Effects of Magnetic Interactions in Superparamagnetic Granular Films

Granular magnetic systems are formed by magnetic grains whose size is of the order of a few nanometers, embedded in a nonmagnetic (insulating or metallic) matrix. These ultrafine particle systems present size, shape, and anisotropy distributions, besides randomly orientated easy directions. Magnetic interactions always exist, being stronger or weaker according to the volume concentration and the matrix type. These systems have shown interesting magnetotransport properties, such as giant magnetoresistance and giant Hall effect. Owing to the inherent complexity of the nanostructure, the magnetization can be analytically calculated only in two limiting cases: when T = 0 (Stoner‐Wohlfarth model) or for high temperatures (Langevin model). The Langevin model presents very good results when applied at temperatures higher than the mean blocking temperature hTBi of the system. However, this adequacy can be just apparent: the obtained structural parameters are very different from the real ones, as we show in this w...