GIANT MAGNETORESISTANCE IN BULK MECHANICALLY ALLOYED CO-AG

Bulk granular magnetic solids have been prepared by the high-energy mechanical alloying of fine cobalt and silver powders, with cobalt concentrations of 5, 15 and 30 at.%. The threshold for giant magnetoresistance in the parent samples was found to lie between 5 and 15 at.% Co, with a maximum effect of 12% at 4 K in the 30 at.% Co sample. The effect of the mechanical alloying process parameters, such as the milling energy, milling time and annealing, on the magnitude of the GMR in this system was also investigated. The sub-micron scale homogeneity of the samples was found to vary with the milling energy, with completely homogeneous samples produced after milling at high energy, and only 60% homogeneity levels achieved in the sample milled at a lower energy. The isotropic and anisotropic contributions to the total magnetoresistance were separated experimentally, and the linear relationship between the two (derived for an ideal superparamagnetic system) was observed experimentally.