Monte Carlo study of the two-dimensional site-diluted dipolar Ising model

By tempered Monte Carlo simulations, we study two-dimensional site-diluted dipolar Ising systems. Dipoles are randomly placed on a fraction x of all L-2 sites in a square lattice and point along a common crystalline axis. For x(c) < x <= 1, where x(c)=0.79(5), we find an antiferromagnetic phase below a temperature which vanishes as x -> x(c) from above. At lower values of x, we study (i) distributions of the spin glass (SG) overlap q, (ii) their relative mean-square deviation Delta(2)(q) and kurtosis, and (iii) xi(L)/L, where xi(L) is a SG correlation length. From their variation with temperature and system size, we find that the paramagnetic phase covers the entire T>0 range. Our results enable us to obtain an estimate of the critical exponent associated to the correlation length at T=0, 1/nu=0.35(10).