Field dependence of magnetoresistance in granular Ag-Co thin films

The proportionality of the giant magnetoresistance (MR) to (cos theta)(2), where theta is the angle between the local magnetization and the applied field, was tested in granular Ag-Co (20 at.%) thin films. The assumption that (cos theta)(2)=(M/Ms)(2) does not give a linear relation. To deduce the granule size distribution, magnetization data for the superparamagnetic state were fitted using a set of Langevin functions on a granule size histogram. Assuming that the scattering rate is proportional to the surface area of each granule in such a system and that the granules are spherical, we weight the average of cos theta in the size distribution by the surface area. If granules containing more than 900 Co atoms are ignored when determining the average value of cos theta: the MR varies linearly with this quantity, which is denoted as (cos theta)(2)(sur.av). We suggest that the surface roughness enhances the contribution of smaller granules to the MR.