Weak coupling effects and re-entrant transitions in ferronematic liquid crystals

In the framework of continuum theory we study the orientational and magnetooptical properties of ferronematics (i.e., colloidal suspensions of magnetic nanoparticles in nematic liquid crystals) with soft planar coupling of a disperse phase with a liquid crystal matrix in a magnetic field. It is shown that there is a threshold energy of coupling between the dispersed phase and the liquid crystal matrix, below which (weak coupling) ferronematic has a complex magnetooptical response to an applied magnetic field. We show that at weak coupling initially a uniform ferronematic structure under the field action undergoes the sequence of re-entrant transitions “uniform phase–non-uniform phase–uniform phase–non-uniform phase”, which can be of the first or second order depending on a segregation parameter value. The tricritical value of the segregation parameter is found analytically as a function of the material parameters of a suspension. We study magnetic field action on the optical phase lag and the capacity of a ferronematic cell. Comparison of results of numerical calculations with experimental data is carried out.