Orientational bistability in ferronematic liquid crystals with negative diamagnetic anisotropy

In the framework of continuum theory we study magnetic field induced orientational transitions in a ferronematic, i.e. suspension of single-domain magnetic particles in a nematic liquid crystal. We consider the case of negative diamagnetic susceptibility anisotropy of a liquid crystal and soft planar coupling of impurity particles with a liquid crystal matrix. We found tricritical behavior of the threshold transition in a magnetic field from perturbed state into uniform planar state. This transition can be the first or second order, depending on the parameter of the magnetic phase segregation. We analytically derive the expression for the tricritical segregation parameter that determines the character of a transition. We show that ferronematic has a large magneto-optical non-linearity which is the result of the director reorientation under external field. Comparison of results of numerical calculations with experimental data has been carried out.