Magnetotransport properties of magnetite-hematite composite oxides.

We have studied transport properties of composite oxides composed of ferrimagnetic conductor Fe 3 O 4 (magnetite) and insulating α -Fe 2 O 3 (hematite) at room temperature. High-density composite oxides with different Fe 3 O 4 /Fe 2 O 3 ratios were successfully synthesized by melting Fe 2 O 3 rods at high temperatures in ambient atmosphere. Transport properties of Fe 3 O 4 -Fe 2 O 3 composites were measured and compared with that of Fe 3 O 4 single crystals. As the ratio of the insulating Fe 2 O 3 concentration to the conductive Fe 3 O 4 one increases, the longitudinal resistivity increases exponentially, while the Seebeck coefficient decreases linearly. The contrasting dependences of electric and thermoelectric transports on the insulating Fe 2 O 3 concentration indicate that the thermoelectric transport is less susceptible to disorder effects than electric ones. The measurement of the Hall effect shows that the anomalous Hall effect due to the ferrimagnetic Fe 3 O 4 phase is clearly observed in the composite oxides. In line with the resistivity increase with the Fe 2 O 3 concentration, the anomalous Hall resistivity increases, following the universal scaling relation established in the dirty limit. The anomalous Nernst effect due to the Fe 3 O 4 phase is also observed in the composite oxides. The anomalous Nernst coefficient decreases with increasing Fe 2 O 3 concentration, and the decrease is found to be related to the anomalous Hall effect through the Mott relation. [ABSTRACT FROM AUTHOR]