4. Origin of coercivity and magnetic hysteresis

For weak fields, hysteresis is due to the irreversible displacement of Bloch walls, whose energy varies with position. For sufficiently large fields, hysteresis is due to anisotropic forces that maintain the direction of spontaneous magnetization in a given direction, after rotation.

The hysteresis exhibited by magnetically hard materials must be sufficiently marked for them to retain significant residual magnetization after the external field, of sufficient strength to saturate the magnet completely, has been removed. Magnetic hardness is characterized by the intrinsic coercive fieldH _cM which is a measure of resistance to demagnetization, i.e. irreversible reversal of magnetization under adverse conditions. It is the minimum value of the applied field capable of causing this change in magnetization. This is one of...