in a ferromagnetic or ferrimagnetic specimen, a direction in which the work performed by an external field in magnetizing the specimen to saturation is a minimum. If no external field acts on the specimen, then each domain of the specimen is magnetized along a direction of easy magnetization.
In ferromagnetic single crystals, the directions of easy magnetization coincide with principal crystallographic axes; for example, in iron they coincide with the tetragonal axes of the type (100), in nickel with the trigonal axes (111), and in cobalt with the hexagonal axes (0001). When external stresses (compression or tension) are applied to a ferromagnetic single crystal, the direction of easy magnetization can be shifted and in a polycrystalline specimen even re-formed in a different direction. The existence of a direction of easy magnetization is a direct consequence of the magnetic anisotropy of ferromagnetic substances.