Spin-valve behaviour of anti-ferromagnetic boundaries in ultrathin magnetite films

Magneto-resistance (MR) measurements on epitaxial Fe₃O₄ films grown on polished MgO have been performed. The measurements presented here are interpreted by a model that describes the MR behaviour as spin-polarised transport across anti-ferromagnetic (AF) interfaces. The Fe₃O₄ films consist of structural domains, separated by anti-phase boundaries where an AF coupling is present. These AF interfaces enhance the resistance of the films. Upon application of a magnetic field the AF-spins rotate towards each other and the resistance decreases. The AF interfaces are thus behaving as spin-valves. In agreement with the model, the observed magneto-resistance is negative and shows linear and quadratic field dependence up to the anisotropy field for fields applied parallel and perpendicular to the film plane respectively. Above the anisotropy field, the slopes of the two MR curves are expected to be equal, which is observed at 60 K. Above the Verwey transition, the shape of the normalised MR curves is independent of temperature. Below the Verwey transition the MR curve becomes more linear with decreasing temperature. A large difference between parallel and perpendicular MR is observed at the Verwey transition.