ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 2239–2244
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Magnetic and magneto-transport properties of double perovskite Ba2xSrxFeMoO6 system Vibhav Pandey a, Vivek Verma a, R.P. Aloysius a, G.L. Bhalla b, V.P.S. Awana a, H. Kishan a, R.K. Kotnala a, a b
National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India Department of Physics & Astrophysics, Delhi University, Delhi, India
a r t i c l e in fo
abstract
Article history: Received 7 November 2008 Received in revised form 23 January 2009 Available online 6 February 2009
The structural magnetic and magneto-transport properties of double perovskite system Ba2xSrxFeMoO6 (0pxp1.0) prepared in bulk polycrystalline form are reported in this paper. X-ray diffraction analysis showed that samples are single phase and the lattice constants decreases with increase in the Sr content. The degree of Fe–Mo ordering has been found decreasing in the series with an increase in the Sr content. Parent compound Ba2FeMoO6 exhibits saturation magnetic moment value of 3.54 mB/f.u. at 85 K in a magnetic field of 6000 Oe. Temperature dependence of resistivity shows metallic behavior for all the samples. The magneto-resistance (MR) of the compound with x ¼ 0.4 is higher than that of the other samples. At room temperature this system shows a saturation magnetization value of 1.73 mB/f.u. and MR value of 7.08% (1 T). The observed variations in the structural and magnetic properties are attributed to the change of chemical pressure due to the substitution of Sr in place of Ba. The effect of antisite disorder (ASD) defects on magneto-transport properties is studied in more detail. & 2009 Elsevier B.V. All rights reserved.
Keywords: Ba2FeMoO6 Double perovskite Magneto-resistance Magnetization
1. Introduction Half metallic transition metal oxides with ordered double perovskite structure of general formula A2B0 B00 O6 (A ¼ alkaline earth element; B0 and B00 are transition metal ions) are promising candidates for magneto-resistive devices owing to their remarkable low-field magneto-resistance (LFMR) characteristic at and above room temperature. The crystal structure consists of alternating B0 O6 and B00 O6 octahedra in the lattice forming a double perovskite structure. Among these double perovskite systems, the A2FeMoO6 (A ¼ Sr, Ba) shows remarkable magnetoresistive property associated with high Curie temperature, 415 1C for Sr2FeMoO6 and 330 1C for Ba2FeMoO6 [1,2]. The origin of magneto-resistance even at low magnetic fields in these materials has been attributed to spin dependent scattering of charge carriers at grain boundaries extended to inter grain-tunneling effect. This inter grain tunneling occurs due to insulating grain boundaries where carrier electrons are spin polarized [3,4]. There are various reports on the effect of substitutions of cations at ‘A’ site with different ionic radii. The variation of ‘A’ site cation will lead to changes in the properties of the system including ferromagnetic transition temperature and magnetic ordering which is having a direct impact on the ultimate magneto-resistive properties of the system [5,6]. The change in
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the properties are caused by the change in chemical pressure brought about by the substitution of cations differing in ionic radii, which in turn affect the extent of orbital overlap and exchange coupling strength through the change in bond length and bond angle of Fe–O–Mo. Also the substitution at ‘A’ site has a direct impact on antisite disorder (ASD) defects. These ASD defects affect half metallic nature, ferromagnetic transition temperature, saturation magnetic moment value and also lowfield magneto-resistance value [7,8]. For an exact estimation of various properties of these double perovskite systems, the contribution of ASD defects can not be ignored. In this paper, we have reported the effect of Sr substitution at A-site in the polycrystalline system Ba2xSrxFeMoO6 (with x ¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). The room-temperature structural, magnetic and magneto-transport properties are discussed in detail. For strontium containing samples at x ¼ 0.4 the change in magneto-resistance has been measured 7.08% at room temperature for an external magnetic field applied 1.0 T, while room-temperature magnetic moment value 1.73 mB/f.u. has been determined by vibrating sample magnetometer. A detailed study of magnetic and magneto-transport properties of ‘A’ site variation in A2FeMoO6 was also conducted by Kim et al. [5] and Habib et al. [6]. As an important result Kim et al. found a maximum roomtemperature LFMR for compound Sr0.4Ba1.6FeMoO6 but Habib et al. found maximum value for compound Sr1.2Ba0.8FeMoO6, also the MR values observed by Habib et al. are lower. The lower values of LFMR observed by Habib et al. may be due to presence of higher ASD defects present in their samples to which they did not
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account for MR values observed in their study. Kim et al. also neglected the effect of ASD defects in their study; however, their results are very noble and present efficiency of these materials as a promising candidate for spintronics and other low-field magneto-resistance applications. Kim et al. explained the maximum LFMR value observed for compound Sr0.4Ba1.6FeMoO6 is the result of competing effect of increasing Curie temperature and decreasing magnetic softness which optimize maximum LFMR for compound Sr0.4Ba1.6FeMoO6. Their results seem to be conclusive but incomplete in the sense that effect of ASD defects was completely neglected. In the present study we have tried to explain LFMR values observed in present samples taking proper consideration of ASD defects.
2. Experimental The polycrystalline samples of Ba2xSrxFeMoO6 with (0.0pxp1.0) were prepared by the standard solid-state reaction method. Powders of high purity BaCO3, SrCO3, Fe2O3 and MoO3 were mixed, ground and calcined at 900 1C in Ar atmosphere for 10 h. After grinding, the calcined powder mixture was pressed into pellets. The pellets were then sintered at 1050 1C for 10 h in a gas flow of 5% H2 and 95% Ar. Resistivity of the samples were measured at room temperature by standard four-probe technique. Structural characterization was done by means of X-ray powder diffractometer at room temperature. The saturation magnetic moment value and Curie temperature were determined by using vibrating sample magnetometer (VSM). The Curie temperature of all the samples was calculated by taking the derivative of the magnetization versus temperature plot. Morphology of the samples was examined by scanning electron microscope (SEM) in back scattered mode.
3. Results and discussion 3.1. X-ray diffraction analysis The XRD patterns of the samples are shown in Fig. 1. The absence of extra peaks confirms that the samples are single phase. The pattern has been indexed assuming a cubic lattice (Fm3m). The presence of (111) and (3 11) peaks represents the Fe–Mo ordering in the lattice. The inset in Fig. 1 clearly shows the presence of ordering peak (3 11) in Ba2FeMoO6 indicating a high level of ordering in Ba2FeMoO6. To get a clear picture on the substitution of Sr on the structural changes, we have done structural refinement of the XRD patterns on the samples using Rietveld analysis. The peak profile was fitted using a pseudo-Voigt function. The analysis shows a regular increase of ASD defects with increasing Sr doping (Table 1). It is known that for double perovskite compounds A2B0 B00 O6 the degree of B-site ordering reduces when charge difference between the B0 and B00 ions decreases. The increased antisite disorder defects in the present case can be thought of as due to the decreased charge difference between Fe and Mo ions caused by the enhanced Fe–Mo orbital hybridization. The orbital hybridization between Fe and Mo enhances due to the substitution of Sr2+ (ionic radii ¼ 1.44 A˚), a smaller ion in place of Ba2+ (ionic radii ¼ 1.61 A˚). The orbital hybridization term for Fe(t2g)–Mo(t2g) varies according to Vdpdpp1/[/dFe–OS4/dMo–OS4], where Vdpdp is hybridization term for Fe(t2g)–Mo(t2g) which account for indirect d–d coupling through oxygen p state, namely the pdd-p coupling, [9]. As the Sr content in Ba2xSrxFeMoO6 increases resulting into a decrease in the average distance between Fe–O and Mo–O ultimately leading to a certain extent increase in the hybridization between
Fig. 1. X-ray diffraction pattern for different composition of Ba2xSrxFeMoO6. Inset shows the ordering peak [3 11] with peak [2 2 2].
Fe(t2g)–Mo(t2g). This increased hybridization changes the valence state of Fe and Mo in the system. The mixed valence state of Fe and Mo in double perovskite A2FeMoO6 has been already reported [10,11]. Nguyen et al. [10] observed a valence state favoring majority of FeII–MoVI pair for double perovskite with large A-site cation (A ¼ Ba; Ba2FeMoO6) while FeIII–MoV pair is favorable for smaller A-site cation (A ¼ Sr; Sr2FeMoO6). They reported a valence distribution of 80% FeII+20% FeIII for Fe in compound Ba2FeMoO6. Similarly, Yasukawa et al. [11] also observed Fe valence state close to FeII in compound Ba2FeMoO6 while mixed valence with higher FeIII in Sr-rich double perovskite A2FeMoO6. Interestingly, Yasukawa et al. also reported higher degree of Fe–Mo ordering in Ba-rich double perovskite phase (with higher FeII–MoVI pair) which is decreased in Sr richer phases (with higher FeIII–MoV pair) as observed in present study also. Moreover, it is known fact that the ordering of Fe and Mo ions in the lattice is the main cause behind the interesting properties including half metallicity, room-temperature ferromagnetism and magneto-resistance of these compounds. Further calculations reveals that a decrease in the value of lattice constant is observed with increase in Sr content in the sample as shown in Table 1, which is obvious due to smaller ionic radius of Sr2+ compared to ionic radius of Ba2+. The lattice compensates the empty space produced by reduction of ‘‘A’’ site cation size when Ba is replaced by Sr. 3.2. Magnetic measurement Magnetic moment measurements were carried out on the samples at room temperature and at 85 K by varying magnetic field up to 0.6 T using VSM. Fig. 2 shows the magnetization versus magnetic field plot of the samples at 85 K. The magnetization loops in Fig. 2 exhibit a clear ferromagnetic nature for all the samples. The saturation magnetic moment value of pure
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Table 1 Properties of Ba2xSrxFeMoO6; (0.0pxp1.0). X:Sr content
0.0 0.2 0.4 0.6 0.8 1.0
Lattice parameter a (A˚)
8.069 8.047 8.023 8.012 7.991 7.977
ASD (%)
3 5 6 9 10 12
Saturation magnetization (mB/f.u.) 300 K 85 K
Coercivity (Oe)
Curie temperature (Tc)
300 K
85 K
(K)
Magneto-resistance (% change) 300 K
1.23 1.64 1.73 1.79 1.80 1.87
19 30 35 45 45 47
71 78 79 90 92 98
317 328 343 353 363 368
4.98 5.28 7.08 6.06 5.82 5.61
3.54 3.39 3.20 3.02 2.86 2.84
Fig. 2. Variation of magnetic moment with applied magnetic field intensity (M–H) plots for Ba2xSrxFeMoO6 at 85 K.
Fig. 3. Variation of magnetic moment with temperature for Ba2xSrxFeMoO6.
Ba2FeMoO6 at room temperature obtained is 1.23 mB/f.u. The Curie temperature of the samples increases with increasing Sr content as shown in Table 1 and Fig. 3. Due to smaller ionic radii of Sr, the substitution of Sr in place of Ba increases the chemical pressure in the lattice. As a result the orbital overlap and exchange coupling increases, resulting into the Curie temperature enhancement. The dependence of Tc on lattice parameter in these systems can be given by relation TcpVdpdpp1/[/dFe–OS4/dMo–OS4]. Since the bond distances for Fe–O and Mo–O are decreasing with increasing Sr content as observed by decreasing lattice parameters, Tc should increase according to above relation which is confirmed by experimental observations. The saturation magnetic moment value measured at 85 K for Ba2FeMoO6 is 3.54 mB/f.u. Saturation magnetic moment value decreases with increasing Sr content in the series [Table 1]. This decrease is due to increase in antisite disorder defects with increasing Sr content in the series. A different trend of saturation magnetic moment value at room temperature is observed which is increasing with increasing Sr concentration in the samples. The
increase in room-temperature saturation magnetic moment value obtained is attributed to increase in Curie temperature. As the difference between Curie temperature and measurement temperature (room temperature 25 1C) is increasing, higher saturation magnetic moment value is obtained. An increase in coercivity has been observed for different composition samples with the increase in Sr concentration as shown in Table 1. The coercivity of the sample Ba2FeMoO6 is 19 Oe at room temperature and it increases up to 47 Oe for the sample Ba1.0Sr1.0FeMoO6. A similar increase in coercivity is also observed at 85 K as shown in Table 1. This indicates that magnetic softness is decreasing with increasing Sr concentration. It is known that magneto-elastic coupling is higher in these double perovskite systems having higher Sr content. Hence a decrease in magnetic softness is expected with increasing Sr content along the series. Also the increasing ASD defects along the series leads to the formation of antiferromagnetic Fe–O–Fe bonds in the system, which then acts as pinning centers for domains and results into increase in coercivity and thus decrease of magnetic softness.
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Fig. 4. Variation of resistivity with temperature for Ba2xSrxFeMoO6.
3.3. Resistivity The resistivity versus temperature plots for different samples is shown in Fig. 4 and these show metallic behavior for all the samples. Resistivity increases with increasing Sr content. Resistivity of double perovskite samples is very sensitive to nature and number of grain boundaries and hence to synthesis conditions .SEM micrographs (Fig. 5) show that average grain size for pure Ba2FeMoO6 is larger than other Sr-containing samples. The average grain size for all Sr-containing samples is not varying much. Higher resistivity of Sr-containing samples over parent compound Ba2FeMoO6 may be due to larger grain boundaries and increased ASD defects, as ASD defects prevent sublattice hopping of charge carriers which is (hopping) the origin of electrical transport in these systems.
3.4. Magneto-resistance Negative magneto-resistance behavior has been observed for all the samples at room temperature (Fig. 6). The magn of magneto-resistance change has been calculated using the relation MR (T, H)% ¼ [r(T, 0)r(T, H)]/r(T, 0), where r(T, 0) is resistivity at zero field and temperature T, r(T, H) is resistivity at field H and temperature T. The change in of magneto-resistance obtained for Ba2FeMoO6 at room temperature is about 5%, which is comparable to reported value [2]. For all the Sr-containing samples small increase in magnitude of MR has been observed. Magneto-resistance increases rapidly for Ho0.5 T but rather moderately for H40.5 T, with increasing magnetic field. No indication of saturation in room-temperature magneto-resistance value is observed up to 1 T field although magneto-resistance response is getting slower as field increases above 0.5 T. For sample Sr0.4Ba1.6FeMoO6 the change in magneto-resistance value obtained is largest, i.e., 7.08% at 1.0 T compared to other samples. We have observed that for samples x ¼ 0.2 and 0.4 low-field
magneto-resistance response has increased. In these double perovskite polycrystalline systems magneto-resistance response originates from tunneling of spin polarized charge carriers through insulating barriers. These barriers may be grain boundaries (giving inter-granular magneto-resistance), Fe–Mo disorder defects and some domain boundaries (giving intra-granular magneto-resistance) [4,12]. Spin polarization of charge carriers plays important role in both of these tunneling magnetoresistances. With this, high magnetic softness of the material is also important as it enhances the low field response of magnetoresistance. The spin polarization in these double perovskite systems is very sensitive to ASD defects and to temperature. Spin polarization decreases rapidly as temperature approaches to Curie temperature. In the present study as the grain size is not changing drastically so grain boundary barriers cannot be solely responsible for this increase observed in magneto-resistance. With Sr doping in Ba2FeMoO6 the Curie temperature increases (above 317 K) and so room-temperature spin polarization value should also increase. This increasing spin polarization value and decreasing magnetic softness as discussed earlier, two competing effects for magnetoresistance response, optimize maximum LFMR for the sample x ¼ 0.4 i.e. for compound Sr0.4Ba1.6FeMoO6. For compounds XX0.4 although Curie temperature is sufficiently higher for a significant spin polarization value but increased amount of disorder defects lowers spin polarization value and also magnetic softness, and hence LFMR again decreases. Thus, ASD defects play a crucial role in determining LFMR response of these systems. To further check the effect of ASD defects we prepared more disordered Sr0.4Ba1.6FeMoO6 sample. Analysis showed an ASD defect 13%, saturation magnetization Ms ¼ 2.76 mB/f.u. and the Curie temperature Tc ¼ 331 K). It yielded a room-temperature magneto-resistance value equal to 2.87% only. This reduced value of MR is obviously the effect of reduced spin polarization which reduces due to ASD defects and slightly lower Curie temperature than previously synthesized sample (Tc ¼ 343 K). This observation also reveals the fact that it is the optimization of spin polarization and magnetic softness which results in increased LFMR response rather than the optimization of Curie temperature and magnetic softness as reported by Kim et al. In case of Habib et al. the low Curie temperature (Tco330 K) and higher ASD defects of their sample optimize spin polarization value for intermediate compound and yielded maximum magneto-resistance for compound Sr0.8Ba1.2FeMoO6 but with reduced value of magneto-resistance (MRo4%). In view of theory given by Garcı´a-Herna´ndez et al. [12] the observed increase of disorder defects may also be responsible for the observed enhancement of low-field magneto-resistance. According to their view, the antisite disorder defects introduce some intra-granular tunneling barriers in the systems which are in the form of antiferromagnetic Fe–O–Fe and paramagnetic Mo–O–Mo patches in the system. For low disorder defects the antiferromagnetic interactions between Fe–O–Fe weakens in the presence of surrounding ferromagnetic interactions in stoichiometric volumes and so the width of tunneling barriers of antiferromagnetic island is low and tunneling between stoichiometric Fe–O–Mo–O–Fe patches is probable as local magnetization lines up. This occurs at low fields because of the domain rotation process in stoichiometric Fe–O–Mo–O–Fe volumes. Hence for low disorder defects there may be an increase in low-field magnetoresistance response, which is expected due to presence of more tunneling barriers. As these disorder defects increases more the antiferromagnetic interaction in core of Fe–O–Fe patches dominates and charge carriers are unable to tunnel through these strong barriers, hence the low-field magneto-resistance response again start decreasing with further increase in antisite disorder defects. With this the presence of large antisite disorder defects
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Fig. 5. SEM micrographs for the Ba2xSrxFeMoO6.
reduces spin polarization of charge carriers. This lowering spin polarization, in addition to strong tunneling barriers, may also be the reason of decreasing low-field magneto-resistance response in higher Sr-containing samples. Huang et al. [13] observed coexistence of inter-granular and intra-granular tunneling magneto-resistances and the competition between two determines the magneto-transport properties of these systems. However, Sarma et al. [4] proved that major part of magneto-resistance in these double
perovskite systems come from intergrain tunneling (tunneling through grain boundaries), and not due to barrier created through ASD defects. Their observation points towards the fact that the cause of higher LFMR in compound Sr0.4Ba1.6FeMoO6 is optimization of competing effect of large spin polarization value and magnetic softness rather than due to effect of ASD barriers although the small contribution of intra-granular barriers can not be neglected in magnetoresistance of the samples.
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The optimization of inter-granular and intra-granular magnetoresistance with high spin polarization of charge carriers and magnetic softness of system gives rise to maximum value of magneto-resistance in Sr0.4Ba1.6FeMoO6 sample. It is also concluded that low-field magneto-resistance response is sensitive to antisite disorder defects which may increase or decrease low-field magneto-resistance for these systems.
Acknowledgements Authors Vibhav Pandey and Vivek Verma acknowledge CSIR, India for their research fellowships. We acknowledge Dr. Ram Kishore and Mr. Sood for their help and support in SEM analysis.
References
Fig. 6. Variation of magneto-resistance with applied magnetic field for Ba2xSrxFeMoO6.
4. Conclusions The substitution of strontium on A-site cation in series Ba2xSrxFeMoO6 increases the Curie temperature, room-temperature saturation magnetic moment and magneto-resistance.
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