Ferromagnetism

The ferromagnetism is the phenomenon by which certain materials — iron, nickel, cobalt, and their alloys — acquire a spontaneous macroscopic magnetic moment below a critical temperature called the Curie temperature. This magnetization is not imposed by an external field; it arises from the quantum mechanical exchange interaction between electron spins, which favors parallel alignment over antiparallel alignment. Ferromagnetism is one of the earliest and most accessible examples of spontaneous symmetry breaking in physics: the equations governing spin interactions are rotationally symmetric, but the ground state selects a particular direction, breaking that symmetry and producing the measurable magnetic field that makes compass needles point north. The same mathematics that describes ferromagnetic phase transitions describes the Higgs mechanism, superconductivity, and even the symmetry-breaking dynamics that may govern the selection of social norms and linguistic conventions in human populations.