Surface induced nucleation of the helifan phase

We calculate the magnetic phases of thin films of helimagnetic materials. The surface of the film is perpendicular to the c-axis so that, for H=0, the moments are in the basal plane. We study the effect of external fields, applied in the basal plane, and find that the field effects are controlled to a large extent by the c-axis anisotropy. If the c-axis anisotropy is small, the effect of the external field is to produce small modifications in the basal plane projections, which remain nearly in a helical pattern, while the c-axis components oscillate throughout the film, with the surface regions displaying stronger oscillations. By increasing the external field strength, this phase evolves to a first order transition to a non-uniform fan phase, with major modifications near the surfaces. If the c-axis anisotropy is strong the out of plane oscillations are inhibited, and depending on the film thickness, a surface helifan phase is nucleated, starting from the gradual alignment of the magnetic moments near the surface region with the external field. The phase has twice the period of the helix and nucleates only in films where the thickness fits the double-period structure.