We combine effective medium theory(EMT) with the two-channel conducting model to study the magnetic granular concentration depen-dence of a giant magnetoresistance (GMR) in magnetic granular composites.The composite is composed of small magnetic granules (such as Co) embeddedin an immiscible nonmagnetic metallic matrix (such as Ag). We consider theinfluence of granule size and concentration respectively. Firstly,granules have adistribution in size,resulting in superparamagnetic (SPM) granuries and ferro-magnetic 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.
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