SPATIAL STRUCTURES OF DIPOLAR FERROMAGNETIC LIQUIDS

We have determined the magnetization structure in a ferromagnetic dipo lar fluid by performing a numerical minimization of a free energy dens ity functional which is highly nonlocal due to the long range of the d ipolar interactions. In a cubic sample a vortex structure is found whi ch consists of four domains and four thick domain walls. Depending on sample size and temperature, near the vortex line we observe an ''esca pe'' of the planar magnetization into the normal direction or a reduct ion of its magnitude inside the vortex core. We describe the temperatu re 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.