EXTRINSIC MAGNETORESISTANCE AND RESISTANCE RELAXATION IN LA0.7CA0.3MNO3 AND FE3O4 FILMS AND HETEROSTRUCTURES

The magnetoresistance and resistance relaxation after a discontinuous field change of La0.7Ca0.3MnO3 (LCMO) and Fe3O4 thin film structures w as investigated. We compare the magnetoresistance and resistance relax ation of epitaxial films, polycrystalline films, mechanically induced grain boundaries and heterostructures in current-perpendicular-to-plan e geometry. We show that the low-field magnetoresistance of various LC MO structures containing strongly scattering interfaces is significant ly 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 compare d with epitaxial films. The resistance relaxation of the LCMO heterost ructures is found to increase with increasing magnetoresistance, where as no relaxation was observed in the Fe3O4 heterostructures. We propos e that the extrinsic magnetoresistance in LCMO structures is due to th e formation of a spin-glass-like state at highly resistive interfaces.