The effect of the size distribution on the giant magnetoresistance in magnetic granular films

In the present paper, assuming that there is a size distribution described by the log‐normal function for the spherical granules in granular films, we take respectively the critical sizes D 1 ( T ) and D 2 ( T ) as phenomenological parameters at different temperatures distinguishing single‐domain ferromagnetic granules from superparamagnetic granules and from multi‐domain granules. Accordingly, magnetic granules can be divided into three categories: superparamagnetic, single‐domain ferromagnetic and multi‐domain ferromagnetic granules. In view of various contributions to the giant magnetoresistance (GMR) effect, the three kinds of granules are separated into two categories: first ferromagnetic granules ( F 1 , consisting of single‐domain ferromagnetic granules), which play a key role in the GMR effect; and second ferromagnetic granules ( F 2 , including superparamagnetic and multi‐domain granules), which play a secondary role in the GMR effect. We combine the effective medium theory with the two‐channel conducting model to study the GMR effect of granular films in detail. As a result, our calculated results are qualitatively in agreement with experimental data, showing that the theoretical method is essentially applicable for magnetic granular composites. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)