An Ising ferromagnet with an antiferromagnetic surface layer: A simple model for magnetic surface reconstruction

Abstract

Simple cubic Ising lattices are studied by Monte Carlo simulation, using a thin film geometry (usually 40 atomic layers thick), with nearest neighbour ferromagnetic exchange J in the bulk and nearest neighbour antiferromagnetic interaction J s between surface spins. Applying a technique of preferential sampling in the surface layers, we investigate the ordering for a variety of values of J s J and for various temperatures. For J s AF < J s < − 0.25 J (where J s AF ≈ − 2.01 J) ferromagnetic ordering occurs at a higher temperature than the antiferromagnetic surface ordering, while for − 0.25 J < J s no antiferromagnetic long range order is possible. For J s < J s AF the surface transition occurs at a higher temperature than the bulk transition. We also study the magnetization profile of the film and find it is hardly affected by the antiferromagnetic ordering in its surface: the system between the surface layers behaves just as a ferromagnet with unchanged interactions at the surfaces. Likewise the antiferromagnetic order in the surface is only affected very little by the underlying layers becoming ferromagnetic, and hence such a system would be a very good approximation for a strictly two-dimensional Ising antiferromagnet. Consequently the multicritical point J s J s AF , T = T cb where surface and bulk order simultaneously is interpreted as a decoupled multicritical point. Extensions to surfaces of isotropic magnets ( XY and Heisenberg model) are discussed qualitatively.