Ferromagnetism

Ferromagnetism is a form of magnetic order in which the magnetic moments of atoms in a material tend to align parallel to each other, resulting in a spontaneous net magnetization even in the absence of an external magnetic field. This long-range order typically occurs only below a material-specific Curie temperature.

The origin lies in the electronic exchange interaction, a quantum mechanical effect that makes parallel alignment energetically favorable for certain partially filled d- or f-electron shells. In many ferromagnets, exchange coupling competes with thermal agitation and with magnetostatic energy, enabling domains of uniform orientation to form and reduce stray fields.

Above the Curie temperature, thermal fluctuations disrupt the alignment and the material becomes paramagnetic, with magnetization proportional to the applied field (Curie-Weiss behavior at high temperature). Theoretical models include the quantum Heisenberg model and the Weiss mean-field theory that approximates the internal molecular field.

Ferromagnetic materials commonly display magnetic domains—regions with uniform magnetization separated by domain walls. The process of magnetization under an external field shows hysteresis, with a characteristic coercive field and saturation magnetization. Magnetocrystalline anisotropy and shape anisotropy influence domain structure and coercivity.

Common ferromagnets include iron, cobalt, nickel and their alloys, as well as certain rare-earth compounds. Applications rely on their strong and persistent magnetization, including permanent magnets, transformer cores, read/write heads, and magnetic memory technologies.