Phase transitions in systems of magnetic dipoles on a square lattice with quenched disorder

We study by Monte Carlo simulations the effect of quenched orientational disorder in systems of interacting classical dipoles on a square lattice. Each dipole can lie along any of the two perpendicular axes that form an angle psi with the principal axes of the lattice. We choose psi at random and without bias from the interval [-Delta, Delta] for each site of the lattice. For 0 <= Delta less than or similar to pi/4 we find a thermally driven second order transition between a paramagnetic and a dipolar antiferromagnetic order phase and critical exponents that change continuously with Lambda. Near the case of maximum disorder Lambda approximate to/4 we still find a second order transition at a finite temperature T-c but our results point to weak instead of strong long-ranged dipolar order for temperatures below T-c. (C) 2009 Elsevier B.V. All rights reserved.