2 COUPLED ISING PLANES - PHASE-DIAGRAM AND INTERPLANAR FORCE

The statistical mechanics of two Ising ferromagnetic planes coupled by a local interlayer two-spin interaction have been studied by means of a variety of calculational methods, including different mean-field ap proximations, Migdal-Kadanoff-type renormalization-group techniques, s caling theory, as well as numerical Monte Carlo simulation techniques. The phase diagram has been derived as a function of the interlayer co upling strength. Furthermore, various thermodynamic variables have bee n determined, including the interlayer correlation function, which is proportional to the interplanar force. It is found that a Migdal-Kadan off renormalization with a decimation procedure which involves the int erlayer coupling in an appropriate fashion provides an accurate descri ption of the phase diagram and that a mean-field description provides a good description of the phase diagram and the interplanar force, exc ept for very low interlayer coupling strengths.