Magnetic properties of granular ferromagnetic materials

Granular ferromagnetic metals are usually regarded as systems with weakly interacting superparamagnetic particles whose magnetization is described by a Langevin model. It is shown that this approach is inadmissible. A correct description of the magnetic properties of granular ferromagnetic metals requires that the magnetic anisotropy of the granules, the spread in their orientations, and the variety of their shapes be taken into account. A model with magnetically anisotropic granules, as opposed to the Langevin model, predicts a weak temperature dependence for the magnetization of granular ferromagnetic metals that is in agreement with experiment and provides averaged information on the grain shapes. The glassy nature of the magnetic state of these systems is demonstrated. The magnetic correlations owing to the dipole interaction of the magnetic moments of the granules are examined.