POSSIBLE PHASE-DIAGRAMS OF SYSTEMS WITH COMPETING ANISOTROPIES

Possible types of the phase diagrams are investigated for mixtures composed of two kinds of spin systems with orthogonal anisotropies. It is shown on the basis of Landau's second order phase transition theory that possible phase diagrams are classified into four types, which are qualitatively different from one another. Calculations by the use of the free energy obtained in a single spin approximation show that all of these four types of the phase diagrams can be realized by suitably choosing the values of the anisotropic exchange couplings and the single ion anisotropy constants.