We combine effective medium theory(EMT) with the two-channel conducting model to study the magnetic granular concentration dependence of a giant magnetoresistance(GMR) in magnetic granular composites.The composite is composed of small magnetic granules (such as Co) embedded in an immiscible nonmagnetic metallic matrix (such as Ag).We consider the influence of granule size and concentration respectively.Firstly,granules have a distribution in size,resulting in superparamagnetic(SPM) granules and ferromagnetic granules which have different contributions to conductance.Secondly,the effective surface scattering is changed with the changes of concentration.The calculated result of GMR is in agreement with theexperimental data.
