  On the basis of the mean spherical approximation of multicomponent dipolar
hard sphere mixtures an analytical expression is proposed for the magnetic
field dependence of the magnetization of size polydisperse ferrofluids. The
polydispersity of the particle diameter is described by the gamma distribution
function. Canonical ensemble Monte Carlo simulations have been performed in
order to test these theoretical results for the initial susceptibility and the
magnetization. The results for the magnetic properties of the polydisperse
systems turn out to be in quantitative agreement with our present simulation
data. In addition, we find good agreement between our theory and experimental
data for magnetite-based ferrofluids.
