title: Phase transitions in a ferrofluid at magnetic-field-induced microphase separation.
abstract: 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 nonmagnetic 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.
authors.affiliation: Department of Physics, University of Pennsylvania, Philadelphia, Pannsylvania 19104-6396, USA.
authors.forename: D
authors.initials: D
authors.lastname: Lacoste
authors.forename: T C
authors.initials: TC
authors.lastname: Lubensky