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 .
In paramagnetic materials, is positive, so . An extreme form of paramagnetism is ferromagnetism . Ferromagnetic materials, such as iron, have a very large, positive , so . A typical value is . Additionally, if we, say, increase then decrease , the domains are not able to realign themselves quickly enough to keep up with the time-varying .