Orientational transitions in ferromagnetic liquid crystals with bistable coupling between colloidal particles and the ma
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TICAL, NONLINEAR, AND SOFT MATTER PHYSICS
Orientational Transitions in Ferromagnetic Liquid Crystals with Bistable Coupling between Colloidal Particles and the Matrix A. N. Zakhlevnykh* and D. A. Petrov Perm State National Research University, Perm, 614990 Russia *e-mail: [email protected] Received March 2, 2016
Abstract—We study the orientational response of a ferromagnetic liquid crystal that is induced by magnetic and electric fields. A modified form of the energy of the orientational interaction between magnetic impurity particles and the liquid crystal matrix that leads to bistable coupling is considered. It is shown that apart from magnetic impurity segregation, first-order orientational transitions can be due to the bistability of the potential of the orientational coupling between the director and the magnetization. The ranges of material parameters that lead to optical bistability are determined. The possibility of first-order orientational transitions is analyzed for the optical phase difference between the ordinary and extraordinary light rays transmitted through a ferronematic cell. It is shown that an electric field applied in the given geometry considerably enhances the magneto-orientational response of the ferronematic. DOI: 10.1134/S1063776116090168
1. INTRODUCTION An important problem in the contemporary physics of liquid crystals (LCs) is the improvement of their properties by physical methods (i.e., without using chemical synthesis of new materials). One such method is the preparation of colloidal suspensions of nanoparticles in LC matrices [1]. The interest in such media is due to the fact that LCs that combine the properties of conventional (isotropic) liquids and crystals exhibit spontaneous orientational ordering and a high sensitivity to external effects. The nonlinear interaction of orientational ordering and the velocity field leads to a variety of physical effects, including the emergence of orientational instabilities of various types. If LC particles that are embedded have a size of 10– 100 nm, such nanoparticles are too small to cause substantial orientational distortions in the LC matrix, but serve as admixtures that can noticeably change the effective properties of the host medium. Suspensions of dipole (ferromagnetic or ferroelectric) particles are of special interest in this respect [1, 3–15]. Experiments show that their extremely low concentrations (on the order of 0.01 vol %) improve the orientational ordering of a composite system, shift the point of transition from the ordered to the isotropic phase, and also increase magnetic and optical anisotropies and enhance the electro/magnetooptical response. For this reason, suspensions of dipole particles in liquid crystals make it possible to improve LC properties in various technical applications.
In this study, we consider suspensions of prolate magnetic nanoparticles in a nematic liquid crystal (NLC). At high temperatures, such suspensions are isotropic. Below the clearing point (the point of transition to the nematic phase), magnetic p
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