Ferromagnet

Ferromagnetism is a form of magnetism in which a material's atomic magnetic moments tend to align parallel to each other, producing a spontaneous net magnetization even without an external magnetic field. The effect arises from quantum mechanical exchange interactions between electrons and is responsible for the strong, permanent magnetization observed in certain elements and alloys.

In ferromagnets, long-range order exists below a characteristic temperature called the Curie temperature. Below Tc, the material can be magnetized to a large extent; above Tc, thermal agitation destroys the parallel alignment and the material becomes paramagnetic. Magnetization can persist after the external field is removed, a behavior described by hysteresis with remanence and coercivity.

Magnetic moments in ferromagnets tend to form regions called domains, each with nearly uniform magnetization. The domain structure minimizes energy; applying a magnetic field can reorient domains, leading to large changes in magnetization with relatively small fields. The strength of ferromagnetism varies with material and composition; common ferromagnets include iron, nickel, and cobalt, as well as various alloys and rare-earth compounds.

Applications include permanent magnets for motors and generators, magnetic storage media, transformers, and sensors. Ferromagnets are distinguished from ferrimagnets and antiferromagnets by the net magnetization in the ordered state; among them, ferromagnets are the most strongly magnetized in zero field. The term also covers materials that exhibit ferromagnetic order only at low temperatures or under special conditions.