Grain boundaries controlled magnetic and magnetotransport properties of reactively sputtered polycrystalline and epitaxial Fe3O4 films

Polycrystalline and epitaxial Fe3O4 films were fabricated by facing-target reactive sputtering on glass and 400 °C heated MgO (1 0 0) substrates. The formation of disordered and amorphous grain boundaries in the polycrystalline Fe3O4 films on glass was suppressed by substrate heating, and antiphase boundaries were found to form in the epitaxial Fe3O4 films grown on lattice matched MgO (1 0 0) substrates, which were directly verified by high-resolution transmission electron microscopy. The magnetic and magnetotransport properties of the Fe3O4 films are controlled by the type of boundaries and the antiferromagnetic coupling strength at the boundaries which are associated with the bond angle subtended by iron–oxygen–iron. The stronger the antiferromagnetic interaction across the boundaries, the slower is the saturation behaviour of magnetization. The magnetoresistance depends mainly on the gradually saturating magnetization at high fields and the antiferromagnetic coupling strength across the boundaries.