for our sample by using: the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

3D-Heisenberg model (β = 0.365, γ = 1.336) (Fig. 2. b); the tricritical mean-field model (β = 0.25, γ = 1) (Fig. 2. c) and the 3D-Ising model (β = 0.325, γ = 1.241) (Fig. 2. d). We can notice that all models yield quasi straight lines and nearly parallel in the high field region. Thus, it becomes hard to compare these results and select the best model. Therefore, we calculate and plot in Fig. 3 their relative slopes (RS) which is defined as: RS = S(T)/S(TC) where S(T) and S(TC) are the slopes deduced from MAP around and at TC, respectively. Perfect parallel lines are described by RS equal to 1. The results in Fig. 3 indicate that the relative slope (RS) reported for 3D-Heisenberg model is the closest to 1, meaning that this model can be suitable for proper choice to deduce the critical exponents. Further to a standard procedure, the values of spontaneous magnetization MS(T) and initial inverse susceptibility  0-1 (T ) are extracted from a linear extrapolation of Arrott plots in high-fields region to the intercept with the M1/β and (H/M)1/γ axes, respectively. Then, we deduce the new values of β, γ and TC on the basis of Eqs. 1 and 2. These last values are reintroduced in the scaling of the MAP. Such iteration process can be applied until the critical exponents β and γ converge, and the temperature dependence of MS(T) and  0-1 (T ) can then be plotted for La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 sample in Fig. 4. Table 1 shows the results of the fits. The values of the critical parameters (β, γ) for La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 sample are between those predicted for a 3D-Heisenberg model and those predicted by 3D-Ising model. To determine the critical exponents as well as TC more accurately, we have analyzed the MS(T) and  0-1 (T ) data using the Kouvel–Fisher (KF) method [38]

M s (T ) dM s (T ) / dT



 (T -TC ) / 

(6)

 0-1 (T ) d  0-1 (T ) / dT   (T -TC ) / 

(7)

-1

-1

According to these equations and using the obtained Ms(T) and  0-1 (T ) curves, a plots 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

of M s (T ) dM s (T ) / dT



-1

-1

and  0-1 (T ) d  0-1 (T ) / dT  versus temperature are reported in Fig.

5. These curves should yield straight lines with slopes 1/β and 1/γ, respectively, and the intercepts on the T axes are equal to TC. The estimated critical exponents and TC are also listed in Table 1. For the exponent δ associated with the isotherms at T ~ TC, its value can be obtained from fitting the M (H, TC). Based on Eq. (3), the linear fit of the data at high-fields region should be a straight line with a slope of 1/δ. In the insets of Fig. 6 we present the critical isotherm on a log-log scale for M (H) curve at TC = 275 K. The results of the analysis are shown in Table 1. From the Widom scaling relation, critical exponents β, γ, and δ are related in the following way [39]: δ = 1 + (γ/β)

(8)

Using this scaling relation and the estimated values of β and γ we obtain δ values which are close to the estimates for δ from the critical isotherms at TC. Thus, the estimates of critical exponents are consistent. The values of δ obtained for our sample are listed in Table 1. For checking the reliability of critical exponents, the scaling theory provides a scope 

according to the magnetic equation of state in critical region using Eq. 4. M /  vs.

H /

 

is plotted in Fig. 7 for our sample. Their values of β and γ are obtained from the KF

method. Both curves represent temperatures below and above TC and the insets show the same plots in log–log scale. Thus, the obtained critical exponents agree well with the prediction of the scaling hypothesis. The results prove the reliability of the critical values determined in our work. For a comparison, the values of the critical exponents obtained for our sample and some of other manganites, as well as the theoretical values based on various models are listed

in Table 1. Clearly, values of the critical exponents (β, γ) reported in this work for 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 are between those predicted for three-dimensional (3D) Heisenberg model and those predicted by 3D-Ising model. Recently, an investigation of the critical behavior in La0.8Ba0.2Mn0.85Fe0.15O3 compound reported by Ghodhbane et al. [31] showed that the values of the critical exponents are close to those predicted by the 3DHeisenberg model. In contrast, the obtained values of Pr0.67Ba0.33Mn0.95Fe0.05O3 [32] are close to the values predicted by the mean-field model. Notice that these two compounds containing one rare earth (La or Pr), while our compound possesses the both. If we consider the previously mentioned La0.8Ba0.2Mn0.85Fe0.15O3 compound [31] as parent compound, it could be reasonably concluded that our work underlines that the substitution of La by Pr do not induce a change in the universality class of such parent compound for the β value. Whereas the change focus to the γ value that has suffered a decrease due to the doping of La by Pr. The critical exponents in our material are governed by the lattice dimension (3D), the dimension of the order parameter (n = 3, magnetization), and the range of interaction (short range, long range, or infinite). Moreover, Fisher et al. [40] performed a normalization group analysis of systems with an exchange interaction of the form J(r) = 1/rd+σ (d is the dimension of the system and σ is the range of the interaction). If σ is less than 3/2, the mean-field exponents are valid. The Heisenberg exponents are valid for σ greater than 2. For intermediate range, i.e., for J(r) ≈ r−3−σ with 3/2 ≤ σ ≤ 2, the exponents belong to a different universality class which depends upon σ. In general, the critical exponent values of this sample are close to those expected for the nearest-neighbor 3D-Heisenberg ferromagnets with short-range interactions. Conclusion The critical phenomena of La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 perovskite have been comprehensively studied by the isothermal dc-magnetization around the Curie point at TC. For

our sample the values of critical exponents (β, γ) are between those predicted for a 3D1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Heisenberg model and those predicted by 3D-Ising model. The reliable exponents (TC, β, δ and γ) are obtained based on various research techniques including modified Arrot plot, Kouvel-Fisher method, and critical isotherm analysis. The field and temperature dependent magnetization follows the scaling theory, and all data fall on two distinct branches, one for T < TC and another for T > TC, indicating that the critical exponents thus obtained in this work are accurate. Acknowledgements This study has been supported by the Tunisian Ministry of Higher Education and Scientific Research, School Phelma Grenoble and the Neel Institute.

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Table Caption 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Table. 1 Comparison of critical exponents of La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound with earlier reports, and with the various theoretical models. Abbreviations: CI, critical isotherm; exp, experimental; cal, calculated; MAP, modified Arrott plots and KF, Kouvel–Fisher.

Figures captions Fig. 1. Isothermal magnetization around TC for La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound at different temperatures with 3 K steps. Fig. 2.(a) Standard Arrott plot (isotherms M2 vs. H/M). Modified Arrott plots: isotherms of M1/β vs. (H/M)1/γ with (b) the 3D-Heisenberg model, (c) the tricritical mean-field model and (d) the 3D-Ising model for La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 compound. Fig. 3. Relative slope (RS) of La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 sample as a function of temperature defined as RS = S(T)/S(TC), using several methods. Fig. 4. Temperature dependence of the spontaneous magnetization MS(T, 0) and the inverse initial susceptibility  0-1 (T ) , along with the fitting curves based on the power laws. Fig. 5. Kouvel–Fisher plots for the spontaneous magnetization and the inverse initial susceptibility for La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 manganite. Fig. 6. Critical isotherms of M vs H corresponding to TC = 275 K for La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 sample. Fig. 7. Scaling plots indicating two universal curves below and above TC for La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3 sample. Inset shows the same plots on a log-log scale.

Table. 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Composition

Technique

TC(K)

β

γ

δ

Ref.

La0.6Pr0.1Ba0.3Mn0.9Fe0.1O3

MAP

273.964 ± 0.232

0.382 ± 0.017

1.219 ± 0.016

4.191

This work

KF

274.287

0.347

1.215

4.507

C. I (exp)

4.528

La0.7Ba0.3MnO3

-

310 ± 0.5

0.35 ± 0.04

1.41 ± 0.02

5.5 ± 0.3

[27]

La0.67Ba0.33MnO3

-

338.1 ± 0.02

0.464 ± 0.003

1.29 ± 0.02

-

[28]

La0.67Ba0.33Mn0.98Ti0.02O3

MAP

310.29 ± 0.26

0.537 ± 0.001

1.015 ± 0.055

2.890

[29]

KF

310.11 ± 0.14

0.551 ± 0.008

1.020 ± 0.024

2.851

C. I (exp) La0.67Ba0.33Mn0.9Cr0.1O3

2.826

MAP

323.03 ± 0.04

0.378 ± 0.005

1.375 ± 0.002

4.589

KF

323.07 ± 0.12

0.380 ± 0.009

1.345 ± 0.039

4.539

C. I (exp) La0.8Ba0.2Mn0.85Fe0.15O3

Pr0.67Ba0.33Mn0.95Fe0.05O3

[30]

4.557

MAP

157.50 ± 0.06

0.379 ± 0.01

1.392 ± 0.03

KF

158.24 ± 0.01

0.370 ± 0.02

1.359 ± 0.02

[31]

C. I (exp)

4.40 ± 0.03

C. I (cal)

4.67 ± 0.01

MAP

128.289 ± 0.110

0.508 ± 0.002

1.003 ± 0.025

KF

128.006 ± 0.431

0.501 ± 0.026

1.007 ± 0.002

[32]

C. I (exp)

2.920

C. I (cal)

5.069

Mean-field model

Theory

-

0.5

1

3

[12]

3D-Heisenberg model

Theory

-

0.365 ± 0.003

1.336 ± 0.004

4.80 ± 0.04

[12]

3D-Ising model

Theory

-

0.325 ± 0.002

1.241 ± 0.002

4.82 ± 0.02

[12]

Tricritical mean-field model

Theory

-

0.25

1

5

[12]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

(a)

(b)

(c)

(d)

Fig. 2 Fig. 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Fig. 7

September 10, 2014 11:46 WSPC/Guidelines-IJMPB

S0217979214502300

International Journal of Modern Physics B Vol. 28 (2014) 1450230 (18 pages) c World Scientific Publishing Company

DOI: 10.1142/S0217979214502300

The effect of Co doping on structural, magnetic and magnetocaloric properties of La0.8Ca0.2 Mn1−x Cox O3 perovskites (0 ≤ x ≤ 0.3)

D. Turki∗ , R. Cherif∗ , E. K. Hlil† , M. Ellouze∗,§ and F. Elhalouani‡

Int. J. Mod. Phys. B Downloaded from www.worldscientific.com by IMAG on 11/24/14. For personal use only.

∗ University

of Sfax, Faculty of Sciences of Sfax, B.P. 1171, 3000, Sfax, Tunisia † Institut N´ eel, CNRS et Universit´ e Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France ‡ University of Sfax, National Engineering School of Sfax, B.P.W, 3038, Sfax, Tunisia § [email protected] Received 26 May 2014 Revised 25 July 2014 Accepted 29 July 2014 Published 10 September 2014 Studies of the structural, magnetic and magnetocaloric properties of polycrystalline La0.8 Ca0.2 Mn1−x Cox O3 compounds (0 ≤ x ≤ 0.3) perovskite manganites were carried out. Samples were synthesized by sol–gel process and then heated at 1000◦ C for 3 h. X-ray powder diffraction and magnetic measurements were used to investigate the structural and magnetic properties. Rietveld analysis shows that the samples crystallize in the orthorhombic structure with Pnma space group. Crystallographic analysis shows a variation in lattice parameters as cobalt substitution increases, accompanied by a variation in the interatomic distances and a small increase in MT–O–MT angles (MT = cobalt and manganese). Magnetocaloric studies on La0.8 Ca0.2 Mn1−x Cox O3 compounds with x = 0 and x = 0.2 have been investigated by measuring the magnetization as a max | is about function of temperature. At 2 T, the maximum magnetic entropy |∆SM 3.09 J · Kg−1 · K−1 and 1.123 J · Kg−1 · K−1 for x = 0 and 0.2, respectively. Besides, the RCP value decreases with increasing Co content from 55.845 J · Kg−1 to 49.971 J · Kg−1 . Also, the existence of Griffiths Phase and its influence in the phase transition is discussed. Keywords: Perovskites; magnetic measurements; magnetocaloric effect; Griffiths phase. PACS numbers: 61.05.C-, 61.05.cp, 75.20.En, 75.30.Cr, 75.30.Gw

1. Introduction Perovskites type manganite T1−y Dy Mn1−x Nx O3 where T is a trivalent lanthanide (La, Pr . . .), D is a divalent like alkaline-earth (Ca, Sr, Ba . . .) and N is a 3d transition metal (Fe, Co, Cr) have been the subject of intensive study due to their § Corresponding

author. 1450230-1


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D. Turki et al.

interesting magnetic and transport properties resulting from the competition of the lattice, charge, spin and orbital degrees of freedom also for their potential technological applications.1 – 6 Several theories such as double-exchange (DE) interaction, polaronic effects and phase separation have been proposed to understand the underlying physics of the manganites.7 – 11 The magnetic and structure properties of La0.8 Ca0.2 MnO3 are strongly dependent on the valence state, spin state of the metal ions and on the defects. They depend on the method of samples preparation and conditions (sol–gel, ball milling and solid state). The La0.8 Ca0.2 MnO3 solid solution has been intensively studied. In 1996, Hamilton et al.12 have measured the specific heat of La0.8 Ca0.2 MnO3 and La0.67 Ba0.33 MnO3 in the ferromagnetic (FM) state well below TC in order to understand the electronic properties of these materials. Later, Walz et al.13 have studied the magnetocaloric effect (MCF) and electric resistivity to better appreciate the magneto-electronic properties of this material type. Afterwards, Ma et al.14 have considered the Young’s modulus and the internal friction measurements under different measuring frequencies to determine whether the information of the lattice variation and the microscopic process is phase transition or relaxation, and some other quantitative information such as activation energy and the relaxation time at infinite temperature. Recently, Khlifi et al.15 focused on the effect of the calcium deficiency and the annealing temperature on both the Curie temperature and the magnetocaloric properties in La0.8 Ca0.2 MnO3 . Also, Xi et al.16 published a systematic investigation of magnetic characteristics of this manganite associated with the finite-size effects and evidenced the existence of surface spin glass in the nano-particle sample. However, the above mentioned studies only focus on La0.8 Ca0.2 MnO3 compound whilethere are no magnetic and magnetocaloric data for doped compound in B-site. Therefore, in this work we investigate the effect of Co doping content on crystal structure, transition temperature, magnetic entropy change and relative cooling power of La0.8 Ca0.2 Mn1−x Cox O3 (x = 0, 0.1, 0.2 and 0.3). The correlation between the Griffiths phase and the phase transition for La0.8 Ca0.2 MnO3 is also analyzed. 2. Experimental La0.8 Ca0.2 Mn1−x Cox O3 with 0 ≤ x ≤ 0.3 compounds were synthesized using sol– gel method by mixing precursors La2 O3 , CaO, MnO2 and Co3 O4 up to 99% purity in appropriate proportion according to the following reaction: 0.4 La2 O3 + 0.2 CaO + (1 − x) MnO2 + x/3 Co3 O4 → La0.8 Ca0.2 Mn1−x Cox O3 + δ CO3 . The precursors were dissolved in water and mixed with nitric acid (HNO3 ) at 95◦ C. After achieving complete dissolution, a suitable ethylene glycol and citric acid were added in the reactor. The solution was agitated and concentrated slowly by evaporating water at 95◦ C and stirred vigorously in order to obtain a transparent 1450230-2


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The effect of Co doping on structural, magnetic and magnetocaloric properties

3. Results and Discussions 3.1. Crystallographic results The XRD patterns of La0.8 Ca0.2 Mn1−x Cox O3 samples are shown in Fig. 1(a). We notice that for all samples, the presence of a second phase (in very small quantities) is attributed to Mn3 O4 as well as seen by Klifi et al.21 Rietveld refinement technique shows that the diffraction peaks corresponding to our samples can be indexed in the orthorhombic structure with Pnma space [

[

,QWHQVLW\DUEXQLWV


viscous solution and formation of a gel.17,18 The gel was dried at 180◦C to get the desired powders. Then, the powders are ground in a mortar for 5 to 10 min to obtain a homogeneous mixture powders. Finally, the pellets were milled and heated at 1000◦ C for 3 h. The phase purity and the structure of the samples were characterized by powder X-ray diffraction (XRD) using a Bruker D8 diffractometer with CuKα radiation source (λCuKα = 1.5405 ˚ A) by step-scanning (0.02◦ ) in the 2θ range from ◦ ◦ 20 to 80 . The data were analyzed by Rietveld method19 using the FULLPROF program.20 Magnetization measurements were carried out by using the BS1 magnetometer developed at Néel institute Laboratory in Grenoble.

[

7KHWD

[

(a) Fig. 1. XRD patterns for La0.8 Ca0.2 Mn1−x Cox O3 (x = 0, 0.1, 0.2, 0.3). (b) Powder XRD pattern and refinement of La0.8 Ca0.2 Mn1−x Cox O3 samples annealed at 1000◦ C. (b) For La0.8 Ca0.2 MnO3 . Inset: Half-width of the more intense peak for x = 0, (c) for La0.8 Ca0.2 Mn0.9 Co0.1 O3. 1450230-3


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D. Turki et al.

,QWHQVLWU\DUEXQLWV

,QWHQVLW\DUEXQLWV

The most intense peak

θ max

β

7KHWD

7KHWD

(b)

/D&D0Q&R2

Yobs Ycal Yobs-Ycal Posr

,QWHQVLW\DUEXQLWV


/D&D0Q2

7KHWD

(c) Fig. 1.

(Continued)

1450230-4


S0217979214502300

The effect of Co doping on structural, magnetic and magnetocaloric properties Table 1.

Cell parameters of La0.8 Ca0.2 Mn1−x Cox O3 .

Samples

a (˚ A)

b (˚ A)

c (˚ A)

V (˚ A3 )

D (nm)

x= x= x= x=

5.4729 5.4745 5.4753 5.4701

7.7408 7.7442 7.7466 7.7414

5.5051 5.5039 5.5042 5.5001

233.14 233.26 233.39 232.88

71.2 66.7 74.7 69.2

0 0.1 0.2 0.3

Table 2. Refined structural parameter for La0.8 Ca0.2 Mn1−x Cox O3 with (x = 0, 0.1, 0.2 and 0.3) annealed at 1000◦ C after the Rietveld refinement of X-ray powder diffraction data.


Samples La/Ca (4c) X Y Z Mn/Co (4b) X Y Z O(1) (4c) X Y Z O(2) (8d) X Y Z χ2 RF

x=0

x = 0.1

x = 0.2

−0.01571 0.25 0.00371

−0.01666 0.25 −0.00678

−0.01721 0.25 −0.00341

x = 0.3 −0.01524 0.25 −0.00291

0 0 0.5

0 0 0.5

0 0 0.5

0 0 0.5

0.51091 0.25 0.04521

0.50271 0.25 0.07094

0.50348 0.25 0.05267

0.52625 0.25 0.06639

0.22191 0.03465 0.79216 2.24 9.07

0.23534 0.03182 0.78515 1.84 7.89

0.20997 0.03673 0.78039 1.71 9.98

0.21995 0.03008 0.78973 1.69 10.6

group [Fig. 1(b) and 1(c)]. Our crystallographic parameters are almost comparable to those obtained by Walz et al.13 and Xi et al.22 The quality of the refinement is evaluated through the goodness of fit indicator (χ2 ). Detailed results of the structural parameters are listed in Tables 1 and 2. We list in Table 1 the unit cell parameters obtained from X-ray for all our samples. The accurate analyses of these results are given with Fig. 1(a). Table 2 gathers the results of our refinements. We note that La/Ca atoms have been located at 4c (0, 0, 0.5) and oxygen atoms occupy two different sites, explicitly O1 at 4c (x, 0.25, z) and O2 at 8d (x, y, z) positions. As shown in Fig. 1(a), the main characteristic peak around 32.5◦ slightly shifts with Co-doped content indicating a change of the lattice parameters. With increasing cobalt concentration, the lattice parameters a and b increase leading to an increase in the unit cell volume from 233.14 ˚ A3 for x = 0 to 233.39 ˚ A3 3 ˚ for x = 0.2 then decrease for x = 0.3 (232.88 A ). However, the lattice parameter c decreases from x = 0 to x = 0.2 then increase for x = 0.3. Wang et al.23 have studied the effect of partial substitution of cobalt in LaMnO3 and reported that 1450230-5


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D. Turki et al.

Fig. 2.

La0.8Ca0.2MnO3

La0.8Ca0.2Mn0.9Co0.1O3

La0.8Ca0.2MnO3

La0.8Ca0.2Mn0.9Co0.1O3

Crystal structure of La0.8 Ca0.2 MnO3 and the coordination polyhedron on Mn.

this shift was linearly correlated with doping content and responsible for the expansion of the unit cell. In order to explain this expansion in our samples, several competitive mechanisms as the ionic radii of the A-site and the B-site24 as well as the mixed valence state of Mn3+ /Mn4+ , Co3+ /Co4+ should be taken into account. In fact,when we substitute Mn3+ (0.65 ˚ A) by Co, this ion enters the solid solution and leads to the formation of an equivalent amount of Mn4+ (0.54 ˚ A) to preserve electro-neutrality of the lattice.25 However, cobalt ion gradually increased toward the prevailing Co3+ (0.61 ˚ A) and formed oxygen vacancies and Mn3+ (0.65 ˚ A).26 – 28 In this case, the mean ionic radius of the B cation hrB i must be increased, causing an increase of the lattice volume. This increase produces a local stress in MnO6 octaedra and so a distortion in the structure. Figure 2 shows the crystal structure 1450230-6


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The effect of Co doping on structural, magnetic and magnetocaloric properties Table 3. Average distance and angles for La0.8 Ca0.2 Mn1−x Cox O3 (x = 0, 0.1, 0.2, 0.3). Samples

x=0

x = 0.1

x = 0.2

x = 0.3

Distances (˚ A) Mn–O (1) Mn–O (2) Mn–O (2)

1.95 1.92 2.03

1.98 1.89 2.05

1.96 1.95 2.02

1.98 1.93 2.01

172.47 168.53 164.93 147.06 157.68

168.59 170.44 157.18 147.7 161.63

171.47 168.71 162.94 147.68 157.01

168.54 169.36 157.08 151.73 159.13


Angle (◦ ) Mn–O (1)–Mn Mn–O (2)–Mn O (1)–Mn–O (1) O (1)–Mn–O (2) O (2)–Mn–O (2)

of La0.8 Ca0.2 Mn1−x Cox O3 (x = 0, 0.1), the coordinate polyhedron for Mn/Co, the distance between Mn/Co and the first neighbor oxygen’s. The Mn/Co atom is coordinated by six oxygen atoms forming in general a regular octahedral. The average of the interatomic distances (hMn(Co)–Oi) and Mn(Co)–O–Mn(Co) angles for the La0.8 Ca0.2 Mn1−x Cox O3 (x = 0, 0.1, 0.2, 0.3) samples, in the orthorhombic structure, are listed in Table 3. The average crystallite sizes D was also calculated using the Scherrer formula29 : D=

Kλ . β cos θ

With K = 0.9 is the particle shape factor, λ = 1.5406 ˚ A is the X-ray wavelength, θ is the diffraction angle for the most intense peak and β is the experimental full width at half-maximum (FWHM) of the same peak [inset Fig. 1(b)]. The average grain size of La0.8 Ca0.2 Mn1−x Cox O3 nanopowders is obtained close to 71.2 nm. 3.2. Magnetic properties Magnetization measurements as a function of temperature in the range 5–350 K under an applied magnetic field of 0.05 T showed that our samples exhibit FM to paramagnetic (PM) transition (Fig. 3) with increasing temperature. We have found that TC is 256 K for La0.8 Ca0.2 MnO3 whereas for the same compound the Curie temperature reported by Walz et al.13 and Ma et al.14 is 198 K and 240 K, respectively. This deviation of our TC value should be due to the difference of the particle sizes (for our compound the size is 71.2 nm) as reported by Xi et al.16 Indeed, Xi et al. suggested that the critical temperatures depend on the particle size. Their reported critical temperatures TC are 234, 252, 214, 237 and 236 K for 17, 19.5, 28, 35.5 and 43 nm samples, respectively.16 The Curie temperature depends also on the unit cell volume as seen from our Table 1 where the change of the unit cell volume is opposite to that of the TC value. 1450230-7


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40

La0.8Ca0.2Mn1-xCoxO3

M (emu/g)

30

x=0 x = 0.1 x = 0.2 x = 0.3

20

0

0

50

100

150

200

250

300

350

T (K) Fig. 3. Magnetization as a function of temperature for La0.8 Ca0.2 Mn1−x Cox O3 (x = 0, 0.1, 0.2, 0.3) at µ0 H = 0.05 T. The inset indicates the plot of dM/dT curve as a function of temperature for x = 0 sample.

80 60

M (emu/g)


10

40 x=0 x = 0.1 x = 0.2 x = 0.3

20 0

0

1

2

3

4

5

6

µ0H (T) Fig. 4. Magnetization evolution versus La0.8 Ca0.2 Mn1−x Cox O3 (0 ≤ x ≤ 0.3).

applied

magnetic

field

at

T

=

5

K

for

3.2.1. Saturated moment Figure 4 shows the magnetic field dependence (µ0 H) in the magnetization (M ) for T = 5 K. An increase in the cobalt content leads to decrease in the saturated moment (Fig. 5). The compound with the chemical formula, 1450230-8


S0217979214502300


Msat (µ B/mol)

3,5

3,0

2,5


2,0

0,0

0,1

0,2

0,3

x (% co) Fig. 5. Saturation magnetization as a function of Co substitution for La0.8 Ca0.2 Mn1−x Cox O3 (0 ≤ x ≤ 0.3) at T = 5 K. Table 4. Experimental and theoretical saturated magnetization moment. Samples x= x= x= x=

0 0.1 0.2 0.3

M exp [µs ] (µB )

M th [µs ] (µB )

3.38 3.34 2.76 2.57

3.8 3.16 2.52 1.88

2+ 3+ 8 La3+ Mn4+ 0.8 Ca0.2 (Mn0.8−x Cox ) 0.2 O3 leads to a theoretical magnetic moments :

µth s = [4(0.8 − 0.8x) − 4(0.8x) + 3(0.2)]µB = (3.8 − 6.4x)µB , where Mn3+ , Mn4+ and Co3+ ions have magnetic moment 4, 3 and 4 µB , respectively. For easy comparison, the temperature dependence of µexp and µth s s for an applied magnetic field of 0.05 T are summarized in Table 4. The difference between the experimental effective PM moment and the theoretical one should be related to the presence of a magnetic inhomogeneity inducing the Griffiths phase in La0.8 Ca0.2 MnO3 as explained below. The presence of such phase induces a decrease of the experimental saturated moment as confirmed in Ref. 29. 3.2.2. Curie–Weiss law In Fig. 6, we report the magnetic susceptibility (χ−1 ) as well as related linear fit from Curie–Weiss law versus temperature. The down turn in the magnetic susceptibility at temperatures above TC for the sample with x = 0 is the signature of the “Griffiths phase”.15,29 The deviation from Curie–Weiss law in the PM region is due to the presence of FM clusters within the PM region. The Griffiths phase 1450230-9


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12 10 x=0 Fit

6 4

χ

-1 ( µ

µ B/mol)

8

0 0

50

100

150 200 T (K)

250

300

350

(a)

7 6

x = 0.1 Fit

4

-1 ( µ

µ B/mol)

5

3

χ


2

2 1 0

0

50

100

150

200

250

300

T (K) (b) Fig. 6. Inverse of magnetic susceptibility (X-1) versus temperature (T) for La0.8 Ca0.2 Mn1−x Cox O3 samples measured at applied magnetic field of 0.05 T. (a) x = 0, (b) x = 0.1.

(TC < T < TG ) is induced by the disorder. In fact, in DE coupled systems local spin-order leading to the stabilization of the large FM clusters above TC .30 For the samples with x = 0.1, 0.2 and 0.3, χ−1 follows the Curie–Weiss law (linear behavior in the PM region). χ=

C , T − θcw

1450230-10


S0217979214502300



where χ is the magnetic susceptibility, θcw is Curie temperature coefficient and C is the constant that is related to the effective magnetic moment. From the linear fitting the PM region of the samples, we can obtain the values of the Curie–Weiss temperature θcw and the Curie constant C. The θcw follows the same trend of TC , its positive values suggest the existence of a FM exchange interaction between the nearest neighbors. The values of θcw obtained are higher than TC ; generally, this difference θcw depends on the substance and is associated to the presence of shortrange slightly ordered above TC , which is related to the presence of a magnetic inhomogeneity.31 In fact, the increase of Co content leads to the suppression of the DE interaction which reduces the FM alignment of Mn ions and so the value of saturate magnetization MS decreases. 3.2.3. Effective paramagnetic moment The experimental effective PM moment can be determined by the relation: NAv 2 C= µ , 3kB eff where NAv = 6.023 × 1023 mol−1 is the number of Avogadro and KB = 1.38016 × 10−23 J/K is the Boltzmann constant. The theoretical effective moment for all samples is calculated using the formula32 : q µth = (0.8 − x)µ2eff(Mn3+ ) + xµ2eff(Co3+ ) + 0.2µ2eff(Mn4+ ) . eff

For comparison, Table 5 gives the summary on temperature dependence of C, th θcw , µexp eff and µeff for an applied magnetic field of 0.05 T. The difference between the experimental effective PM moment and the theoretical one can be related to the presence of FM polarons in the PM state which can be described by Zener model.33 3.2.4. Magnetic transition In order to evaluate the influence of the Griffiths phase and the disorder on the type of the magnetic phase transitions in La0.8 Ca0.2 MnO3 compound, we have performed magnetization measurements as a function of magnetic applied field µ0 H up to 5 T at different temperatures (Fig. 7). We have used the criterion given by Banerjee34 that consists in inspecting the slope of isotherm plots of µ0 H/M versus M 2 . Results are presented in inset of Fig. 8. According to this criterion, the Table 5. Curie temperature Tc , Curie constant C, Curie–Weiss temperature θcw and effective paramagnetic moment µeff . Samples

TC (K)

C (µB KT −1 mol−1 )

θcw (K)

exp (µB )

th (µB )

x=0 x = 0.1 x = 0.2 x = 0.3

256 180 175 176

10.12 10.05 8.81 3.76

246.19 198.28 189.49 196.55

8.996 8.967 8.805 5.483

4.713 4.718 4.711 4.709

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D. Turki et al.

80 T = 200 K

x=0

δT=2K

M (emu/g)

60

40

T = 292 K

0

0

1

2

3

4

5

6

µ0 H (T) (a)

80 x = 0.2 T = 130 K δT=2K

60 M (emu/g)


20

40

20

0

T = 232 K

0

1

2

3

4

5

6

H (T) (b) Fig. 7. Isothermal magnetization as a function of applied magnetic field at different temperatures, (a) x = 0 and (b) x = 0.2.

magnetic transition is of second-order if all the curves have positive slopes. On the other hand, if some of these curves show a negative slope, the transition is of firstorder. From Fig. 8 it can be observed that µ0 H/M versus M 2 curves show negative slopes, implying that this sample obey first-order transition. In fact, the formation 1450230-12


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0,20

T = 200 K

x=0


µ0 H/M (T.g /emu)

0,15

δT=2K 0,10

T = 290 K

0,05

0,00 0

2000

4000

6000

M 2(emu/g) 2 Fig. 8.

µ0 H/M versus M 2 curves (Arrott plots) for La0.8 Ca0.2 MnO3 .

of polarons that characterizes the DE ferromagnetic phase is responsible for the discontinuous transition (first-order transition). 3.3. Magnetocaloric study According to the Maxwell relations based on the thermodynamical theory, the MCE is defined as the heating or cooling of a magnetic material due to the application of a magnetic field H. The magnetic entropy change ∆SM , which results from the spin ordering, can be evaluated by processing the temperature and field dependent magnetization curves in the vicinity of the temperature TC , using the Eq. (1). Z Hmax ∂M (T, H) ∆SM (T, ∆H) = dH . (1) ∂T 0 H Using isothermal magnetization plots (Fig. 7) we plot in Fig. 9 the magnetic entropy change −∆SM (T ) calculated using Eq. (1) under different magnetic field values (0 − 5 T), for both La0.8 Ca0.2 MnO3 (LCMO) and La0.8 Ca0.2 Mn0.8 Co0.2 O3 (LCMCO) samples. As shown in Fig. 9, the maximum values of −∆SM is found to be around TC and it increases with increasing the ∆H variation of the applied magnetic field. In Table 6 we listed the MCE performance of our samples with some other perovskite manganites reported in the literature. According to Othmani et al.4 the annealing at low temperature enhances the value of the maximum of entropy change. In fact, we have found that for LCMO annealed at 1000◦ C the −1 maximum −∆S max ·K−1 . Whereas Khlifi et al.35 reported that M value is 3.09 J · Kg for La0.8 Ca0.2 MnO3 elaborated by solid–solid reaction and then annealed at 800◦ C and 1200◦C the −∆S max is 5.7 J · Kg−1 · K−1 and 2.23 J · Kg−1 · K−1 , respectively. M 1450230-13


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D. Turki et al.

6 µ0∆H = 1T

x=0

µ0∆H = 2T

2

0 220

230

240

250

260

270

280

290

T (K) (a)

3,0 2,5

µ0∆H = 1T

x = 0.2

µ0∆H = 2T

-1

.kg

-1

)

µ0∆H = 3T

(−∆ SM (J.K


µ0∆H = 4T µ0∆H = 5T

-1

-1

−∆ SM (J.K .kg )

µ0∆H = 3T

4

2,0

µ0∆H = 4T µ0∆H = 5T

1,5 1,0 0,5 0,0 120

140

160

180

200

220

240

T (K) (b) Fig. 9. Temperature dependence of the magnetic entropy change (−∆SM ) at different applied magnetic field, (a) for LCMO and (b) for LCMCO.

For LCMCO the maximum of entropy change corresponding to a magnetic field variation of 5 T is 3.21 J · Kg−1 · K−1 . Similar results were also observed by Nisha et al. for La0.67 Ca0.33 Mn0.85 Co0.15 O3 and for La0.67 Ca0.33 Mn0.9 Cr0.1 O3 .10,36 From Fig. 10 we can deduce that with increasing Co substitution the maximum of −∆SM versus T decreases. We noted that at a magnetic field variation of 5 T the −1 |∆max · K −1 and 2.568 J · Kg−1 · K −1 for x = 0 and 0.2, reM | is about 5.595 J · Kg spectively. As it well recognized,30 the DE interaction controls the close relationship 1450230-14


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The effect of Co doping on structural, magnetic and magnetocaloric properties Table 6.

Magnetocaloric properties for our compounds compared with other works.

Tc

∆SM (J/K · Kg)

µ0 ∆H(T )

RCP (J/Kg)

Reference

252 256 182 186 241

3.09 5.595 1.123 2.568 5.7

2 5 2 5 2

55.845 153.862 49.971 144.545 96

Our work Our work Our work Our work 36

241

8.1

5

251

36

183

2.23

2

112.36

36

176

3.67

1.5

99.09

38

234

0.25 0.62 7.5

2 4.5 4.5

53.5 137 348.6

16 16 16

6.41 8.63 2.06 3.97 3.5 3.21

2 4.5 5 5 5 5

100 225 — 114 147 —

16 16 39 10 37 10

200

250

300

La0.8 Ca0.2 MnO3 La0.8 Ca0.2 MnO3 La0.8 Ca0.2 Mn0.8 Co0.2 O3 La0.8 Ca0.2 Mn0.8 Co0.2 O3 La0.8 Ca0.2 MnO3 (annealed at 800◦ C) La0.8 Ca0.2 MnO3 (annealed at 800◦ C) La0.8 Ca0.2 MnO3 (annealed at 1200◦ C) La0.8 Ca0.2 MnO3 (monocristal) La0.8 Ca0.2 MnO3 (D = 17 nm) La0.8 Ca0.2 MnO3 (D = 28 nm) La0.8 Ca0.2 MnO3 (D = 43 nm) La0.67 Ca0.33 MnO3 La0.67 Ca0.33 Mn0.9 Co0.1 O3 La0.67 Ca0.33 Mn0.9 Cr0.1 O3 La0.67 Ca0.33 Mn0.85 Co0.15 O3

214 236 252 170 232.5 147

6

-1

-1

−∆SM (J.K .kg )


Composition

5

x=0 x = 0.2

4

µ0 H = 5 T

3 2 1 0

150

T (K) Fig. 10. Temperature dependence of the magnetic entropy change (−(−∆SM ) at different measured for an applied field 5 T for La0.8 Ca0.2 Mn1−x Cox O3 (x = 0 and 0.2) compounds.

between the structure and the magnetic properties in this kind of perovskite. When we substitute Mn4+ by Co3+ the amount of Mn4+ /Mn3+ decreases so it causes the decreases of the number of hopping electrons and the available hopping sites between Mn4+ and Mn3+ , then, it weakens the DE interaction of Mn4+ –O–Mn3+ . For 1450230-15


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D. Turki et al.

180 x=0 x = 0.2

150

fit x = 0 fit x = 0.2

120 90 60 30 0 1

2

3

4

5

µ0 ∆H (T) Fig. 11.

Field dependence of relative cooling power of LCMO and LCMCO.

the above reasons, the FM coupling is weakened, therefore, the maximum −∆SM max decreases. Though the Co doping reduces the |∆SM | values; we can also deduce the type of the transition. In fact, a transition of first-order is characterized by a narrow magnetic transition and an important value of −∆S max M . However, a transition of a second-order presents a large magnetic transition (high value of δTFWHM ) while the maximum of the magnetic entropy change is reduced. From a cooling perspective, it is important to consider not only the magnitude of the magnetic entropy change but also the refrigerant capacity (RC) or the so-called relative cooling power RCP, which is evaluated from the product of the peak entropy changes and the FWHM as described in Eq. (2) max RCP = −∆SM × δTFWHM .

(2)

The value of RCP quantifying the performance of the system in refrigeration, appear to change almost linearly with increasing field. The results are shown in Fig. 11. The RCP value of La0.8 Ca0.2 Mn0.8 Co0.2 O3 measured at 5 T is 144.545 J · Kg−1 . This value is comparable to the RCP value of La0.67 Ca0.33 Mn0.9 Cr0.1 O3 (147 J ·Kg−1 ) given by Nisha.10 4. Conclusion We have investigated the effect of cobalt substitution in the magnetic properties of La0.8 Ca0.2 Mn1−x Cox O3 compounds. These compounds are prepared by sol–gel method. We found that all our samples crystallize in the orthorhombic structure with Pnma space group. The applied magnetic field (µ0 H) dependence of the magnetization reveals that the magnetization decreases when increasing x for our samples. The results shows that the Curie temperature (TC ) depends on the x com1450230-16


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position and decreases from 256 K for x = 0 to 176 K for x = 0.3. Furthermore, we have focused on the influence of the Griffiths phase in the discontinuous phase transition. The magnetic results show that the substitution of Co by Mn sites reduces the magnetic entropy change as well as the RCP. Acknowledgments This work has been supported by the Tunisian Ministry of Scientific Research and Technology and Institute Néel at Grenoble.


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