Spontaneous magnetic order in strongly coupled ferrofluids

Using the dipolar hard-sphere fluid as a model, we study the spontaneous magnetization of ferrofluids. Mean field theory suggests magnetic order will occur in a strongly coupled ferrofluid. At fixed low density, the system phase separates at low temperature into a dilute nonmagnetic gas and a dense, magnetically ordered liquid. We focus our study on the effect of positional randomness, which is not included in conventional mean field studies. Our results show that a positionally unstructured ferrofluid exhibits magnetic order at low temperatures, provided its density is high enough. Freezing the solvent inhibits phase separation. In this case, positional randomness prevents magnetic order in a dilute frozen ferrofluid.