Introduction

The shape of the B versus H characteristic of a material reveals whether its magnetism is based upon magnetocrystalline or shape anisotropy. In either case, the ideal characteristics described in Chapter 1 were founded on the concept of spontaneous magnetization, and this theory is certainly a good approximation for single crystals; measurements in the preferred [1,0,0] direction for iron shown in Figure 1.6 confirm this. However, practical materials do not follow this theoretical ideal, as shown for comparison by the initial magnetization curves for a real sample of iron in Figure 2.1. A measurable external applied field H is required to magnetize and saturate this material, so this sample does not exhibit spontaneous magnetization. The same is true for samples of nickel, cobalt and all alloys that are used to produce permanent magnets.

While the magnetization curves for the single crystal in Figure 1.6 are dependent upon the crystallographic direction, the curve for a bulk sample of iron is not. A simple ferromagnetic material such as this is therefore isotropic with no preferred axis, and any enhanced properties in a specific direction will only be imparted to a magnet during its production process. Spontaneous magnetization still exists in the crystal lattice structure, so it must be explained how the theory of magneto crystalline anisotropy must be modified to predict the actual characteristics of permanent magnets. To do this, the model of a magnetic material must first be enhanced.