A new multicritical point in anisotropic magnets. III. Ferromagnet in both a random and a uniform field

Abstract

For pt.II see ibid., vol.14, p.3603 (1981). Uniaxially anisotropic magnetic systems with both a random and a uniform magnetic field along the easy axis exhibit rather exotic phase diagrams as function of the anisotropy (a), temperature(T), random field (H 0 ) and uniform field (H). For fixed strong anisotropies, below the tricritical temperature. the coexistence surface (H=0) bifurcates for H not=0 into a pair of symmetrical 'wings' of first-order transitions, bounded by two critical loci which meet at the tricritical point at H=0. On the other side, for fixed weak anisotropies the spin-flop phase spreads out into two symmetrical 'horns' (for H>0 and H<0) containing two tricritical lines which meet at the bicritical point at H=0. At intermediate fixed anisotropies, for large uniform fields, the upper parts of the 'horns' overlap the 'wings'. The global four-dimensional phase diagram has been explicitly constructed using mean-field theory. In the vicinity of the new multicritical point, there is a large variety of critical (random Ising-like, pure xy-like), normal and special bicritical (random Ising-like), tricritical (random Ising-like, pure xy-like), fourth-order (random Ising-like) behaviours, as well as crossovers between these behaviours. The results are applicable to the description of dilute antiferromagnets in a uniform field.