Phase diagrams of weakly anisotropic Heisenberg antiferromagnets, nonlinear excitations (solitons) and random-field effects

Abstract

An overview is presented on the phase diagrams of weakly anisotropic Heisenberg antiferromagnets, for the 3-dimensional, the quasi 2-dimensional and the quasi 1-dimensional case, and for both uniaxial and orthorhombic symmetry. The concept of effective, field-dependent anisotropy is used and explained in detail. Some typical experimental examples are discussed. In the quasi 1-d case the basic features can be explained in terms of nonlinear excitations (1-d domain walls or solitons). In the quasi 2-d case random-field effects are of importance, especially regarding the 1st order character of the spinflop transition, which is completely destroyed by the random fields even in nominally pure systems, in contrast with the 3-d case. This provides strong experimental evidence for a lower critical dimensionality d lc = 2 for the random field problem. Finally, the correspondence between the 2-d antiferromagnet in an applied field and the commensurate-incommensurate transition for a noble gas monolayer on a substrate is pointed out.