Interplay among helical order, surface effects, and range of interacting layers in ultrathin films

The properties of helical thin films have been thoroughly investigated by classical Monte Carlo simulations. The employed model assumes classical planar spins in a body-centered tetragonal lattice, where the helical arrangement along the film growth direction has been modeled by nearest-neighbor and next-nearest-neighbor competing interactions, the minimal requirement to get helical order. We obtain that, while the in-plane transition temperatures remain essentially unchanged with respect to the bulk ones, the helical or fan arrangement is stabilized at lower and lower temperatures when the film thickness, n, decreases; in the ordered phase, increasing the temperature, a softening of the helix pitch wave vector is also observed. Moreover, we show also that the simulation data around both transition temperatures lead us to exclude the presence of a first-order transition for all analyzed sizes. Finally, by comparing the results of the present work to those obtained for other models previously adopted in literature, we can get a deeper insight about the entwined role played by the number (range) of interlayer interactions and surface effects in noncollinear thin films.