REENTRANT DIMENSIONAL CROSSOVER IN PLANAR ISING SUPERLATTICES

The (d=2)-dimensional spin model composed of alternating strips of two different Ising magnets is revisited. Application of modern technique s results in explicit exact solution for the free energy and correlati on lengths of the continuous (field-theoretic) limit of the model. In agreement with earlier results, the specific heat generally exhibits t hree different critical features: two near the respective critical tem peratures T-c1 and T-c2 of the composing models, as well as a new supe rlattice transition at T-c1<T-c<T-c2. Further analysis shows that at a ll temperatures between T-c1 and T-c2 the correlations in the system i nclude a low-amplitude but very long-range component reflecting fluctu ations in a large-scale domain wall network. The essential features of the solution can be explained by a reentrant dimensional crossover fr om the d=2, bulk behavior within the strips, to one-dimensional critic ality in the individual strips, and finally back to the two-dimensiona l behavior on a new, superlattice level. This qualitative understandin g of the physical content of the model allows for semiquantitative des cription of the temperature dependence of spontaneous magnetization an d magnetic susceptibility, which have been previously obscure.