Statistical mechanics of itinerant-electron magnets

The transition metal iron is the archetypal magnetic material. However some aspects of its magnetic behaviour are still not fully understood. The conceptual difficulty is that, while unpaired electrons responsible for magnetism in an insulator are localised on the atomic sites, in a metal they have extended states. In iron these itinerant 3-D electrons may conspire to form well-defined local magnetic moments. In this limit one can treat metals such as iron in a similar fashion to insulators in the framework of the Heisenberg model. The local moments align at low temperatures giving rise to ferromagnetic behaviour and above the Curie temperature, Tc, misalign to destroy long range order. The correlation between the directions of the neighbouring moments in the paramagnetic phase remains a subject of controversy. Neutron scattering data would indicate that the magnetic moments are more correlated than would be expected in an insulator. This is generally referred to as magnetic short range order. [Continues.]