Monte Carlo simulations of rare-earth holmium ultrathin films

Motivated by recent experimental results in ultrathin helimagnetic holmium films, we have performed an extensive classical Monte Carlo simulation of films of different thickness, assuming a Hamiltonian with six interlayer exchange constants. Both magnetic structure and critical properties have been analyzed. For n>16, n being the number of spin layers in the film, a correct bulk limit is reached, while for lower n the film properties are clearly affected by the strong competition among the helical pitch and the surface effects, which involve the majority of the spin layers. In the thickness range n=9−16 three different magnetic phases emerge, with the high-temperature, disordered, paramagnetic phase and the low-temperature, long-range ordered one separated by an intriguing intermediate-temperature block phase, where outer ordered layers coexist with some inner disordered ones. The phase transition of these inner layers displays the signatures of a Kosterlitz- Thouless one. Finally, for n<=7 the film collapses once and for all to a quasicollinear order. A comparison of our Monte Carlo simulation outcomes to available experimental data is also proposed and further experimental investigations are suggested.