Spatial Structures of Dipolar Ferromagnetic Liquids

We have determined the magnetization structure in a ferromagnetic dipolar fluid by performing a numerical minimization of a free energy density functional which is highly nonlocal due to the long range of the dipolar interactions. In a cubic sample a vortex structure is found which consists of four domains and four thick domain walls. Depending on sample size and temperature, near the vortex line we observe an ``escape'' of the planar magnetization into the normal direction or a reduction of its magnitude inside the vortex core. We describe the temperature dependence of the structure upon approaching the Curie point. If an external field is applied, a transition to a homogeneously magnetized state takes place at a critical field strength.