Approach to Saturation in Inhomogeneous Magnetic Materials

Abstract

The Néel theory of the approach to saturation in inhomogeneous magnetic materials is critically reviewed. It is found that the theory is rigorously applicable when the local saturation magnetization differs only slightly from its spatial average, but that this condition is not satisfied in porous materials or in material containing inclusions of a second magnetic phase having a saturation magnetization twice that of the host material or larger. The Néel theory is extended by (i) calculating the high‐field behavior of the magnetization curve for arbitrary inhomogeneity, and (ii) calculating the magnetization curve for arbitrary field strength to a higher order of approximation than in Néel's theory. The results show that for porous materials, as well as for materials containing a second magnetic phase, the higher‐order corrections are quite significant. The higher‐order corrections to the Néel theory are found to depend quite strongly on the shape of the inclusions, being smallest for spherical inclusions and approximately six times larger for layerlike inclusions.