Spin Configurations in an Antiferromagnetic Slab

Abstract

The low-energy spin configurations of a two sublattice antiferromagnet have been examined for a sample of finite thickness. The model consists of layers of spins connected by isotropic nearest-neighbor exchange interactions. The applied field and the anisotropy field are assumed to be perpendicular to the layers; the molecular-field approximation is used to relate the orientations of the spins within a layer. The equilibrium equations and boundary conditions are found as a continuum limit of difference equations obtained by minimizing the energy with respect to the directions of the spins. It is found that a nonuniform spin configuration has a lower energy than the uniform antiferromagnetic state for applied fields above (2)−1/2Hc, where Hc is the critical field for the spin-flop transition that occurs in the usual treatment of the two sublattice model. In the new equilibrium state, the spins at the center of the slab are arranged in a spin-flopped configuration. At applied fields well below Hc, the configuration rapidly approaches the antiferromagnetic state with increasing distance away from the center of the slab. As the applied field is increased through Hc, the width of the ``flopped'' region near the center increases rapidly and the configuration approaches a uniform spin-flop state except in the regions near the surface. It is found that the surface spin-flop state1 does not occur as an equilibrium solution of the problem if all important contributions to the difference equations are retained.