Magnetic phase transition in ultrathin helimagnetic Ho films

We present Monte Carlo simulations of Homagnetic thin films. We consider a Heisenberg model with easy-plane single ion anisotropy and seven coupling constant, obtained by experimental neutron scattering measurements. For the analysed thicknesses (n = 9-6), increasing the temperature the films show, after the distorted helical configuration, a temperature range where the inner planes are disordered, while the surface ones appear in a block ordered state. At low temperatures, we also show that the progressive thickness reduction does not entail a sudden helical order suppression, but rather a gradual passage to a fan-like order is observed, driven by the helix distortion related to the surface effects. (C) 2009 Elsevier B.V. All rights reserved.