Inhomogeneous magnetization in dipolar ferromagnetic liquids

TL;DR

This paper investigates the inhomogeneous magnetization structures in dipolar ferromagnetic liquids, revealing vortex formations, domain walls, and the effects of external fields through microscopic density functional minimization.

Contribution

It provides a detailed analysis of the equilibrium magnetization structures in dipolar liquids, including shape-dependent inhomogeneities and the impact of external magnetic fields.

Findings

Vortex structures with domain walls scale with system size.

Magnetization escapes into the third dimension near the vortex axis.

Strong external fields lead to homogeneous magnetization.

Abstract

At high densities fluids of strongly dipolar spherical particles exhibit spontaneous long-ranged orientational order. Typically, due to demagnetization effects induced by the long range of the dipolar interactions, the magnetization structure is spatially inhomogeneous and depends 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 resembling four domains separated by four domain walls whose thickness increases proportional to the system size L. There are indications that for large L the whole configuration scales with the system size. Near the axis of the mainly planar vortex structure the direction of the magnetization 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.