Direct measurement of the dependence of granular giant magnetoresistance on the relative orientation of magnetic granules

Experiments have been designed to vary the relative angle between the magne tic moments of different Co granules in Cu80Co20 granular system. The momen ts of granules are mostly aligned in the same direction by field cooling to low temperature in a high magnetic field. A small field applied at an angl e relative to the cooling field rotates the moments of a portion of the gra nules that have small particle size and coercivity. It is found that the gi ant magnetoresistance (GMR) varies linearly with cos phi, where phi is the relative angle between the magnetic axes of granules. This behavior disappe ars if the sample is cooled in zero fields, or if the rotating field is too large or small, or if the measuring temperature is higher than the blockin g temperature. Our results show that the GMR in granular structures has the same angular dependence as the layered films and confirm the existing theo ries and recent microscopic models of granular GMR suggesting a crucial rol e of the relative orientations of the magnetic granules in determining the spin dependent scattering. (C) 2000 American Institute of Physics. [S0003-6 951(00)01326-7].