Magnetoresistance of magnetite

The resistivity and magnetoresistance of a magnetite single crystal and Fe3O4 films of various thicknesses were measured in the temperature range 70 K < T < 300 K and in magnetic fields in the range -1 T less than or equal to mu(0)H less than or equal to 1 T. The magnetoresistance depends on both current and magnetic field direction. The anisotropic magnetoresistance is determined as the difference of the magnetoresistances in longitudinal and transverse geometry. The data were analysed within a phenomenological model above the Verwey temperature. The anisotropic magnetoresistance for currents along [100] was found to show a sign change simultaneously with that of the crystalline anisotropy constant K-1. Whereas the magnetoresistance of the single crystal saturates above the anisotropy field, the Fe3O4 films show a significant high-field magnetoresistance depending linearly on the applied field. This behaviour was attributed to carrier transport across antiphase boundaries. A simple model was proposed that is in good qualitative agreement with the data. The single crystal shows a significant decrease of the Verwey transition in magnetic fields applied along [110]; this leads to a magnetoresistance of 70% in an external field of 1 T.