Ferromagnetisme

Ferromagnetism, sometimes called ferromagnetisme in French, is a form of magnetism in which certain materials exhibit a large and spontaneous magnetic moment. This behavior arises from the quantum mechanical exchange interaction between electron spins, which favors parallel alignment of neighboring spins and lowers the system’s energy when spins are aligned. As a result, a ferromagnetic material can develop a net magnetization even in the absence of an external magnetic field.

Below a characteristic temperature called the Curie temperature, the exchange-driven order leads to spontaneous magnetization. Above the Curie temperature, thermal agitation destroys the long-range order and the material becomes paramagnetic. In real solids, magnetization organizes into domains—regions with roughly uniform magnetic orientation. The overall magnetization depends on the domain structure and on the ability of an external field to reorient domains, often producing hysteresis, remanence, and coercivity.

Common ferromagnetic elements include iron, cobalt, and nickel, as well as many alloys and oxides such as magnetite. Ferromagnetism is described by quantum models such as the Heisenberg model, in which spins on lattice sites interact via exchange coupling J. If J is positive, parallel alignment is favored, yielding ferromagnetic order; simplified approaches include the Ising model and mean-field theory.

Applications arise from the persistent magnetization of ferromagnets, including permanent magnets for motors and generators, magnetic recording, transformers, and various sensors. Ferromagnetism is distinct from ferrimagnetism and antiferromagnetism, where opposing sublattices produce partial or complete cancellation of macroscopic magnetization.