Ferromagnet

A ferromagnet is a material in which atomic magnetic moments spontaneously align with one another, producing a net magnetisation that persists after any applied field is removed. Iron, nickel, cobalt and various alloys are the familiar examples. The alignment is not caused by the moments' magnetic attraction, which is far too weak at the relevant temperatures; it is caused by the quantum-mechanical exchange interaction, which arises from the Pauli exclusion principle acting on overlapping electron orbitals and energetically favours parallel spins.

Heating destroys the order at a sharp temperature — the Curie temperature, 1043 K for iron — above which the material becomes paramagnetic and retains no magnetisation of its own. This is a continuous phase transition, and the magnetisation is its order parameter: it falls to zero at T_c not abruptly but as a power law, (T_c − T)^β. In three dimensions β ≈ 0.326, the same exponent that governs the liquid–vapour critical point of a fluid, which is universality at its most striking.

A bulk piece of iron is usually unmagnetised despite being a ferromagnet, because it breaks into domains — regions individually aligned but pointing in different directions, an arrangement that lowers the energy stored in the external field. Applying a field grows the favourably oriented domains at the expense of the others, and the irreversibility of that process produces the hysteresis loop on which magnetic recording depends.

Known since antiquity through lodestone; explained qualitatively by Pierre Weiss's molecular-field theory of 1907, which introduced domains and the Curie temperature. The exchange origin of the alignment was identified by Heisenberg and Dirac in 1926–1928, and the Lenz–Ising model was proposed in 1920 as the simplest possible description of the transition.