Mean field theories and ferromagnetic ordering in ferrofluids

Abstract This paper addresses basic questions concerning spontaneous “ferromagnetic” order in ferrofluids. We obtain the exact equation connecting the one-particle distribution function with the pair correlation function. We study four expressions for the pair correlation function in terms of the one-particle distribution function, and only one of them results in the spontaneous orientational order. This self-consistent uncoupling is used in the density functional approach and proves to be equivalent to the mean field model. In this case an influence of dipole–dipole interaction is overestimated and so the theoretical prediction of ferromagnetic order in ferrofluids has to be considered as questionable.