The effect of the preparation method and grain morphology on the physical properties of A 2 FeMoO 6 (A=Sr,Ba) E.K. Hemery 1,2, G.V.M. Williams 1 and H.J. Trodahl 2 The effect of the preparation method and grain morphology on the physical properties of A 2 FeMoO 6 (A=Sr,Ba) E.K. Hemery 1,2, G.V.M. Williams 1 and H.J. Trodahl 2 1 MacDiarmid Institute for Advanced Materials and Nanotechnology, Industrial Research, Lower Hutt, New Zealand, 2 MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University, Wellington, New Zealand Introduction Several double perovskites are thought to have half metallic ground states, with the magnetic order persisting well above ambient temperature; they have consequently gained attention for their potential as a source of spin polarized electrons for spintronic applications. The most heavily studied in this context has been Sr 2 FeMoO 6 (SFMO), though more recently attention has also been afforded to other members of the class, including Ba 2 FeMoO 6 (BFMO) and various mixed and oxygen deficient double perovskites. Synthesis The double perovskites were prepared by solid reaction from stoichiometric mixes of A(NO 3 ), Fe 2 O 3 and MoO 3. Sintered pellets of the double perovskites AFMO were made by the conventional method and via an intermediate step involving the formation of AFeO 3-x and AMoO 4. For this step the pressed pellets were heated at 1200 o C in air for 4 h. All samples were finally sintered at 1100 o C for 6 h in a reducing atmosphere of 5% H 2 – 95% N 2. The barium samples needed a longer sintering time in order to obtain single-phase material. X-ray diffraction measurements were made to determine the phase purity. Acknowledgments This program is funded by the NZ Mardsen fund and the MacDiarmid Institute Collaboration: L. Dupont and J. Crisford. Structural measurement SEM/EDS measurements were performed on AFMO samples. EDS measurements showed that the samples were microscopically single phase and the atomic fractions of A, Fe, Mo, and O were 20, 10, 10 and 60 % respectively. Samples prepared without the intermediate step and left in air for more than 6 months showed the presence of ~1 μm Fe-O islands. SEM measurements revealed that Ba 2 FeMoO 6 samples sintered via different routes possess different grain morphologies: small or large grains. In terms of the electrical transport, the sample with larger grains had a resistivity that was ~10 4 times greater than the sample with smaller grains. This may be due to the conductivity being limited by inter-grain magneto-tunnelling. Figure 2. SEM pictures of two Ba 2 FeMoO 6 samples. (a) consists of small grains (low resistivity) while (b) has large grains (high resistivity). Transport properties Magnetic response Magnetic data have been taken using a SQUID Magnetometer. The black curve is the magnetic response of the sample shown in Figure 2 (b) while the dotted one represents Figure 2 (a). They have the same saturation magnetization (i.e. no Fe/Mo site disorder) but possess a different hysteresis loss and low field gradient. This shows that the grain morphology plays an important role in the magnetic response. Conclusions The resistivity strongly depends on the grain morphology and it is dominated by the grain boundaries. The charge transport from one grain to another depends on the magnetic orientation of the domains. There is no correlation between the thermopower and resistivity at room temperature. This reinforces the idea that the charge transport in this material is mainly dominated by intergrain tunnelling. The saturation magnetic moment is independent of the ceramics synthesis method but there is a change in the hysteresis loss. This suggests that the sample with small grains has either more magnetic domain wall pinning centres or the pinning potential is stronger. Although the main thrust of our research in these materials is toward their half-metallicity, it is interesting to note that at present the most advanced proposals for their exploitation is associated with a very strong low-field negative magnetoresistance. It is found in ceramic polycrystalline material, and is clearly associated with a field-sensitive tunnelling between grains, through a non-conducting SrMoO 4 film that forms on their interfaces. (a)(b) Figure 1. Partial and total density of states for the majority up spins and minority down spins 1. Figure 3. Resistivity versus temperature for Sr 2 FeMoO 6 samples. Figure 4. Thermopower versus temperature for Sr 2 FeMoO 6. By using Mott’s Law and the gradient of the linear part we obtained a Fermi energy of 0.8 eV. The inset is a plot of the thermopower at RT versus log(ρ) for Sr 2 FeMoO 6 (filled circles) and Ba 2 FeMoO 6 (open circles). Figure 5. Ba 2 FeMoO 6 Hysteresis loops at 5 K; The saturation magnetization for both curves is 3.5 μ B /f.u. obtained below 1 T. The magnitude and gradient of the thermopower has a typical metallic behaviour. One can see from the inset of Figure 4 that the thermopower is not correlated with the resistance; this shows that the thermopower is an intra-granular effect compared with the electrical resistivity which is determined by the inter-grain region. Figure 3 shows the temperature variation of the resistivity, ρ, for two Sr 2 FeMoO 6 samples. Curve (a) shows a semiconductor-like behaviour while (b) is neither metallic-like or semiconductor- like. Metallic-like behaviour has been reported by some researchers. The different temperature dependences of the resistivity may be due to different grain morphologies and different distributions of magneto-tunnel junctions where the resistivity is limited by the grain boundaries. 1 Kobayashi et al (1998), Nature 395: 677