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 relati on. To deduce the granule size distribution, magnetization data for th e superparamagnetic state were fitted using a set of Langevin function s on a granule size histogram. Assuming that the scattering rate is pr oportional to the surface area of each granule in such a system and th at the granules are spherical, we weight the average of cos theta in t he size distribution by the surface area. If granules containing more than 900 Co atoms are ignored when determining the average value of co s theta: the MR varies linearly with this quantity, which is denoted a s (cos theta)(2)(sur.av). We suggest that the surface roughness enhanc es the contribution of smaller granules to the MR.