Phase diagram of ultrathin ferromagnetic films with perpendicular anisotropy

Ultrathin ferromagnetic films with perpendicular spin anisotropy can possess an alternating up-down stripe-domain structure, with widths L±δ. 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 (${\mathit{H}}_{\mathrm{\perp }}$, ${\mathit{H}}_{\mathrm{\parallel }}$, T) space. This sequence of phases can occur whether or not the system ultimately undergoes a spin reorientation transition to a planar phase.