Orientational Instability and Hysteresis Phenomena in a Ferronematic Liquid Crystal in a Magnetic Field
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TICAL, NONLINEAR, AND SOFT MATTER PHYSICS
Orientational Instability and Hysteresis Phenomena in a Ferronematic Liquid Crystal in a Magnetic Field D. V. Makarova, M. A. Utkina, and A. N. Zakhlevnykha,* a
Perm State University, Perm, 614990 Russia *e-mail: [email protected] Received December 18, 2017
Abstract—A magnetic-field-induced orientational structure in a ferronematic (FN) liquid crystal (LC) layer is studied within the continuum theory. The rotation angles of the director and the magnetization and the concentration of magnetic impurity corresponding to a supertwisted orientational structure of the suspension are calculated. It is shown that the deviation angle of the director from the direction of the external field has the hysteresis region in which the orientational structure of the FN changes stepwise from a state with a positive twist of the director to a state with a negative twist. A value of the magnetic field strength is found above which orientational bistability regions arise. It is shown that orientational instability under the rotation of the field most clearly manifests itself in FNs with strong anchoring of particles to the LC matrix. It is established that the effect of magnetic segregation responsible for the redistribution of magnetic particles in the layer leads to the expansion of the hysteresis region and to a decrease in the field at which orientational instability arises. It is shown that, in FNs with soft anchoring between magnetic and LC subsystems, there exist several response modes to a quasistatic rotation of the magnetic field. DOI: 10.1134/S1063776118060055
1. INTRODUCTION In recent years, great interest has been shown in composite media based on liquid crystals (LCs) [1, 2]. This is associated with the high sensitivity of LCs to external mechanical, electromagnetic, acoustic, and temperature fields. The presence of a spontaneous orientational order in the arrangement of LC molecules allows one to orient anisometric particles embedded into LCs. Of special interest are LC composites with ferroelectric or ferromagnetic particles [1]. Suspensions of anisometric ferromagnetic particles in which the role of the carrier medium is played by a nematic liquid crystal (NLC) are called ferronematics (FNs) [3]. In such systems, due to the anchoring of LC molecules to the surface of magnetic particles, one can achieve orientational coupling between the magnetic and the LC subsystems, so that the initial magnetic susceptibility of the suspension increases by a few orders of magnitude compared with the pure nematic [4–9]. To theoretically describe equilibrium distortions of the orientational structure of FNs, in [3], the authors proposed a continuum approach for the case of strong anchoring between ferroparticles and the LC matrix, which was later generalized to the case of soft (finite) anchoring in [5, 10]. In an LC, the orientation of the director in a magnetic field is determined by the anisotropy of diamag-
netic susceptibility (quadrupolar mechanism). In an FN, the magnetic field acts not only
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