Exotic magnetic structures in ultrathin helimagnetic holmium films

The problem of spin arrangement in ultrathin films of materials with competing medium-range interactions has been investigated by classical Monte Carlo simulations, assuming the model pertinent to bulk holmium.We find that in addition to distorted helical configurations at low temperature, unusual blocked phases—with some inner disordered planes intercalating ordered blocks and with the inner planes undergoing a Kosterlitz-Thouless phase transition as the temperature rises—may appear at intermediate temperature. The blocked phases turn out to be much more structured than those predicted by mean-field calculations, and their contribution to the static structure factor appear substantially indistinguishable from that corresponding to helical order thus constituting a challenge for their experimental probing by conventional elastic scattering techniques.