Magnetic granular alloy thin films are composite materials which consist of nanoscale ferromagnetic granules embedded in the immiscible nonmagnetic matrix. Since Xiao et al. and Thomas et al. simultaneously first observed giant magnetoresistance (GMR) effect in Co-Cu system in 1992, much attention has been focused on this area for understanding the GMR mechanism as well as the development for use as magnetic sensors and read-out heads.
Magnitude of the MR and the saturation field depends strongly on the size distribution of ferromagnetic granules. So the appropriate ferromagnetic volume fraction and the post treatment is an important requirement for obtaining a high performance GMR material. We investigated the effect of ferromagnetic composition and annealing process on the GMR behavior. By controlling the specific ferromagnetic volume ratio and optimizing the annealing process and controlling the distribution of ferromagnetic granules, It was possible to make more effective spin dependent scattering occur. Consequently the MR ratio is improved.
Ferromagnetic granules grew with increasing ferromagnetic volume fraction and at higher annealing temperatures. Through TEM BF images, the growth of granules was identified. The thermal fluctuation strongly obstructs the spin rotation of ferromagnetic particles when particle size is less than the superparamagnetic ciritical size. The CoFe granules in the nonmagnetic Ag matrix which are grown over superparamagnetic critical size do not behave as superparamagnetic particles any more. The theoretical size of superparamagnetic single domain of $Co_{0.5}Fe_{0.5}$ can be approximately calculated. In the reference this value and through TEM analysis, it is construed that the decrease of saturation field can be attributed to the disappearance of superparamagnetic particles which were annealed at elevated temperature.
Saturation field is decreased by shape as well as size of ferromagnetic granules. FMR (ferromagnetic resonance) experiment shows that the oblate spheroid shape of CoFe granule evolve mainly in plane like pancake shape during annealing treatment. So CoFe granules can be magnetized more easily to parallel direction, because demagnetizing field increases perpendicular to the plane of the sample and decreases parallel to the plane.
Thickness of granular thin films also influenced on the GMR behavior. As the thin film thickness decreased, so did the MR ratio. These results originated not from the decrease of Δρ, but from the increase of the sheet resistivity. The Δρ value was almost same. It is considered that the decrease of Δρ which mainly originated from the relative increase of spin flipping in the surface is negligible until the film reaches 500Å thickness.
