Extrinsic magnetoresistance and resistance relaxation in La_0.7Ca_0.3MnO₃ and Fe₃O₄ films and heterostructures

The magnetoresistance and resistance relaxation after a discontinuous field change of La_0.7Ca_0.3MnO₃ (LCMO) and Fe₃O₄ thin film structures was investigated. We compare the magnetoresistance and resistance relaxation of epitaxial films, polycrystalline films, mechanically induced grain boundaries and heterostructures in current-perpendicular-to-plane geometry. We show that the low-field magnetoresistance of various LCMO structures containing strongly scattering interfaces is significantly larger than the intrinsic magnetoresistance of epitaxial LCMO films. On the other hand, the corresponding structures made from magnetite (Fe₃O₄) show only a small enhancement of the magnetoresistance compared with epitaxial films. The resistance relaxation of the LCMO heterostructures is found to increase with increasing magnetoresistance, whereas no relaxation was observed in the Fe₃O₄ heterostructures. We propose that the extrinsic magnetoresistance in LCMO structures is due to the formation of a spin-glass-like state at highly resistive interfaces.