Crystal structure, magnetic and electrical transport properties of CeKFe_1.2Mo_0.8O₆ double perovskite

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

1. INTRODUCTION
Recently, more attention was focused on the double perovskite oxides (DP) and investigated their properties. DP Sr₂FeMoO₆ is half metallic ferromagnetic and its Curie temperature is higher than room temperature (RT) ^[1]. When doping with La and Ca in the sample of A_xSr_2-xFeMoO₆ (A: La and Ca), the Curie temperature (T_C) raises in the La series and slightly decreases in the Ca ones ^[2], another report also supports that doping with La will result in an increase of T_C and reduces the degree of ordering on the Fe and Mo sites, which causes a reduction of the saturation magnetization ^[3], while doping with K will decrease T_C ^[4].
   In this paper we investigate the crystal structure, magnetic and transport of DP CeKFe_1.2Mo_0.8O₆ polycrystalline sample.

2. EXPERIMENTAL
The polycrystalline sample CeKFe_1.2Mo_0.8O₆ has been synthesized by standard solid state reaction technique. The raw materials: CeO₂, K₂CO₃ , Fe₂O₃ and (NH₄)₆Mo₇O₂₄·4H₂O, of high purity (more than 99.99%) were mixed in an agate mortar for at least 45 min. Then it was pressed into pellets under 10 Mpa pressure for 1 min. following preheating in air at 800^oC for 10 h, The calcined mixture was pulverized and pressed into pellets. The pellets were sintered at 900^oC for 16 h in a stream of 6% H₂/Ar with intermediate grindings and 1100^oC for 8 h in a stream of 7% H₂/Ar. Phase analysis and characterization were carried out by X-ray diffraction(XRD) using CuKa radiation on X'TRA model X-ray diffractometer. Temperature dependence of magnetization curves was measured by a vibrating-sample magnetometer (VSM) in field of 0.5T over the temperature range 80-500 K. Transport properties were measured by a standard four-probe DC method in the temperature range from 80 to 300 K.

3. RESULTS AND DISCUSSION
3.1 Crystal structure             
The XRD data of powdered sample were collected at room temperature and the pattern is shown in Fig. 1 which shows that the sample is single phase and no impurities were detected. The peaks diffraction could be indexed in the monoclinic system with space group P2₁/n. The lattice parameters are measured by pirum program and found to be: a=0.5555 nm, b=0.5553 nm, c=0.7890 nm and b=89.99°. The diffraction peak 101* shows that the cations of Fe and Mo in the sample occupy on site B and B' orderly.

Fig.1 XRD pattern of CeKFe_1.2Mo_0.8O₆ sample

Fig.2 M-T curve at ZFC and FC 

3.2 Magnetic properties
Magnetization measurement made as a function of temperature M (T) on warming the CeKFe_1.2Mo_0.8O₆ sample from 80K to 500K in a magnetic field of 0.5 T is shown in Fig. 2. FC curve: cooling the sample in a magnetic field of 0.5T, ZFC: cooling the sample in zero magnetic fields. The magnetic moment decreases only 0.0063 Am²Kg^-1K^-1 in the temperature range from 80 K to 300 K. It is evident to see from the M–T curve that no T_C appears in this temperature range. We have also measured magnetic moment in a magnetic field of 0.5T on warming the sample with the temperature from 300-500K, it also shows no T_C in the course (not shown). All of these indicate that the Curie temperature of CeKFe_1.2Mo_0.8O₆ sample is higher than 500 K. It also can be seen from FC and ZFC curve that FC magnetization intensity is slightly higher than that in ZFC, This indicates that it exists a little glass state in the sample, if we keep 0.5 T magnetic field in the course of cooling the sample, the glass state will be weaken and the magnetization intensity is slightly increased. It is probably because of tilting of the octahedral BO₆ with doping Ce³⁺ and K⁺.
    The isothermal magnetization M (T, H) at 300 K with magnetic field up to 0.7 T was measured and is shown in Fig.3. The magnetic moment is not saturated in magnetic field of 0.7 T. A linear extrapolation at 1/H=0 allows us to obtain the saturation magnetization intensity, 1.15 mB (300 K).

Fig.3 M-H curve at 300 K

3.3 Electrical properties
The r(T) curves registered upon warming in 0.0 T and 0.5 T magnetic fields are shown in Fig. 4. One can see from Fig. 4 that CeKFe_1.2Mo_0.8O₆ compound demonstrates semiconducting behavior under zero and 0.5 T magnetic fields over the temperature range from 80 to 300 K, and shows a significant magnetoresistance (MR) effect in 0.5 T applied magnetic field. 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. In Fig.4, it also shows the temperature dependence of the –MR in 0.5 T magnetic field. The –MR of this sample under 0.5 T magnetic field varies from 0.65 to 0.30 with increasing temperature range from 80 to 300 K. Besides, we make the resistivity data fit for various models in order to understand the electronic transport processes in the sample, and get the best model : small polaron variable range hopping model ^[5]: r /r₀T ∝ exp(T₀/T)^1/4

Fig.4 r-T curve and MR curve from 80-300 K

REFERENCES
[1] K.I. Kobayashi, T. Kimura, H. Sawada, et al. Nature, 1998,395: 677–680.
[2] C. Frontera, D. Rubí, J. Navarro, et al. Physica B: Condensed Matter, 2004,350(1-3): E285-E288
[3] A. Kahoul, A. Azizi, S. Colis, et al. J. Appl. Phys., 2008, 104: 123903 (8 pages)
[4] J. Kim, J.G. Sung, H.M. Yang, et al. Journal of Magnetism and Magnetic Materials, 2005, 290-291(2): 1009-1011.
[5] M.Jaime, M.B.Salamon, K.Pettit, et al. Appl. Phys. Lett. 1996, 68(11): 1576-1578.

双钙钛矿型化合物CeKFe_1.2Mo_0.8O₆晶体结构、磁性及电输运性质的研究
霍国燕，王小晴，张红蕊，施鹏斐
（河北大学 化学与环境科学学院，河北 保定 071002）
摘要 用固相法合成了双钙钛矿型氧化物CeKFe_1.2Mo_0.8O₆，并对它的结构、电磁性质等方面进行了研究。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, Fe和Mo 离子有序占有B and B'位，通过MT曲线可以看出，居里温度高于500K。样品的饱和磁化强度达到1.15 mB (300 K)，磁化强度随温度的升高缓慢减小。电输运性质符合小极化子变程跃迁模型，在温度为124 K，磁场为0.5 T时，磁电阻效应最高达-0.65。
关键词 磁性材料；电输运性质；晶体结构