Large room-temperature spin-dependent tunneling magnetoresistance in polycrystalline Fe3O4 films

Abstract

Polycrystalline ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films have been prepared by reactive sputtering at room temperature. Transmission electron microscopy images show that the films consist of quite uniform ${\mathrm{Fe}}_3{\mathrm{O}}_4$ grains well separated by grain boundaries. It was found that the tunneling of spin-polarized electrons across the antiferromagnetic coupled grain boundaries dominates the transport properties of the films. Magnetoresistance (MR) $\text{{=[ρ(H)−ρ(0)]/ρ(0)}}$ shows linear and quadratic magnetic-field dependence in the low-field range when the field is applied parallel and perpendicular to film plane, which is similar to the behaviors observed in the epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films consisting of a large fraction of antiferromagnetic antiphase domain boundaries. At 300 K, the size of the MR reaches −7.4% under a 50-kOe magnetic field, which is a very large MR for polycrystalline ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films.