Interplay of dipolar interactions and grain-size distribution in the giantmagnetoresistance of granular metals

The giant magnetoresistance (GMR) of a granular metal containing interactin g magnetic particles with disperse sizes and shapes is studied numerically using a tight binding Hamiltonian with spin-dependent potentials. Dipolar i nteractions between the magnetic particles are assumed and the equilibrium configuration of the system is obtained by a classical Monte Carlo simulati on. The conductance of the system is calculated using the Kubo-Greenwood fo rmula and real space Green function techniques. Due to the dipolar interact ions acting between the grains the maximum GMR value is reduced and the sat uration field is increased. When the coalescence between particles is intro duced the concentration dependence of the GMR develops an optimum value clo se to the percolation threshold, where the effect of dipolar interactions i s mostly pronounced, causing serious deviations from the predictions for no ninteracting grains. Both dipolar interactions and grain size distribution are responsible for the deviations from the parabolic dependence of the GMR on the reduced magnetization at low fields. The relative importance of the se two factors is investigated. Our numerical results are compared with exp erimental findings in CoxCu1-x granular alloys.