Phase diagram of ultrathin ferromagnetic films with perpendicular anisotropy.

Abstract

Ultrathin ferromagnetic films with perpendicular spin anisotropy can possess an alternating up-down stripe-domain structure, with widths L\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}. Considering the two inequivalent types of stripe domains to form a single unit, this structure may be thought of as a two-dimensional smectic crystal. It is subject to a weak stripe orientation energy. With increasing temperature the domain system changes from a smectic crystal phase to an ``Ising nematic'' phase, and then to a ``tetragonal liquid'' phase. We discuss its possible phase diagrams, in (${\mathit{H}}_{\mathrm{\ensuremath{\perp}}}$, ${\mathit{H}}_{\mathrm{\ensuremath{\parallel}}}$, T) space. This sequence of phases can occur whether or not the system ultimately undergoes a spin reorientation transition to a planar phase.