Effects of random fields on bicritical phase diagrams in two and three dimensions

Stimulated by the pioneering work of Michael Fisher and collaborators on bicritical phase diagrams in pure systems, we consider the corresponding behavior in systems with uniaxial random fields. We discuss experiments in the two- and three-dimensional n = 3 systems Rb2Mn0.7Mg0.3F4 and Mn0.75Zn0.25F2, respectively. We also report a new theory for the 2D n = 3 system, which predicts a novel phase boundary geometry. In both two and three dimensions the Ising component is dominated by metastability effects. However, the XY component shows a reversible transition to long range order. Experiments in the bicritical region in Mn0.75Zn0.25F2 are inconclusive. However, the theory describes the measured XY phase boundary in Rb2Mn0.7Mg0.3F4 quite well.