Huo Guoyan, Wang Xiaoqing, Zhang Hongrui, ShiPengfei
(School of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)
AbstractThe crystal structure, magnetic and electrical transport properties of CeKFe1.2Mo0.8 double perovskite have been investigated. The structure of this double perovskite compound is assigned tomonoclinic system with space group P2/n and structural analysis suggests that cationic Fe and Mo alternately occupy B- and B’- sites. Thermal magnetization demonstrates that the magnetic transition temperature is higher than 500 K. Isothermal magnetization shows that the saturation magnetization 1.15B (300 K). The magnization intensity decreases slowly with temperature increasing, The electrical transport process can be described by small polaron variable range hopping model, Large magnetoresistance, -0.65, can be observed under low magnetic field of 0.5 T with the temperature of 124 K.
Recently, more attention was focused on the double perovskite oxides (DP) and investigated their properties. DP SrFeMoO is half metallic ferromagnetic and its Curie temperature is higher than room temperature (RT) . When doping with La and Ca in the sample of ASr2-xFeMoO (A: La and Ca), the Curie temperature (T) raises in the La series and slightly decreases in the Ca ones , another report also supports that doping with La will result in an increase of T andreduces the degree of ordering on the Fe and Mo sites, which causes a reduction of the saturation magnetization, while doping with K will decrease T
In this paper we investigate the crystal structure, magnetic and transport of DP CeKFe1.2Mo0.8 polycrystalline sample.
The polycrystalline sample CeKFe1.2Mo0.8 has been synthesized by standard solid state reaction technique. The raw materials: CeO, KCO , Fe and (NHMo24·4HO, 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 800C for 10 h, The calcined mixture was pulverized and pressed into pellets. The pellets were sintered at 900C for 16 h in a stream of 6% H/Ar with intermediate grindings and 1100C for 8 h in a stream of 7% H/Ar. Phase analysis and characterization were carried out by X-ray diffraction(XRD) using CuKradiation 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. 1which 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 P2/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=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.1XRD pattern of CeKFe1.2Mo0.8 sample
Fig.2 M-T curve at ZFC and FC
Magnetization measurement made as a function of temperature M (T) on warming the CeKFe1.2Mo0.8 sample from 80K to 500K in a magnetic field of 0.5 T is shown in Fig. 2. FC curve: cooling the samplein amagnetic field of 0.5T, ZFC: cooling the sample in zero magnetic fields. The magnetic moment decreases only 0.0063 AmKg-1-1 in the temperature range from 80 K to 300 K. It is evident to see from the M–T curve that no T 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 in the course (not shown). All of these indicate that the Curie temperature of CeKFe1.2Mo0.8 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 oftilting of the octahedral BO 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 magnetizationintensity, 1.15 B (300 K).
Fig.3 M-H curve at 300 K
3.3 Electrical properties
The(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. 4that CeKFe1.2Mo0.8 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= [(H) -(0)]/(0), where(H) and(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 ∝ exp(T/T)1/4
Fig.4-T curve and MR curve from 80-300 K
We have investigated the structure, magnetoresistance, magnetic and electrical transport properties of double perovskite CeKFe1.2Mo0.8. The X-ray diffraction pattern shows that the structure of the sample is monoclinic system with space group P2/n and cations of Fe and Mo orderly occupy on B and B’sites. Thermal magnetization indicates that magnetic transition temperature is above 500 K. The saturation magnetization is 1.15B. The magnetoresistance measured shows large values up to 0.65 under a 0.5 T magnetic field and varies with temperature from 80 to 300 K. The electrical transport process is fitted for small polaron variable range hopping model.
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（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CeKFe1.2Mo0.8，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P2/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1.15B (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。