Thickness Dependence of Giant Magnetoresistance in Granular Materials

Abstract

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.

Magnetic relaxation in the nanoscale granular alloyFe20Cu20Ag6

“…Both derived an activation law for the relaxation time of the magnetization of the form 20 Cu 20 Ag 60 granular alloy as determined by Rietveld analysis of x-ray data collected on a synchrotron radiation source of wavelength ϭ0.6920 Å. Two main phases were refined, an fcc Ag-Cu and a bcc Fe-Cu substitutional alloy, plus a minority amorphous component that accounted for ϳ1.7 wt % of the sample.…”

“…Superparamagnetism is generally observed in those materials that exhibit GMR, [12][13][14][15][16][17][18] and has been nominated as the cause of GMR in some granular systems. 20 However, present studies of the role of superparamagnetism in granular alloys have focused on simplified model systems rather than more complex, but technologically relevant, systems. For example, there have been studies of superparamagnetism in nearly monodisperse ensembles of ␣-Fe particle deposited on nonmetallic supports, such as carbon, 21,22 MgO, 23 and Faujasite-type zeolites.…”