Stripe-tetragonal first order phase transition in ultra-thin magnetic films

We analyze the nature of the phase transition from a smectic stripe phase to a tetragonal phase predicted in analytic studies by Abanov et al. (Phys. Rev. B 51, 1023 (1995)) and observed in experiments on ultra-thin magnetic films by Vaterlaus et al.(Phys. Rev. Lett. 84, 2247 (2000)). At variance with existent numerical evidence, for the first time we show results of Monte Carlo simulations on a two-dimensional model with competing exchange and dipolar interactions showing strong evidence that the transition is a weak first order one, in agreement with the theoretical predictions of Abanov et al. Besides the numerical evidence, we give further support to the first order nature of the transition analyzing a continuum version of the model and showing that it belongs to a large family of systems, or universality class, in which a first order transition driven by fluctuations is expected on quite general grounds.