Interplay between dipole and quadrupole modes of field influence in liquid-crystalline suspensions of ferromagnetic particles

摘要

In the framework of continuum theory we study orientational transitionsinduced by electric and magnetic fields in ferronematics, i.e., inliquid-crystalline suspensions of ferromagnetic particles. We have shown thatin a certain electric field range the magnetic field can induce a sequence ofre-entrant orientational transitions in ferronematic layer: nonuniform phase--- uniform phase --- nonuniform phase. This phenomenon is caused by theinterplay between the dipole (ferromagnetic) and quadrupole (dielectric anddiamagnetic) mechanisms of the field influence on a ferronematic structure. Wehave found that these re-entrant Freedericksz transitions exhibit tricriticalbehavior, i.e., they can be of the first or the second order. The character ofthe transitions depends on a degree of redistribution of magnetic admixture inthe sample exposed to uniform magnetic field (magnetic segregation). Wedemonstrate how electric and magnetic fields can change the order oforientational transitions in ferronematics. We show that electric Freedericksztransitions in ferronematics subjected to magnetic field have no re-entrantnature. Tricritical segregation parameters for the transitions induced byelectric or magnetic fields are obtained analytically. We demonstrate there-entrant behavior of ferronematic by numerical simulations of themagnetization and optical phase lag.