NON-SIMPLE MAGNETIC ORDER FOR SIMPLE HAMILTONIANS

The phase diagram at T = 0 K and the spin wave spectrum is studied for magnetic models appropriate for certain layer compounds of transition metals. Heisenberg and XY exchange is considered with interaction up to third neighbours in the basal plane. We consider only the case of nearest neighbour (n.n.) ferromagnetic interaction. Noncollinear order (helical phases) can easily occur when the interaction between more distant neighbours is antiferromagnetic. The following lattices are considered: square, triangular, honeycomb, tetragonal, exagonal and rhombohedral. Under certain conditions the spin wave spectrum of the helical phases has a large density of states at low frequency due to the presence of saddle points in the spectrum or of a soft branch". Models for NiBr2 and BaCo2(AsO4)2 are considered. © 1979."