3rd International Conference on Electrical & Computer Engineering ICECE 2004, 28-30 December 2004, Dhaka, Bangladesh
MAGNETORESISTANCE IN DOUBLE LAYERED PEROVSKITE MANGANITES M.A. Basith1 and M.Huq2 Department of Physics, Dhaka University of Engineering & Technology, Gazipur-1700, Bangladesh. 2 Department of Physics, Bangladesh University of Engineering & Technology, Dhaka-1000, Bangladesh. E-mail:
[email protected];
[email protected] 1
ABSTRACT Magnetoresistive properties of R2-2xSr1+2xMn2O7 (R= La, Gd) bulk polycrystalline samples prepared for doping levels x = 0.3 and 0.4 and sintered at temperature 11000C for 24 hours in air have been investigated from room temperature down to liquid nitrogen temperature using standard four-probe technique. All investigated samples yielded metalinsulator transition and exhibited magnetoresistance (MR) effects. The results of replacing Gd by La atoms is found to lower the ferromagnetic (or metalinsulator) transition temperature, an effect which would be due to bond bending caused by the lattice adjusting to the size differential between the La and Gd ions. The exhibited large MR effects in these compounds at low temperature and very low field might be associated with magnetic-domain based scattering or spin-polarized tunneling between grains. It was observed that the higher percentage of Sr concentration favours insulating paramagnetic phase. On the contrary, when Sr is replaced with Gd atom the transition temperature is found to rise dramatically favoring a metallic phase. The study also revealed that the presence of 0.7T magnetic field increases M-I transition temperature. This is due to the suppression of spin fluctuations with the applied field in the paramagnetic region. The applied magnetic field accelerates magnetic ordering which causes the transition temperature enhancement by few Kelvins. An applied magnetic field also resulted in a reduction in resistivity due to the reduction of spin fluctuations. The sample characterization was carried out by x-ray diffraction analysis and the samples were found to be homogeneous and single phase.
1. INTRODUCTION Perovskite manganites, which have the general formula R1-xAxMnO3 (R=La, Pr or Nd and A= Ca,
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Sr, Ba, or Pb), have generated much interest due to their so-called colossal magnetoresistance (CMR) properties. Among a number of perovskite compounds, hole doped manganese oxide systems have attracted great attention because of their particular magnetotransport phenomena resulting from strong spin-charge coupling [1]. The ABO3 type compounds with the three-dimensional Mn-OMn networks such as La1-xSrxMnO3 have long been known to be conducting ferromagnets. The coexistence of metallic conductivity and ferromagnetic coupling in these materials has been explained in terms of this double exchange (DE) mechanism [2,3] based on the mixed Mn3+/Mn4+ valence state. Experimental evidence suggest [4,5] that the exchange mechanism alone is not sufficient for metal-insulator transition and the lattice distortion seems to play a vital role in metalinsulator transition and in the appearance of colossal magnetoresistance. With the ABO3 type manganese oxides, a ferromagnetic metal to a paramagnetic insulator transition take place at a magnetic transition temperature Tc at which large negative magnetoresistance effects have been observed [6-7]. Recent attention in the field of mixed-valence manganese perovskites has focused on the layered perovskites (R1-xDx)n+1MnnO3n+1 , where R is rare earth trivalent cation and D is divalent cation. The n = 1 compound is similar to the La2CuO4 structure, and the n = 2 compound is analogous to the Sr3Ti2O7 structure [8-9]. The n =1 series of compounds (R1xDx)MnO4 have been well-studied [10,11]. These compounds exhibit insulating behavior for all x, and in the region x ≅ 0.5, a charge ordering state appears around T = 250 K [10-12]. The aim of this research work is to investigate the electric and magnetic properties of the less investigated layered structure perovskite (R1-xDx)n+1MnnO3n+1 with n =2 , R = La, Gd and D = Sr.
9
9
La1.4Sr1.6Mn2O7
8
8
La1.2Sr1.8Mn2O7
7
La1.2Gd0.2Sr1.6Mn2O7
7
6
La1.0Gd0.2Sr1.8Mn2O7
ρ(T)/ρ(RT)
ρ(T)/ρ(RT)
10
5 4 3
La1.2Sr1.8Mn2O7 La1.2Gd0.2Sr1.6Mn2O7
6
La1.0Gd0.2Sr1.8Mn2O7
5 4 3
2
2
Appl. Magnetic Field = 0T
1
Appl. Magnetic Field =0.86 T
1
0 50
75
100
125
150
175
200
225
Temperature (K)
250
275
300
0
325
50
Fig. 1 (a) Normalized resistivity as a function of temperature with the applied magnetic field 0 T 8
L a 1 .0 G
0 .6
S r 1 .4 M n 2 O
100
150
200
Temperature (K)
250
300
Fig. 1 (b) Normalized resistivity as a function of temperature with the applied magnetic field 0 .86T
L a 1 .0 G d 0 .4 S r 1 .6 M n 2 O
7
7
6
7
L a 1 .0 G d 0 .8 S r 1 .2 M n 2 O
7
6
L a 1 .0 G d 0 .4 S r 1 .6 M n 2 O
7
L a 1 .0 G d 0 .6 S r 1 .4 M n 2 O
7
L a 1 .0 G d 0 .8 S r 1 .2 M n 2 O
7
ρ(T)/ρ(RT)
5
5
ρ(T)/ρ(RT)
La1.4Sr1.6Mn2O7
4
4
3
3
2
2
A p p l. M a g n e tic F ie ld = 0 T
1
A p p l. M a g n e tic F ie ld = 0 .8 6 T
1 0 50
100
150
200
250
300
50
T e m p e ra tu r e (K )
Fig. 2 (a) Normalized resistivity as a function of temperature with the applied magnetic field 0 T
100
150
200
T e m p e ra tu re (K )
250
300
Fig. 2 (b) Normalized resistivity as a function of temperature with the applied magnetic field 0.86T
2. SYNTHESIS AND CHARACTERIZATION OF THE SAMPLES Polycrystalline samples R2-2xSr1+2xMn2O7 (R=La, Gd) with various x were prepared using the conventional solid-state reaction technique. Stoichiometric amounts of raw materials La2O3 (99.99%), SrCO3 (99.99%), Gd2O3 (99.99%) and MnCO3 (99.99%) were well mixed, then calcined at 11000 C in air for 24 hours. The resulting powder samples were then reground and sintered at 11000 C for 48-50 hrs in air with one intermediate grinding. Before the final sintering step at 11000 C for 24 hrs, the samples were pressed into pellets. The resulting pellets were subjected to electric and magnetic investigation. The specimen’s crystallinity & structure were checked by x-ray diffractometry. The DC electrical resistivity for various polycrystalline
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samples was measured from room temperature down to liquid nitrogen temperature by standard four-probe method using Van der Pauw technique. The temperature dependence of normalized resistivity, ρ (T)/ ρ (RT) at zero applied magnetic field for various polycrystalline samples and the corresponding behavior in presence of 0.86 T applied magnetic field have been investigated. Magnetoresistance measurements were carried out in a magnetic field of around 0.86 Tesla in the temperature range 80K to 300K.
3. RESULTS AND DISCUSSUION The temperature dependence of normalized resistivity of polycrystalline bulk samples La2-2xSr1+2xMn2O7 for x = 0.3 and x = 0.4 and also the normalized resistivity for Gd doping on the La and Sr sites of these compounds in zero
0.0 -0.2
0
La1.4Sr1.6Mn2O7
-0.4
MR % at RT
La1.2Sr1.8Mn2O7
MR % at RT
-0.6 -0.8
La1.4Sr1.6Mn2O7
-1.4
La1.2Sr1.8Mn2O7
-1.6
La1.2Gd0.2Sr1.6Mn2O7
-1.8
La1.0Gd0.2Sr1.8Mn2O7 0
200
400
La1.0Gd0.2Sr1.8Mn2O7
La and -10 Sr site. It is well known that hole doping in ABO3-type perovskite manganese oxide produces -15 conductivity and metallic
-1.0 -1.2
La1.2Gd0.2Sr1.6Mn2O7
-5
H* = 160 mT -20
600
800
1000
0
Magnetic Field (mT)
200
400
Fig. 3(a) Magnetoresistance as a function of magnetic field at RT
800
1000
Fig. 3(b) Magnetoresistance as a function of magne field at 78 K.
0.0
0
L a 1 .0 G d 0 .4 S r 1 .6 M n 2 O 7
-2
L a 1 .0 G d 0 .6 S r 1 .4 M n 2 O 7
-0.5
MR % at RT
MR % at RT
600
Magnetic Field (mT)
-1.0
-1.5
L a 1 .0 G d 0 .8 S r 1 .2 M n 2 O 7
-4 -6 -8
-1 0
-2.0
La1.0Gd0.4Sr1.6Mn2O7
-1 2
La1.0Gd0.6Sr1.4Mn2O7
-1 4
H* = 152 m T
La1.0Gd0.8Sr1.2Mn2O7
-2.5 0
200
400
600
-1 6
800
0
1000
200
400
600
800
1000
M a g n e tic F ie ld (m T )
Magnetic Field (mT)
Fig. 4(a) Magnetoresistance as a function of magnetic field at RT
Fig. 4(b) Magnetoresistance as a function of magne field at 78 K and ferromagnetic where the magnetic interaction
magnetic field and in a magnetic field of 0.86 T is plotted in Fig. 1(a), 1(b) and 2(a), 2(b), respectively. All the manganites show an insulator-metal transition with a peak in the electrical resistivity, ρP, at a temperature Tp (Table 1). The transition temperature varies from 108 K to 165 K with composition and doping percentage of Gd on the La and Sr sites. It is well known that hole doping in ABO3-type perovskite manganese oxide produces metallic conductivity and ferromagnetism where the magnetic
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interaction is mediated by the transfer of the holes (the double exchange interaction in the mixed Mn3+ /Mn4+ valence state). With La2-2xSr1+2xMn2O7, the fundamental mechanism leading to the appearance of metallic ferromagnetism would be the same. The parent material of La2-2xSr1+2xMn2O7 is a charge transfer insulator of La2SrMn2O7. In hole undoped La2SrMn2O7, the Mn3+ ion has the electron configuration of t32ge1g. Among the four 3d electrons on the Mn site, t32gelectrons can be
low field was quite sufficient to align the domain spins and thus a sharp decrease in MR was observed but to align the spins at the domain boundary requires much larger field leading to weak field dependence.
Table 1 Transition temperature for various samples with 0 T and 0.86 T applied magnetic field. Sample compositions La1.4Sr1.6Mn2O7 La1.2Sr1.8Mn2O7 La1.2Gd0.2Sr1.6Mn2O7 La1.0Gd0.2Sr1.8Mn2O7 La1.0Gd0.4Sr1.6Mn2O7 La1.0Gd0.6Sr1.4Mn2O7 La1.0Gd0.8Sr1.2Mn2O7
Tp (with 0 T field) 118.68 K 111.65 K 111.65 K 108.20 K 114.07 K 142.32 K 152.21 K
Tp (with 0.86 T field) 132.55 K 116.59 K 128.73 K 117.89 K 126.30 K 150.35 K 165.81 K
4. CONCLUSIONS
viewed as localized spins (S = 3/2), while the e1g electron is either itinerant or localized. Substitution of La3+ by Sr2+ introduces holes into the eg state ( some Mn3+ ions convert into the Mn4+ state without e1g electrons). We have found hole doping with x = 0.3 favoring metallic phase compare to x = 0.4. The substitution of 14% to 16% Gd in place of La keeping the Sr site unchanged results a lowering of metal insulator transition temperature. As the atomic size of Gd is less than that of La, the substituted Gd ions lower the Mn-Mn exchange interaction substantially by bending the Mn-O-Mn bond angle. From the ρ-T curve it is clear that the presence of Sr enhances paramagnetic insulating phase. But when non-magnetic Sr is replaced with magnetic Gd atom the transition temperature is found to increase dramatically favoring metallic phase. The magnetic property of Gd is thought to be responsible for higher transition temperature. Typical magetoresistance curve obtained for the samples at room temperature are shown in Fig. 3(a) and 4(a) as a function of magnetic field. Room temperature MR is found to be very low almost 1.5% ~ 2% and is almost linear with field. But the MR at 78 K for these polycrsytalline samples shown in Fig. 03(b) and 04(b) exhibited a large value in presence of low applied magnetic field. At low temperature (78K), the field dependence of MR exists for an applied field of upto H* as shown in figures 03(b) and 04(b). The magnetic field H* designates the boundary of the two slopes. Beyond H* the magnetoresistance is a weak function of the applied magnetic field. In this work, about 12 % ~15 % of the MR is observed at H* = 0.15 T~ 0.16 T. This may be due to the reason that as the materials are subdivided into domains,
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The polycrystalline samples under investigation undergo a metal insulator transition, which can be explained within the framework of double exchange theory. The effect of Gd doping on the La site influences the transport properties by distorting the lattice due to the disparity between the Gd3+ and La3+ ionic radii. The result of Gd doping on the Sr site results in the increase of transition temperature dramatically. The low field and low temperature magnetoresistance in these materials is most significant whereas in room temperature the MR is negligible. The low-field MR is attributed to the effect of grain boundaries.
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