Association phenomena in a ferromagnetic colloid

We discuss the equation of state and the static correlations in a system of spherical ferromagnetic grains suspended in a magnetically passive fluid. In the domain where dipole-dipole interactions between the grains are large (low temperature) we show that the variation of the small-angle X-ray scattering with magnetic field provides a sensitive test of the model proposed to treat this system. We discuss the condensation of the grains into linear chains at low density, high magnetic field, and low temperature. The possibility of resonances in the small-angle X-ray scattering is demonstrated and certain other consequences of chain formation are indicated.