Preparation and characterization of the double perovskite Sr1.10.9FeMoO compound

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

Abstract The double-perovskite Sr1.10.9FeMoO ceramics have been successfully synthesized. The structural, Curie temperature and transport properties of this compound have been studied. Its structure belongs to monoclinic system with P2/n space group. The magnetic moment is high and slowly reduced with the increase of temperature, and large magnetoresistance of the sample is found under 0.5 T and 1 T field.
Keywords double-perovskite Sr1.10.9FeMoO compound; large magnetoresistance; magnetic properties; crystal structure

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 SrFeMoO[1-3] and SrFeReO[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 SrFeMoO has an ordered structure with alternating localized up spin Fe3+ (3d; t2g2g) and itinerant down spin Mo5+ (4d; t2g). The parallel magnetic moments of Fe3+, antiferromagnetically coupled with the spins of Mo5+ induce an ideal saturation magnetic moment (Ms) of 4 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 AFeMoO compounds (where A is the alkaline-earth metals). For example, it was reported that the substitution of La3+ for Sr2+ in SrFeMoO promotes a rising of the Curie temperature (T) and a decreasing of M as doping level increases[7], and Kim et al. found that the replacement of Ba2+ by a small amount of K in BaFeMoO reduces the T[8], and it was also reported that the substitution of Ba2+ and Ca2+ for part of Sr2+ also reduce the T[9]. With the La3+ or K doping in AFeMoO, carriers doping are also existence in these materials.
References on K doping at Sr site in SrFeMoO have not been reported until now, in this paper we investigate the structure, magnetic and magnetoresistance properties of the double-perovskite SrFeMoO compounds with K replaces 45% of the Sr2 +

The polycrystalline sample Sr1.10.9FeMoO has been synthesized by standard solid state reaction technique. The raw materials, SrCO, KCO, Fe and (NHMo24·4HO, 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 C for 8 h. The calcined mixture was pulverized and pressed into pellets. The pellets were sintered at 800 C for 8 h and 900C for 9 h in a stream of 4.8 % H/Ar, respectively. Phase analysis and characterization were carried out by X-ray diffraction using CuKradiation 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.

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 P2/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
=93.9856°. The diffraction peaks of (1 0 1) and (0 1 3) display that, in superlattice Sr1.10.9FeMoO structure, part of Fe3+ and Mo5+ ions orderly occupy on B and B’sites, respectively.
Fig.1 X-ray diffraction pattern of Sr1.10.9FeMoO sample

Magnetic moment measurements made as function of temperature M (T) on warming the Sr1.10.9FeMoO sample from 80 K to 300 K in a magnetic field of 0.5 Tare 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”O can be viewed as a regular arrangement of corner-sharing B’O and B”O 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’O and B”O 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 Sr1.10.9FeMoO sample is higher than 300 K. We calculated that reduction rate with temperatures of magnetic moment of the sample is 0.050 Am/( kg·K)(a) and 0.043 Am/( kg·K) (b) from 80 K to 300 K.


Fig.2 Temperature dependent magnetization of Sr1.10.9FeMoO 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, 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, lower than theoretical value, 4.90. We analyze that the reason is there may be a disorder of the regular arrangement of FeO and MoO octahedra in double-perovskite Sr1.10.9FeMoO 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.


Fig.3 Isothermal magnetization with magnetic fields at 300 K

The(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 Sr1.10.9FeMoO sample exhibits metallic behavior (d/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=[(H)(0)]/ρ(0), where(H) and(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

We have successfully synthesized and investigated the structure, magnetic and electrical transport properties of double perovskite Sr1.10.9FeMoO specimen which structure belongs to monoclinic system with P2/n space group. The magnetic moment is high and slowly reduced with the increase of temperature, and thermal magnetization indicates that magnetic transition temperature of this compound is above 300 K. The net magnetic moment per unit formula in the direction of the magnetization is 3.25 μ. The resistivities are monotonously increasing over the temperature range from 80 K to 300 K under zero, 0.5 T and 1.0 T magnetic fields. Large magnetoresistances of the sample are found under 0.5 T and 1 T field, respectively.