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RESEARCH PRODUCT

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

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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 Js between surface spins. Applying a technique of preferential sampling in the surface layers, we investigate the ordering for a variety of values of JsJ and for various temperatures. For JsAF < Js < − 0.25J (where JsAF ≈ − 2.01J) ferromagnetic ordering occurs at a higher temperature than the antiferromagnetic surface ordering, while for − 0.25J < Js no antiferromagnetic long range order is possible. For Js < JsAF 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 JsJsAF, T = Tcb 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.