Origin of giant magnetoresistance effect in granular thin films

Abstract

Though the demagnetization process of granular Co-Ag films became steep by adding permalloy layer, slow saturation in the magnetoresistance (MR) curves was maintained. In addition, the MR characteristics of granular Fe-Ag films prepared under a magnetic field, in which strong magnetic anisotropy was induced, were isotropic. The disagreement between the MR and magnetic characteristics implies that ferromagnetic granules are not responsible for the giant MR (GMR) effect. The MR ratio of the granular Fe-Ag films considerably increased at thicknesses less than 20 nm. In such ultrathin films, the features of the MR curves corresponded well with those of the magnetization curves with slow saturation and no hysteresis. These results suggest that the GMR effect in the granular systems is attributable to superparamagnetism.