Direct and inverse domain structures in ferrofluids

The work deals with the theoretical study of domain structures in a ferrofluid, filling a thin flat gap with hard non-magnetic walls, placed into magnetic field, perpendicular to the gap plane. Two experimentally observed types of these structures are considered. The first one is the direct domain structure, presenting a system of dense cylindrical-like domains in a relatively dilute surrounding ferrofluid. The second one is the inverse structure, i.e., the system of dilute cylindrical holes in the dense simply connected environment. We estimate dependencies of diameter of the domains on applied magnetic field, on the gap thickness and other parameters of the system. The diagrams, corresponding to the transition from the direct to inverse structure, are calculated.