Surface induced nucleation of the helifan phase

We calculate the magnetic phases of thin films of helimagnetic materials. T he surface of the film is perpendicular to the c-axis so that, for H = 0, t he 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 sma ll, the effect of the external field is to produce small modifications in t he basal plane projections, which remain nearly in a helical pattern, while the c-axis components oscillate throughout the film, with the surface regi ons displaying stronger oscillations. By increasing the external field stre ngth, this phase evolves to a first order transition to a non-uniform fan p hase, 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 gr adual alignment of the magnetic moments near the surface region with the ex ternal field. The phase has twice the period of the helix and nucleates onl y in films where the thickness fits the double-period structure. (C) 2001 E lsevier Science B.V. All rights reserved.