Quantum theory of easy plane antiferromagnets in a magnetic field

The spin excitations of easy plane antiferromagnets with Dzyaloshinskii interactions in an external magnetic field of arbitrary direction are considered using the solution of the self-consistent quantum problem. The one-ion magnetization in the magnetic field is shown to be independent of the field only when S = 1/2; but when S = 1 the quantum equation for the spin configurations differs from the quasiclassical one. A theory of the interaction of different spin excitation branches is given without limitations on the parameters involved in the problem.