Preparation and characterization of the double perovskite Sr_1.1K_0.9FeMoO₆ compound

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

1 INTRODUCTION
In recent years, double-perovskite oxide materials have been concerned by the scientists due to the discovery of magnetoresistance effect at room temperature of double perovskite compounds Sr₂FeMoO₆^[1-3] and Sr₂FeReO₆^[4]. Fe / Mo double-perovskite compounds have more obvious magnetoresistance effect at room temperature and higher Curie temperature than other known magnetoresistance materials, consequently, they have become the most likely new materials to be used as the magnetoresistance materials at room temperature^[5]. The double-perovskite Sr₂FeMoO₆ has an ordered structure with alternating localized up spin Fe³⁺ (3d⁵; t³_2ge_2g) and itinerant down spin Mo⁵⁺ (4d¹; t¹_2ge⁰_g). The parallel magnetic moments of Fe³⁺, antiferromagnetically coupled with the spins of Mo⁵⁺ induce an ideal saturation magnetic moment (Ms) of 4 m_B per formula unit (f.u.)^[6]. In order to explore the influence of doping on magnetoresistance and obtain better magnetoresistance effect at room temperature, a lot of investigations on cationic doping at Sr site have been done in double-perovskite A₂FeMoO₆ compounds (where A is the alkaline-earth metals). For example, it was reported that the substitution of La³⁺ for Sr²⁺ in Sr₂FeMoO₆ promotes a rising of the Curie temperature (T_C) and a decreasing of M_S as doping level increases^[7], and Kim et al. found that the replacement of Ba²⁺ by a small amount of K⁺ in Ba₂FeMoO₆ reduces the T_C^[8], and it was also reported that the substitution of Ba²⁺ and Ca²⁺ for part of Sr²⁺ also reduce the T_C^[9]. With the La³⁺ or K⁺ doping in A₂FeMoO₆, carriers doping are also existence in these materials.
    References on K⁺ doping at Sr site in Sr₂FeMoO₆ have not been reported until now, in this paper we investigate the structure, magnetic and magnetoresistance properties of the double-perovskite Sr₂FeMoO₆ compounds with K ⁺ replaces 45% of the Sr^{2 +}.

2. EXPERIMENTAL
The polycrystalline sample Sr_1.1K_0.9FeMoO₆ has been synthesized by standard solid state reaction technique. The raw materials, SrCO₃, K₂CO₃, Fe₂O₃ and (NH₄)₆Mo₇O₂₄· 4H₂O, of high purity (more than 99.99%) were mixed by hand in an agate mortar for at least 40 min. Then it was pressed into pellets under 10 Mpa pressure for 1 min, following preheating in air at 800 ^oC for 8 h. The calcined mixture was pulverized and pressed into pellets. The pellets were sintered at 800 ^oC for 8 h and 900^oC for 9 h in a stream of 4.8 % H₂/Ar, respectively. 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
The reaction products were obtained as black and well crystallized powders. The XRD data of sample were collected at room temperature and the patterns are shown in Fig. 1 which shows that the sample is single phase. No impurities were detected based on XRD pattern. The diffraction peaks 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=0.5883 nm, b=0.5676 nm, c=0.7886 nm and b=93.9856°. The diffraction peaks of (1 0 1) and (0 1 3) display that, in superlattice Sr_1.1K_0.9FeMoO₆ structure, part of Fe³⁺ and Mo⁵⁺ ions orderly occupy on B and B' sites, respectively.

Fig.1 X-ray diffraction pattern of Sr_1.1K_0.9FeMoO₆ sample

    Magnetic moment measurements made as function of temperature M (T) on warming the Sr_1.1K_0.9FeMoO₆ sample from 80 K to 300 K in a magnetic field of 0.5 T are shown in Fig. 2. Curve (a) in Fig. 2 shows the M–T curves with the process of cooling in magnetic field, while curve (b) in Fig. 2 in zero field. It is evident to see from the M–T curves that the magnetic moment of sample with a magnetic field in the process of cooling (a) is higher than that without a magnetic field (b), the reason seems to be as follows: The crystal structure of double-perovskite A'A''B'B''O₆ can be viewed as a regular arrangement of corner-sharing B'O₆ and B''O₆ octahedra, alternating along the three directions of the crystal, with the voluminous A' and A'' cations occupying the voids in between the octahedra. But the B'O₆ and B''O₆ octahedra will tilt to give rise to fitting space for A' and A'' cations, in the case of the mismatch between the ionic radii of A' and A'' ions is large. The tilting will leads to deflections of B' and B'' ions spin aspects, and some glass state will be produced which result in the decrease of effective magnetic moment. The consistency of B' and B'' ions spin aspects can be increased and the glass state can be reduced by an applied magnetic field on the sample in the process of cooling, consequently, the magnetic moment of sample with a magnetic field in the process of cooling is higher. It can also be seen from Fig. 2 that the magnetic moment reduces slowly with temperature warming from 80 K to 300 K and no magnetic transition temperature appears in this temperature range. This indicates that the Curie temperature of Sr_1.1K_0.9FeMoO₆ sample is higher than 300 K. We calculated that reduction rate with temperatures of magnetic moment of the sample is 0.050 Am²/( kg· K)(a) and 0.043 Am²/( kg· K) (b) from 80 K to 300 K.

Fig.2 Temperature dependent magnetization of Sr_1.1K_0.9FeMoO₆ double perovskite registered on warming in 0.5 T magnetic field

    The magnetic moment versus magnetic field at 300 K is represented in Fig. 3. The magnetic field increases from 0 T to 1 T gradually. It shows that the magnetic moment is nearly saturated in magnetic field higher than 0.7 T. The spontaneous moment M (300 K) =2.58 m_B, can be obtained by linear extrapolation high field slope to H=0.0 T in M-H curve. It also shows that M (300 K, H) changes linearly with reciprocal of the magnetic field. A linear extrapolation at 1/H=0 allows us to obtain the saturation magnetic moment^[10], M (300 K) and thus, the net magnetic moment per unit formula in the direction of the magnetization, 3.25 μ_B, lower than theoretical value, 4.90m_B. We analyze that the reason is there may be a disorder of the regular arrangement of FeO₆ and MoO₆ octahedra in double-perovskite Sr_1.1K_0.9FeMoO₆ due to the large mismatch between the ionic radii of Sr²⁺ (0.144nm) and K⁺ (0.164nm) ions, and a antiferromagnetic interaction appearing between the irregular magnetic ions Fe-Fe and Mo-Mo lowers the ferromagnetic interaction, consequently, the saturation magnetic moment is lower than the theoretical value.

    The r(T) curves registered upon warming in zero fields, 0.5 T and 1.0 T magnetic fields are shown in Fig. 4. It can be seen from Fig. 3 that Sr_1.1K_0.9FeMoO₆ sample exhibits metallic behavior (dr/dT＞0) under zero, 0.5 T and 1.0 T magnetic fields over the temperature range, from 80 K to 300 K, and shows a distinct magnetoresistance effects in 0.5 T and 1.0 T applied magnetic fields, respectively. We define MR=[r(H)- r(0)]/ρ(0), where r(H) and r(0) are the resistivity in a magnetic field and without a magnetic field, respectively. The -MR of this sample is up to 16.0 % under 0.5 T field while 20.5 % under 1.0 T field at 300 K.

Fig.4 Temperature dependent resistivity registered upon warming in zero, 0.5 T and 1.0 T magnetic fields

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