The effect of the preparation method and grain The effect of the preparation method and grain morphology on the physical properties of Amorphology on the physical properties of A

22FeMoOFeMoO66

(A=Sr,Ba)(A=Sr,Ba)

E.K. Hemery E.K. Hemery 1,21,2, G.V.M. Williams , G.V.M. Williams 11 and H.J. Trodahl and H.J. Trodahl 22

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

IntroductionSeveral 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 Sr2FeMoO6 (SFMO), though more recently attention has

also been afforded to other members of the class, including Ba2FeMoO6 (BFMO) and various

mixed and oxygen deficient double perovskites.

SynthesisThe double perovskites were prepared by solid reaction from stoichiometric mixes of A(NO3),

Fe2O3 and MoO3. Sintered pellets of the double perovskites AFMO were made by the

conventional method and via an intermediate step involving the formation of AFeO3-x and

AMoO4. For this step the pressed pellets were heated at 1200 oC in air for 4 h. All samples were

finally sintered at 1100 oC for 6 h in a reducing atmosphere of 5% H2 – 95% N2. 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.

AcknowledgmentsThis program is funded by the NZ Mardsen fund and the MacDiarmid InstituteCollaboration: L. Dupont and J. Crisford.

Structural measurementSEM/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 Ba2FeMoO6 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 ~104 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 Ba2FeMoO6 samples. (a) consists of small grains (low resistivity) while (b) has large grains (high resistivity).

Transport properties

Magnetic responseMagnetic 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 SrMoO4 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 Sr2FeMoO6 samples.

Figure 4. Thermopower versus temperature for Sr2FeMoO6. 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 Sr2FeMoO6 (filled circles) and Ba2FeMoO6 (open

circles).

Figure 5. Ba2FeMoO6 Hysteresis loops at 5 K; The saturation magnetization for both curves is 3.5 μB/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 Sr2FeMoO6 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