  In the presence of a magnetic field applied perpendicular to a thin sample
layer, a suspension of magnetic colloidal particles (ferrofluid) can form
spatially modulated phases with a characteristic length determined by the
competition between dipolar forces and short-range forces opposing density
variations. We introduce models for thin-film ferrofluids in which
magnetization and particle density are viewed as independent variables and in
which the non-magnetic properties of the colloidal particles are described
either by a lattice-gas entropy or by the Carnahan-Starling free energy. Our
description is particularly well suited to the low-particle density regions
studied in many experiments. Within mean-field theory, we find isotropic,
hexagonal and stripe phases, separated in general by first-order phase
boundaries.
