Surface spin instabilities in the Heisenberg ferromagnet

Abstract

We examine the nature of the spin order in the surface of a semi-infinite Heisenberg magnet. When both ferromagnetic and antiferromagnetic exchange couplings are present, and the ground-state spin configuration of the bulk is ferromagnetic, we find the spin arrangement in and near the surface can be more complex in nature. A magnetic analog of surface reconstruction occurs. For the (100) surface of an fcc crystal with nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic exchange in the bulk, we present detailed studies of the surface spin arrangements. The possibility that the exchange interactions in the surface may differ from those in the bulk is taken into account. A stability diagram for the ground state is constructed through study of the excitation spectrum of surface spin waves. For a selected set of parameters, we present results of a mean-field-theory description of the dependence of the surface spin configuration on temperature and external magnetic field.