Exchange spring magnet
The magnetostatic energy is the energy stored in the field generated by a material's magnetic moments. The magnet's field reaches its maximum intensity if all the magnetic moments orient in one direction; this is what occurs in a hard magnet. In order to prevent building up the magnetic field, sometimes magnetic moments tend to form loops. That way, the energy stored in the magnetic field can be constrained; this is what occurs in a soft magnet. What determines whether a magnet is hard or soft is the dominant term of its magnetic energy. For hard magnets, the anisotropy constant is relatively large, making the magnetic moments align with easy axis. The opposite case applies to soft magnets, in which the magnetostatic energy is dominant.
Classification of electromagnetic fields
A non-null field is characterised by formula_10. If formula_11, there exists an inertial reference frame for which either the electric or magnetic field vanishes. (These correspond respectively to "magnetostatic" and "electrostatic" fields.) If formula_12, there exists an inertial frame in which electric and magnetic fields are proportional.