FINITE-TEMPERATURE TRANSITIONS IN 2D DIPOLAR SYSTEMS WITH UNIAXIAL ANISOTROPY

We discuss the finite temperature transitions of 2D dipoles with short range exchange (J), long range dipolar (g), and z-axis anisotropy (K) interactions. Under some conditions there can be three transitions so that at low temperatures only the z magnetization Mz is nonzero. As the temperature is increased, (1) Mx becomes nonzero. Eventually, (2) Mz becomes zero and then (3) Mx becomes zero. We discuss renormalization group, mean field, and Monte Carlo calculations for this system. Possible relevance of this to recent experimental results on ultrathin magnetic films is discussed. © 1995 The American Physical Society.