Effect of annealing treatment on structure, magnetic and electrical properties of double perovskite LaKFeMoO₆

Huo Guoyan, Zhang Hongrui, Shi Pengfei, Wang Xiaoqing
(College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)

Abstract The double perovskite LaKFeMoO₆ ceramics have been successfully synthesized. The structure, magnetic and electronic properties of this compound and the sample after annealing treatment were studied. The structure belongs to monoclinic system with space group P2₁/n. The magnetic moment is slowly reduced with the increase of temperature, and annealing treatment at 900⁰C makes the magnetoresistance (MR) of the sample elevated.
Keywords Double perovskite, LaKFeMoO₆ compound, Annealing treatment，Structure, Magnetic property, Electrical property

1. INTRODUCTION
Recently, much interest has been focused on the half-metallic double perovskite Sr₂FeMoO₆ because of its tunneling magnetoresistance (TMR) at room temperature^[1-3]. The double perovskite Sr₂FeMoO₆ is an ordered structure of the A₂BB'O₆ type with Fe and Mo atoms alternating on the B and B' sites, respectively. The ferrimagnetic structure can be described as an ordered array of parallel Fe³⁺ (3d⁵; t³_2ge_2g) magnetic moments, antiferromagnetically coupled with Mo⁵⁺ (4d¹; t¹_2ge⁰_g) spins. Recently, it has been reported that the substitution of La³⁺ for Sr²⁺ in Sr₂FeMoO₆ promotes a rising of the T_C and a decreasing of M_S as doping level increases^[4]. And Kim et al.^[5] reported that replacement of Ba²⁺ by K⁺ in Ba₂FeMoO₆ reduces the T_C as K⁺ content increases. For Ba₂FeMoO₆, annealing at 1273 K in H₂/Ar leads to a large increase of the resistivity compared with the as-synthesized material and the low field MR is significantly increased as well^[6].
    Therefore，with the La³⁺ and K⁺ doping in double perovskite A₂FeMoO₆ compound carriers doping are inevitable in these materials. In this paper, we investigate the influence of annealing treatment on structure, magnetic and electrical properties of double perovskite LaKFeMoO₆.

2. EXPERIMENTAL
The polycrystalline LaKFeMoO₆ sample has been synthesized using Sol-Gel method followed by heat treatment. La₂O₃ was mixed stoichiometrically with nitric acid and added KNO₃, Fe (NO₃)·9H₂O and (NH₄)₆Mo₇O₂₄·4H₂O, heated until a gel formed^[7]. The gel was dried, then preheated at 600ºC for 8h in air. The calcined mixture was pulverized and pressed into pellets followed by sintering at 900ºC for 8h in the stream of 6.25% and 4.76% H₂/Ar, respectively (sample A). The sample was annealed for 8h in Ar atmosphere at 900 ºC (sample B). Phase analysis and characterization were carried out by X-ray diffraction using CuKa radiation with a graphite monochromator on Rigaku model D/max-2400 X-ray diffractometer.
    Temperature dependence of magnetization curves was measured by a vibrating-sample magnetometer (VSM) in field of 0.5 T over the temperature range 80-300 K. Isothermal magnetization curve was performed on VSM apparatus. Transport properties were determined by a standard four-probe DC method in the temperature range 80-300 K.

3. RESULTS AND DISCUSSION
Powder XRD patterns of sample A and sample B are shown in Fig. 1 which shows that the specimens are single phase. No impurity was detected based on powder XRD pattern. The diffraction peaks could be indexed in the monoclinic system with space group P2₁/n. The lattice parameters are found to be: sample A, a=0.5620 nm, b=0.5573 nm, c=0.7942 nm and b=96.7796⁰; sample B, a=0.5579 nm, b=0.5572 nm, c=0.7737 nm and b=92.3245⁰. The grain size of sample B is decreasing. The diffraction lines of (0 1 1) and (1 0 3) display that Fe and Mo ions partly order to occupy B and B' sites, respectively.

Fig.1 X-ray diffraction pattern of LaKFeMoO₆ sample

    Magnetization measurement made as a function of temperature M (T) on warming the sample A and sample B from 80 to 300K in a magnetic field of 0.5 T are plotted in Fig. 2. It can be seen from Fig. 2 that the magnetization reduces slowly with temperature warming from 80 to 300K, furthermore, the magnetic moment of sample B is higher than sample A. The reason seems to be as follows: the disorder on the B and B' sites^[8,9]. In LaKFeMoO₆ not all Fe/Mo retain their own site, the anti-site occupancy of Mo at Fe positions and vice versal would give a variation of the magnetization, where anti-site is the fraction of Fe atoms replaced by Mo. In our samples, annealing treatment makes Fe and Mo atoms alternating on the B and B' sites, and decrease the anti-site occupancy. So the magnetic moment of sample B is higher. In Fig. 2 no magnetic transition temperature appears in this temperature range. This indicates that the Curie temperature of LaKFeMoO₆ sample is higher than 300 K. We calculated that reduction rate with temperatures of magnetic moment of the samples are 0.005emu/(g·K) (sample A) and 0.008 emu/(g·K) (sample B) from 80 K to 300 K.

Fig.2 Temperature dependent magnetization of LaKFeMoO₆ registered on warming in 0.5 T magnetic field

    The r(T) curves registered upon warming in zero and 1.0T magnetic fields are shown in Fig. 3. One can see from Fig. 3 that sample A and sample B demonstrate semiconducting behavior under zero and 1.0T magnetic fields over the temperature range from 80 to 300K. The resistivities of the annealed sample is increased compared to the as-synthesized sample. In XRD patterns (Fig. 1) we found that the FWHM (full width at half maximum) increases after annealing, to the peak of about 67⁰(2q) as an example (Fig. 4). The FWHM may indicate that the grain size decreases with the heat treatment. The decreasing of grain size leads to the more grain boundaries and to the increase of the resistivity^[10].

Fig. 3 Temperature dependent resistivity registered upon warming in zero and 1.0 T magnetic fields

Fig. 4 Comparison of peak profile (about 67°)

We define MR=[r(H)-r(0)]/r(0), where r(H) and r(0) are the resistivity in a magnetic field and without a magnetic field, respectively. Fig. 5 shows the temperature dependence of the magnetoresistance (MR) of sample A and sample B in 1.0T magnetic fields. The -MR of sample B is larger than sample A. We concluded that the grain size of the annealed sample decreased, which means more grain boundaries introduced and the intergranular tunneling magnetoresistance was modified, so the -MR of sample B elevated^[10].

Fig. 5 Temperature dependence of magnetoresistance in 1.0 T magnetic fields

4. CONCLUSION
In conclusion, annealing treatment influences the character of LaKFeMoO₆ compound. The magnetic moment of the annealed sample is higher, about 12.5 emu/g. The grain size decreases with the annealing treatment. The decreasing of grain size leads to the intergranular tunneling magnetoresistance was modified，therefore the magnetoresistance of the sample elevated.

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退火处理对双钙钛矿型化合物LaKFeMoO₆结构、磁性和电学性质的影响
霍国燕, 张红蕊,施鹏斐,王小晴
(河北大学化学与环境科学学院,河北保定 071002)
摘要 溶胶-凝胶法合成了双钙钛矿性化合物LaKFeMoO₆，将未退火样品和退火处理的样品在结构、磁性和电学性质方面进行比较研究。实验表明，此化合物的晶体结构属于单斜晶系，空间群为P2₁/n；具有较高的磁化强度，并且随温度的升高缓慢下降；经过900ºC退火的样品磁阻效应有所增大。
关键词 双钙钛矿；LaKFeMoO₆；退火；晶体结构；磁学性质；电学性质