In ferromagnetic materials, for a given value of H, B is not unique but depends on previous history of the sample. This phenomenon is called hysterisis.
Retentivity and coercivity
The value of B at H = 0 is called retentivity or remanence. The value of H at c is called coercivity.
Hysteresis loss is due to the reversal of magnetization of transformer core whenever it is subjected to alternating nature of magnetizing force .Whenever the core is subjected to an alternating magnetic field, the domain present in the material will change their orientation after every half cycle. The power consumed by the magnetic domains for changing the orientation after every half cycle is called Hysteresis loss.
Selection of magnetic materials for a specific purpose
Permanent magnets: Desired properties: 1. High retentivity for increasing strength of the magnet 2. High coercivity for sustaining magnetisation for a longer time. Example: steel, cobalt steel etc. Electromagnets: Desired properties: 1. High permeability (or susceptibility) to increase the magnetic field strength in the core. 2. Low retentivity to quickly kill the magnetic effect on the removal of excitation current. Example: soft iron Transformer cores and Telephone diaphragms: Materials goes through continuous complete cycles of magnetisation continuously. Desired properties: 1. Low hysteresis loss to reduce loss of energy. 2. High permeability (or susceptibility) to obtain large flux density 3. High resistivity to reduce eddy-current losses. Example: Soft iron, permalloys, etc