XY model with weak random anisotropy in a symmetry-breaking magnetic field

We present a numerical study of the two-dimensional classical XY model with weak random anisotropy at zero temperature. Zero-field configurations obtained by ultrafast cooling, first-magnetization curves, and hysteresis loops have been calculated for different random-anisotropy-to-exchange ratios. In zero field, a pinning of vortices by the random-anisotropy field occurs. It prevents the binding then collapsing of pairs of opposite charges and thus leads to a nonferromagnetic ground state. Applying a magnetic field causes a progressive depinning of vortices that disappear in pairs until saturation. Starting from saturation and decreasing the applied field leads, in zero-field, to a magnetic state of large remanent magnetization. However, an important aftereffect is observed. It should give smaller remanence after much longer computer time. Then, the reversal of magnetization in negative fields occurs through a peculiar process that involves the formation and collapse of new kinds of topological defects (infinite strings). These linear defects are in fact the ultimate stage in the shrinking of domains oriented in the initial direction of saturation. Their collapse occurs abruptly through the creation and propagation in opposite directions, along the defect, of an unbound vortex-antivortex pair.