The characterization of the interaction of materials with magnetic fields closely parallels the interactions with the electric field discussed in the previous section. Once again, we have two views for . Equivalently, we can view as a volume density of magnetic dipole moments. The presence of magnetic dipoles in a material is a consequence of quantum mechanics. Unlike the electric dipole model in the previous section, it is not suitable.
Many materials exhibit complex and unusual magnetic properties.
In general, a crystalline material is comprised of many regions, called domains, such that the magnetic dipole moments are uniform throughout the domain. The various domains will have different orientations, and so the bulk material will exhibit no net magnetic field. However, if an external magnetic field is applied, the domains orient themselves along the applied field, reinforcing it. The terms homogeneous and isotropic apply the magnetic materials, and in particular linear isotropic materials are characterized by scalar and . Diamagnetic materials have small negative , so but .