Landau description of ferrofluid to ferronematic phase transition.

H. Pleiner¹ - E. Jarkova¹ - H. W. Muler¹ - H. R. Brand²

¹ Max-Planck-Institute for Polymer Research, POBox 3148, 55021 Mainz, Germany
² Theoret. Physik III, Universität Bayreuth, 95440 Bayreuth, Germany

Abstract
A Landau-type free energy function is presented to describe the phase transitions from an isotropic (superparamagnetic) ferrofluid to a ferromagnetic nematic liquid crystal either directly or via a superparamagnetic nematic liquid crystal. These two nematic phases are usually both called 'ferronematic', although they are distinct phases. Both show nematic ordering, but only the ferromagnetic phase shows spontaneous magnetic ordering, additionally. The interplay of nematic with magnetic order is of special interest. A phase with ferromagnetic order but no nematic order is not possible in the present model, since the former always implies the latter (but not vice versa). In the presence of a strong external magnetic field these transitions are smeared out and the different ferronematic phases become more similar to each other. Figs 2, Refs 24.