A permanent magnet should produce a high magnetic field with a low mass, and should be stable against the external influences which would demagnetize it. Permanent magnet strength depends upon the material used in its preparation.

When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization until we remove magnetizing field. The amount of magnetization must be driven back to zero by a field in the opposite direction. The amount of reverse driving field required to demagnetize it. If an alternating magnetic field is applied to the material, its magnetization will trace out. Once the magnetic domains are reoriented, it takes some energy to turn them back again. This property of ferrromagnetic materials is useful as a magnetic memory. Some compositions of ferromagnetic materials will retain an imposed magnetization indefinitely and are useful as permanent magnets.