MAGNETIC SIZE EFFECTS IN ULTRATHIN LAYERS

We study N-layer samples (N less-than-or-equal-to 10) for the Heisenbe rg model, with ferro- and antiferromagnetic exchange couplings, using a modified version of the Onsager reaction field approximation. The pr esent scheme includes short-range spin-spin correlations, and allows f or layer-dependent order parameters when free surface boundary conditi ons are imposed. The limits N = 1 (two dimensions) and N --> infinity (three dimensions) can be solved analytically, while systems with seve ral layers have to be numerically calculated. We found no indication o f a phase transition at finite temperature up to the sizes investigate d (N = 10), the layered systems behaving essentially as two-dimensiona l. A phase transition is only obtained for the three-dimensional limit .