INHOMOGENEOUS MAGNETIZATION IN DIPOLAR FERROMAGNETIC LIQUIDS

At high densities fluids of strongly dipolar spherical particles exhib it spontaneous long-ranged orientational order. Typically, due to dema gnetization effects induced by the long range of the dipolar interacti ons, the magnetization structure is spatially inhomogeneous and depend s on the shape of the sample. We determine this structure for a cubic sample by the free minimization of an appropriate microscopic density functional using simulated annealing. We find a vortex structure resem bling four domains separated by four domain walls whose thickness incr eases proportional to the system size L. There are indications that fo r large L the whole configuration scales with the system size. Near th e axis of the mainly planar vortex structure the direction of the magn etization escapes into the third dimension or, at higher temperatures, the absolute value of the magnetization is strongly reduced. Thus the orientational order is characterized by two point defects at the top and the bottom of the;sample, respectively. The equilibrium structure in an external field and the transition to a homogeneous magnetization for strong fields are analyzed, too.