Phase Diagram Of Dipolar-Coupled Xy Moments On Disordered Square Lattices

The effects of dilution disorder and random-displacement disorder are analyzed for dipolar-coupled magnetic moments confined in a plane, which were originally placed on the square lattice. In order to distinguish the different phases, new order parameters are derived and parallel tempering Monte Carlo simulations are performed for a truncated dipolar Hamiltonian to obtain the phase diagrams for both types of disorder. We find that both dilution disorder and random-displacement disorder give similar phase diagrams, namely, disorder at small enough temperatures favors a so-called microvortex phase. This can be understood in terms of the flux closure present in dipolar-coupled systems.