The Morin phase transition in haematite (α-Fe2O3) I. A group-theoretic model

A group-theoretic technique for simplifying spin-wave calculations in complicated magnetic structures is developed in this paper. The technique gives the free spin-wave energies exactly at the centre of the Brillouin zone and at certain other symmetry points. By simplifying the canonical transformation to magnon operators, it reduces the labour required to analyse magnon interaction effects, and brings complex crystals within the scope of the Green function method.