Defect-Induced Long-Range Order and the Imry–Ma State in 2-D O(n) Models

The anisotropic distribution of the defect-induced random local field directions in 2-D systems with continuous symmetry of the order parameter (O(n) models) can induce a global anisotropy of the “easy axis” type which transfers the system to the universality class of Ising models and the long-range order arises in it at finite temperature. At the same time, statistical fluctuations of random defect fields cause the appearance of the disordered Imry-Ma phase in which the order parameter follows fluctuations of the random field. The confrontation of these two opposite trends in 2-D O(n) models with defects of the “random local field” type was investigated, and their phase diagrams were constructed.