Thickness Dependence of Giant Magnetoresistance in Granular Materials

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Granular Co–Ag films show a steep low-field magnetization process when they are covered by a permalloy layer, but their magnetoresistance (MR) curves show slow saturation. In granular Fe–Ag films prepared under a magnetic field, moreover, strong magnetic anisotropy is induced in the magnetization curves, while the MR curves are isotropic. The disagreement between the MR and magnetization curves suggests that ferromagnetic granules are not responsible for the giant MR (GMR) effect. The MR ratio of the granular Fe–Ag film is much enhanced when the thickness of the film is less than 20 nm. In such ultra-thin films, the MR and magnetization curves show slow saturation and no hysteresis. These results indicate that superparamagnetism causes the GMR effect in granular systems.