Ferromagnetism

All substances react in some degree to a magnetic field, but only iron and those substances designated ferromagnetic by virtue of their similarity to iron become strongly enough magnetized in ordinary magnetic fields to be popularly regarded as magnetic. Ferromagnetic materials are generally characterized by (a) saturation in readily obtainable magnetic fields, (b) a magnetization curve exhibiting a hysteresis loop (see Fig. 1), and (c) a temperature, known as the Curie temperature, above which ferromagnetic effects disappear.