Journal of the Korean Physical Society, Vol. 60, No. 10, May 2012, pp. 1587∼1592
Above Room Temperature Magnetic Transition and Magnetocaloric Effect in La0.66 Sr0.34 MnO3 M. S. Anwar, Shalendra Kumar, Faheem Ahmed, Nishat Arshi, G. W. Kim and Bon Heun Koo∗ School of Nano and Advanced Materials Engineering, Changwon National University, Changwon 641-773, Korea (Received 8 July 2011) In this study, the magnetic and the magnetocaloric properties of the La0.66 Sr0.34 MnO3 (LSMO) compound were investigated. The X-ray diffraction result indicates that the LSMO sample has a single phase of rhombohedral symmetry without any impurity phase. The magnetic study reveals that the specimen La0.66 Sr0.34 MnO3 exhibits a ferromagnetic-paramagnetic transition at TC ∼ 376 K. Using Arrott plots, the phase transition from ferromagnetic to paramagnetic is found to be of second order. A maximum magnetic entropy change of 1.25 J/kgK has been observed for a low applied magnetic field of 1T. The relative cooling power values exhibit a nearly linear dependence on the applied magnetic field. Moreover, the analysis of the magnetocaloric effect (MCE) using the Landau theory of phase transitions shows good agreement with the experimental results, confirming the importance of magnetoelastic coupling and electron interactions in the magnetocaloric properties of perovskite manganites. This investigation suggests that La0.66 Sr0.34 MnO3 can be used as a potential magnetic refrigeration material. PACS numbers: 75.50.Lk, 75.30.Sg, 74.25.Op Keywords: Magnetic refrigerant, Magnetocaloric effect, Landau theory, Magnetic entropy DOI: 10.3938/jkps.60.1587
I. INTRODUCTION In recent years, the interest in ferromagnetic perovskite maganites with a general formula of Ln(1−X) AX MnO3 (Ln = rare-earth ions and A = divalent ions) has grown considerably in view of their special electronic and magnetic properties, including colossal magnetoresistance (CMR), as well as their potentials applications [1–4]. Recently, magnetocaloric effect (MCE) studies have revealed their potential in magnetic refrigeration [5,6]. Among the magnetic materials with potential for magnetic refrigeration, the perovskite maganites have attracted much attention due to their low production cost, chemical stability and higher resistivity, which is favorable for reducing eddy currents. These compounds also present a relatively large magnetic entropy change [6– 9], which is very useful for magnetic refrigeration. The most straightforward application of the MCE, magnetic refrigeration, is becoming a field of increasing interest because researchers believe that this subject will give rise to energy-efficient, environmentally friendly technological application. On the other hand, the study of some model materials has given more insight into the phenomenon and the physics of materials during the application of a magnetic field [10]. ∗ E-mail:
[email protected], Shafi
[email protected]; Tel: +82-55-264-5431; Fax: +82-55-262-6486
Valuable advances have been made in understanding the magnetic properties of manganites. The transport and the magnetic properties of manganites can be explained by means of the double exchange (DE) and the super exchange (SE) mechanisms, and the competition between the DE and the SE mechanisms is known to be trivial in determining the magnetic properties of the system [11–13]. The DE and the SE mechanism are known to be sensitive to variations in the Mn-O bond length and the Mn-O-Mn bond angle, both controlled by the average ionic radius of the A- or the B-site ions whereas the density of charge carriers is controlled by the Mn3+ /Mn4+ ratio [11–13]. In the last decade, significant progress has been made in interpreting the magnetocaloric properties of manganites [14–16]. The large magnetic entropy change in perovskite manganites is believed to originate from spin-lattice coupling in the magnetic ordering process [17,18]. Due to strong coupling between the spin and the lattice, significant lattice change has been observed accompany the magnetic transition in perovskite manganites [17,18]. The use of phenomenological theories has given valuable insight into this subject [19–21]. In this work, we study the MCE of the polycrystalline La0.66 Sr0.34 MnO3 compound. Three different methods were used to determine the Curie temperature (TC ). The entropy change was determined from magnetic measurements. The Landau theory for the phase transition was applied to describe the MCE in our sample with magne-
-1587-
-1588-
Journal of the Korean Physical Society, Vol. 60, No. 10, May 2012 Table 1. Curie temperature, TC , maximum entropy change, and applied magnetic field. ∆H (T) La0.67 Ba0.33 MnO3 5 3 La0.67 Ca0.33 MnO3 La.7 Sr0.3 Mn0.9 Ti0.1 O3 5 2 La0.75 Sr0.25 MnO3 Materials
Fig. 1. (a) X-ray diffraction pattern and (b) typical SEM micrograph of La0.66 Sr0.34 MnO3 .
La0.66 Sr0.34 MnO3
1
TC −∆S max RCP References (K) (J/kgK) (J/kg) 292 1.48 161 28 259 2.60 114 29 210 2.94 288 15 340 1.50 65 30 Present 375 1.25 45.44 work
toelastic and magnetoelectronic couplings. To estimate the refrigeration capacity of the sample we calculated the Relative cooling power (RCP ) by using the maximum magnetic entropy change at different applied magnetic fields for the La0.66 Sr0.34 MnO3 compound. II. EXPERIMENTAL A ceramic sample with a nominal composition of La0.66 Sr0.34 MnO3 was synthesized using the standard solid state reaction method. Stoichiometric amounts of high-purity analytical-grade La2 O3 , SrO, and Mn2 O3 (all chemicals were of 99.99% purity and purchased from Sigma Aldrich) were mixed using alcohol and were ball milled for 10 h. The mixed slurry was dried for 10 h at 80 ◦ C and then first heated at 800 ◦ C in air for calcination for 10 h. After grinding, the mixed powder was pressed into a disk-shape with a diameter of 7 mm and a thickness of about 2 mm. The disk sample was first sintered at 1100 ◦ C for 40 h followed by repeated grinding. Final sintering was performed at 1250 ◦ C for 24 h in air. Finally, the sample was cooled to room temperature at a cooling rate of 4 ◦ C/min. The structure and the phase purity of the sample was checked at room temperature by means of X-ray powder diffraction (XRD) using a Phillips X’pert (MPD 3040) X-ray diffractometer with Cu Kα radiation (λ = 1.5406 ˚ A) operated at a voltage of 40 kV and a current of 30 mA. The morphology of the grain boundaries and surfaces were investigated by using scanning electron microscopy (SEM, JSM5610). The magnetic measurements of these samples were carried out in the temperature range of 100 – 390 K at a frequency of 40 Hz by using a quantum design vibrating sample magnetometer (PPMS, 6000 VSM). In order to evaluate the magnetic entropy, we recorded magnetization isotherms at small temperature intervals around TC in magnetic fields up to 4 T. III. RESULTS AND DISCUSSION The X-ray diffraction pattern of the La0.66 Sr0.34 MnO3 (LSMO) compound is shown in Fig. 1(a). From the XRD
Fig. 2. (Color online) Temperature dependence of the magnetization for La0.66 Sr0.34 MnO3 at a magnetic field of 0.5 T (a) plot of dM/dT curve for determining TC (b) plot of the Curie Weiss Law.
pattern, LSMO clearly exhibits a polycrystalline behavior with maximum intensity for the (104) reflection. All the reflections were indexed to the rhombohedral structure by using POWDER-X software with lattice parameters a = b = 5.5013 ˚ A and c = 13.2980 ˚ A. The characteristics reflections corresponding to the (012), (104), (202), (024), (300), (208), (312), and (134) planes are located at 2θ = 22.90, 32.51, 40.09, 46.63, 58.0, 68.35, 72.96, and 77.75, respectively, which is in good agreement with the literature (JCPDS, 50-0308). No impurity peaks are observed in the pattern, indicating a single-phase formation of the LSMO compound. Figure 1(b) shows the polycrystalline structure in a typical SEM micrograph for the LSMO compound. The LSMO compound was observed mainly to have strongly connected large grains. The average grain size was estimated using the line-intercept method and was found to be ∼11 µm. Figure 2 displays the field-cooled temperature dependence of the magnetization of the LSMO sample taken at 5000 Oe. In the field-cooled process, the sample was cooled in the presence of a 5000 Oe applied magnetic field from 305 K to 100 K; and the magnetization mea-
Above Room Temperature Magnetic Transition and Magnetocaloric Effect · · · – M. S. Anwar et al.
-1589-
Fig. 4. (Color online) Temperature dependence of the magnetic entropy change under different applied magnetic fields for the La0.66 Sr0.34 MnO3 compound.
Fig. 3. (Color online) (a) Magnetization vs. applied magnetic field H, measured at different temperatures, for the La0.66 Sr0.34 MnO3 and (b) H/M vs. M 2 isotherms. The temperatures of the isotherms are indicated.
surements were performed in the presence of the same field during the warming-up cycle from 100 K to 400 K. The magnetization vs. temperature (M -T ) reveals that the LSMO sample exhibits a sharp ferromagneticparamagnetic transition occurring at TC . To determine the value of TC , we used three methods: a linear extrapolation of the M -T curve to zero magnetization, as shown in Fig. 2, a determination of minimum of the derivative dM/dT of the M -T curve, as indicated in the a-inset of Fig. 2, and a linear fitting of the Curie Weiss Law [22], 1 1 θ = T− , χ c c
(1)
in the paramagnetic region, as shown in the b-inset of Fig. 2, where χ is the magnetic susceptibility, θ is the Curie Weiss temperature and C is Curie constant defined as [22] C=
µ0 2 g S(S + 1)µ2B , 3KB
(2)
with µ0 = 4π × 10−7 H/m being the permeability of free space, g the gyromagnetic ratio, KB the Boltzmann constant, S the spin momentum and µB = 9.27 × 10−24 J/T the Bohr magnetron. The values of TC determined by
the using three methods mentioned above were nearly the same. The as-obtained values of TC were 382 K, 376 K, and 379 K, respectively, higher than other reported results (Table 1). Interestingly, a sharp jump in magnetization (Fig. 2) with respect to temperature appeared to occur in the magnetic phase transition range, indicating a large change of magnetic entropy around TC . Therefore, the isothermal magnetization curves near TC , ranging from 280 K to 390 K for the LSMO sample, were measured and are shown in Fig. 3(a). The temperature step is 5 K in the range from 360 K to 390 K and 10 to 20 K for the other temperature ranges. The magnetization was found to increase with decreasing temperature in the temperature range 280 – 390 K, where thermal fluctuation of spins decreases with decreasing temperature. In order to get more insight into the nature of the magnetic phase transition, we used the Banerjee criterion [23], according to which the slope of the H/M vs. M 2 curves (Arrott plots) connotes whether the observed magnetic transition is of the first order (negative slope) or second order (positive slope). The Arrott plots shown in Fig. 3(b) clearly indicate a positive slope in the complete M 2 range, and confirming the transition to be of the second order. The magnetic entropy of a material is associated with the magnetic degree of freedom, which varies as the field changes the magnetic order of the material. According to the classical thermodynamical theory, the isothermal magnetic entropy change together with a magnetic field variation is given by [24] ∆SM (T, H) = SM (T, H)−SM (T, 0) =
0
H
∂S ∂H
dH T
(3)
-1590-
Journal of the Korean Physical Society, Vol. 60, No. 10, May 2012
Fig. 5. (Color online) Maximum magnetic entropy change and RCP values as functions of the applied magnetic field for the La0.66 Sr0.34 MnO3 compound. Fig. 6. Temperature dependence of the Landau coefficients a, b, and c for the La0.66 Sr0.34 MnO3 compound.
with Maxwell’s equation ∂M ∂S = . ∂H T ∂T H
(4)
Thus, the magnetic entropy change can be written as H ∂M ∆SM (T, H) = dH. (5) ∂T H 0 Experimentally, there are two ways to evaluate the magnetic entropy change. The first one is measurement of the M -H curve at different temperatures. The second one is measurement of the M -T curve under different applied magnetic fields. In this paper, we use the first method to evaluate the magnetic entropy change. In the case of magnetization measurements at small discrete fields and temperature intervals, the integral in Eq. (5) can be numerically approximated as −∆SM (T, H) =
Mi − Mi+1 i
Ti+1 − Ti
∆H.
(6)
where, Mi and Mi+1 are the experimental values of the magnetization at Ti and Ti+1 , respectively, under a magnetic field Hi . Figure 4 shows the magnetic entropy change as a function of temperature at different applied magnetic fields for the LSMO sample. The sign of the magnetic entropy change is negative, which means that heat is liberated when the magnetic field is changed adiabatically. The LSMO sample exhibits the MCE around TC at different applied magnetic fields. The ∆SM near TC is arises from interactions between the Mn and the La spin systems. The contribution of the rare-earth ions spins to the change in the magnetic entropy in perovskite manganese oxides is also confirmed by Chen and Du [25]. Our sample exhibits maximum entropy changes, max |, equal to 1.25, 2.01, 2.51, and 3.01 J/kgK for |∆SM magnetic field of 1 T, 2 T, 3 T, and 4 T, respectively.
max As seen from Fig. 5, |∆SM | increasing linearly with increasing applied magnetic field, which is indicative of a much larger entropy change being expected at higher magnetic fields. The most meaningful parameter that provides a measure of the effectiveness of magnetic materials for applications in magnetic refrigeration is the relative cooling power (RCP ) [26]. The RCP based on the ∆SM is demax | × δTF W HM , where δTF W HM is fined as RCP = |∆SM the full-width at half maximum of the magnetic entropy change curve. Figure 5 shows the RCP data plotted as a function of the applied magnetic field. The RCP values exhibit a nearly linear dependence on the applied magnetic field. The obtained RCP values at 1 T, 2 T, 3 T, and 4 T are 45.44, 86.9, 124.37, and 165.49 J/kg, respectively. From table 1, these values are observed to be comparable to the results of other groups with higher TC value. In general, materials with second-order magnetic phase transitions exhibits a smaller MCE than the materials with first-order magnetic phase transitions [1]. Based on the Landau theory of phase transitions [27], we attempted to explain the magnetic entropy change obtained from the experimental data. The magnetic free energy G(M, T ) can be expanded as a Landau expansion in powers of the magnetization M (up to sixth power of M );
G(M, T ) =
a(T ) 2 b(T ) 4 c(T ) 6 M + M + M − M H. (7) 2 4 6
Here a, b, and c are known as Landau coefficients. These coefficients are temperature-dependent parameters containing the elastic and the magnetoelastic terms of the free energy [27]. The sign of b(T ) the of M 4 term determines the type of magnetic phase transition. From the
Above Room Temperature Magnetic Transition and Magnetocaloric Effect · · · – M. S. Anwar et al.
-1591-
IV. CONCLUSION
Fig. 7. (Color online) Experimental and theoretical magnetic entropy changes (∆SM ) for the La0.66 Sr0.34 MnO3 at magnetic fields of 0 – 1 T.
condition of equilibrium
∂G(M,T ) ∂M
= 0, we obtain
H = a(T ) + b(T )M 2 + c(T )M 4 . M
We have made a systematic investigation of the magnetic and the magnetocaloric properties of the La0.66 Sr0.34 MnO3 compound. The magnetization vs. temperature curve showed a sharp transition from a paramagnetic to a ferromagnetic phase at temperature above room temperature (376 K). The magnetocaloric properties of this compound were examined from isothermal magnetization versus magnetic field data measured at different temperatures in the vicinity of TC . The magnetic phase transition from a ferromagnetic to a paramagnetic phase was observed to be is of second order. The maximum value of the magnetic entropy change increased from 1.25 to 3.01 J/kgK when the applied magnetic field was increased from 1 T to 4 T. We have calculated the ∆SM using two different methods, Maxwell relation and landau theory, and we found good agreement between the two values. The analysis of the La0.66 Sr0.34 MnO3 compound using landau theory confirmed the influence of both magnetoelastic coupling and electron interactions on the MCE properties.
(8)
The values of the Landau coefficients were obtained from polynomial fittings of the Arrott plots (Fig. 3(b)). The values of the Landau coefficients as functions of temperature are shown in Fig. 6. b(T ) is observed to be negative (−0.64 × 10−3 T4 kg3 J−3 at 330 K) and positive (0.81 × 10−3 T4 kg3 J−3 at 375 K) for the LSMO sample. The change in the sign of the b(T ) coefficient from negative to positive again suggests that the phase transition in the sample is second order. The corresponding magnetic entropy is obtained from differentiation of the magnetic free energy with respect to the temperature and can be written as ∂G −∆SM (T, H) = ∂T H 1 1 1 = a1 (T )M 2 + b1 (T )M 4 + c1 (T )M 6 , 2 4 6 (9) where a1 (T ), b1 (T ), and c1 (T ) are the temperature derivatives of the Landau coefficients. Using the values of a1 (T ), b1 (T ) and c1 (T ), we calculated the temperature dependence of the magnetic entropy change from Eq. (9) at an applied magnetic field of 1T, as shown in Fig. 7. Good agreement is found between the experimental entropy change and the one estimated by using Landau theory. It is important to note that this theory predicts the fairly well observed temperature dependence of the magnetic entropy change of the La0.66 Sr0.34 MnO3 compound without considering the effect of the Jahn-Teller distortion and the exchange interaction. Further, this indicates the influence of both magnetoelastic coupling and the electron interaction on the MCE properties.
ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0002448). This Research was also financially supported by the MKE (The Ministry of Knowledge Economy), Korea, under the ITRC (Information Technology Research Center) support program supervised by the NIPA (National IT Industry Promotion Agency) (NIPA-2011-C1090-11210015).
REFERENCES [1] H. Terashita, B. Myer and J. J. Neumeier, Phys. Rev. B 72, 132415 (2005). [2] J. Yang, W. H. Song, Y. Q. Ma, R. L. Zhang, B. C. Zhao, Z. G. Sheng, G. H. Zheng, J. M. Dai and Y. P. Sun, Phys. Rev. B 70, 092504 (2004). [3] C. L. Lu, K. F. Wang, S. Dong, J. G. Wan, J. M. Liu and Z. F. Ren, J. Appl. Phys. 103, 07F714 (2008). [4] R. N. Mahato, K. Sethupathi, V. Sankaranarayanan and R. Nirmala, J. Appl. Phys. 107, 09A943 (2010). [5] V. B. Naik, S. K. Barik, R. Mahendiran and B. Raveau, Appl. Phys. Lett. 98, 112506 (2011). [6] P. Sarkar, P. Mandal and P. Choudhury, Appl. Phys. Lett. 92, 182506 (2008). [7] V. S, Kumar and R. Mahendiran, J. Appl. Phys. 109, 023903 (2011). [8] Z. B. Guo, Y. W. Du, J. S. Zhu, H. Huang, W. P. Ding and D. Feng, Phys. Rev. Lett. 78, 1142 (1997).
-1592-
Journal of the Korean Physical Society, Vol. 60, No. 10, May 2012
[9] M. S. Anwar, S. Kumar, N. Arshi, F. Ahmed, G. W. Kim, C. G. Lee and B. H. Koo, J. Magnetics 16, 457 (2011). [10] R. Caballero-Flores, V. Franco, A. Conde, K. E. Knipling and M. A. Willard, Appl. Phys. Lett. 98, 102505 (2011). [11] S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R. Ramesh and C. H. Chen, Science 264, 413 (1994). [12] R. V. Holmolt, J. Wocker, B. Holzapfel, L. Schultz and K. Samwer, Phys. Rev. Lett. 71, 2331 (1993). [13] A. P. Ramirez, J. Phys. Condens. Matter 9, 8171 (1997). [14] J. Fan, L. Pi, L. Zhang, W. Tong, L. Ling, B. Hong, Y. Shi, W. Zhang, D. Lu and Y. Zhang, Appl. Phys. Lett. 98, 072508 (2011). [15] S. Kallel, N. Kallel, O. Pena and M. Oumezzine, Mater. Lett. 64, 1045 (2010). [16] R. Szymczak, R. Kolano, A. Kolano-Burian, J. Pietosa and H. Szymczak, J. Magn. Magn. Mater. 322, 1589 (2010). [17] V. S. Kumar and R. Mahendiran, Solid State Commun. 150, 1445 (2010). [18] P. Sande, L. E. Hueso, D. R. Miguens, J. Rivas, F. Rivadulla and M. A. Lopez-Quintela, Appl. Phys. Lett. 79, 2040 (2001).
[19] A. Tozri, E. Dhahri and E. K. Hlil, Mater. Lett. 64, 2138 (2010). [20] L. Jia, G. J. Liu, J. Z. wang, J. R. sun, H. W. Zhang and B. G. Shen, Appl. Phys. Lett. 89, 122515 (2006). [21] V. S. Kolat, T. Izgi, A. O. Kaya, N. Bayri, H. Gencer and S. Atalay, J. Magn. Magn. Mater. 322, 427 (2010). [22] B. D. Cullity and C. D. Graham, Introduction to Magnetic Materials (Wiley, New Jersey, 2009). [23] S. K. Banerjee, Phys. Lett. 12, 16 (1964). [24] S. Othmani, R. Blel, M. Bejar, M. Sajieddine, E. Dhahri and E. K. Hlil, Solid State Commun. 149, 969 (2009). [25] P. Chen and Y. W. Du, Chin. J. Phys. 39, 357 (2001). [26] M. Koubaa, W. C-R. Koubaa and A. Cheikhrouhou, J. Magn. Magn. Mater. 321, 3578 (2009). [27] V. S. Amaral and J. S. Amaral, J. Magn. Magn. Mater. 272, 2104 (2004). [28] D. T. Morelli, A. M. Mance, J. V. Mantese and A. L. Micheli, J. Appl. Phys. 79, 373 (1996). [29] X. X. Zhang, J. Tejada, Y. Xin, G. F. Sun, K. W. Wong and X. Bohigas, Appl. Phys. Lett. 69, 3596 (1996). [30] Z. B. Guo, W. Yang, Y. T. Shen and Y. W. Du, Solid State Commun. 105, 89 (1998).
