Structural and thermal properties of orientationally ordered dipolar fluids

At high densities dipolar fluids with strong dipole moments exhibit an orientationally ordered phase. Using a density-functional theory for the corresponding Stockmayer model we study the properties of this ferromagnetic phase both in the absence of and as a function of an external field. In a needle-shaped volume the magnetization is homogeneous due to the lack of a demagnetization field. For other sample shapes in zero field inhomogeneous magnetization distributions are formed, leading to a shape-independent free energy that equals that of the needle-shaped volume. We discuss general properties of the resulting domain configurations and analyze some simple special cases in more detail. In an external field a phase transition occurs between homogeneously and inhomogeneously magnetized states, resulting in phase diagrams that depend on the shape of the sample.