Orientational transitions in antiferromagnetic liquid crystals

The orientational phases in an antiferromagnetic liquid crystal (ferronematic) based on the nematic liquid crystal with the negative anisotropy of diamagnetic susceptibility are studied in the framework of the continuum theory. The ferronematic was assumed to be compensated; i.e., in zero field, impurity ferroparticles with the magnetic moments directed parallel and antiparallel to the director are equiprobably distributed in it. It is established that under the action of a magnetic field the ferronematic undergoes orientational transitions compensated (antiferromagnetic) phase–non-uniform phase–saturation (ferrimagnetic) phase. The analytical expressions for threshold fields of the transitions as functions of material parameters are obtained. It is shown that with increasing magnetic impurity segregation parameter, the threshold fields of the transitions significantly decrease. The bifurcation diagram of the ferronematic orientational phases is built in terms of the energy of anchoring of magnetic particles with the liquid-crystal matrix and magnetic field. It is established that the Freedericksz transition is the second-order phase transition, while the transition to the saturation state can be second- or first-order. In the latter case, the suspension exhibits orientational bistability. The orientational and magnetooptical properties of the ferronematic in different applied magnetic fields are studied.     

The structural instabilities in ferronematic based on liquid crystal with negative diamagnetic susceptibility anisotropy

The studied ferronematic is a nematic liquid crystal (ZLI1695) of low negative anisotropy of the diamagnetic susceptibility (χ_a<0) doped with the magnetic particles Fe₃O₄. Structural instabilities are interpreted within Burylov and Raikher's theory. The high magnetic fields were oriented perpendicular (Freedericksz transition) or parallel to the initial director. Using capacitance measurements the Freedericksz threshold magnetic field of the ferronematic B_FN, and the critical magnetic field B_max, at which the initial parallel orientation between the director and the magnetic moment of magnetic particles breaks down, have been determined. The values of these quantities have been used to estimate the surface density of the anchoring energy W of liquid crystal molecules on the surface of the magnetic particles. The obtained values indicate a soft anchoring of the liquid crystal on the magnetic particles with a preferred parallel orientation of the magnetic moment of magnetic particles and the director.     

Orientational transitions in ferromagnetic liquid crystals with bistable coupling between colloidal particles and the matrix

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.     

Orientational Instability and Hysteresis Phenomena in a Ferronematic Liquid Crystal in a Magnetic Field

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.     

Weak Coupling Effect on the Magnetic Freedericksz Transition in a Ferronematic Liquid Crystal

Physics of the Solid State - The magnetic Freedericksz transition in a ferronematic representing a diluted suspension of magnetic nanoparticles in an N-(4-methoxybenzylidene)-4-butylaniline (MBBA)...     

Magnetic properties of ferrocholesterics with soft particle anchoring

The influence of the external magnetic field on the orientational structure and magnetic properties of the ferrocholesteric is analyzed. A soft homeotropic coupling between the magnetic particles and the cholesteric molecules is assumed. The diamagnetic anisotropy of the matrix is chosen to be positive. In this case, the dipolar and quadrupolar mechanisms of orientational interaction with the external field compete with each other. The field being applied normal to the helix. Using the continuum theory, the occurrence of magnetic-field-induced ferrocholesteric–ferronematic transition is studied. The transition field as a function of the material parameters of a ferrocholesteric is found. It is shown that rising the field strength in the ferronematic phase leads to a change in the coupling between the particles and the director from homeotropic to planar one. A study on the structure of the domain walls in ferronematic phase is undertaken.     

Investigation of Nematic Liquid Crystals Doped with Spherical Multiferroic Nanoparticles in the Presence of a Magnetic Field

Recently, multiferroic nanoparticles (MFNPs) have attracted significant attention as dopant in the nematic liquid crystal (NLC) medium. Here, behavior of the NLC molecules and MFNPs in the presence of a magnetic field is theoretically investigated in the framework of continuum theory, based on our knowledge. The electric field of the MFNPs is taken into account in the calculations. Freedericksz transition (FT) threshold field in the presence of spherical nanoparticles is determined. Dependence of the FT threshold to the diameter and volume fraction of the nanoparticles was studied. Also, the NLC director and nanoparticle reorientations for the magnetic field larger than the FT threshold field are studied. It was shown that the FT threshold of NLC is decreased by doping of spherical MFNPs. Migration of the nanoparticles to the central layers of the cell (segregation effect) is investigated.     

Magnetic field-induced orientational phases of ferronematics in shear flow

We examine the effect of shear flow on the orientational phase transitions induced by a magnetic field in ferronematic liquid crystals. Continuum approach based on the generalized Leslie–Ericksen theory is used to describe the dynamics of ferronematic liquid crystals. We consider three orientations of the magnetic field in a plane of shear flow. Stationary solutions for the director and the magnetization are obtained as functions of the magnetic field strength for different values of material parameters. Our results show that shear flow can lead to the shift of the field thresholds or to a “smoothing” of the magnetic field-induced transitions in ferronematics. In the limiting case of pure nematic liquid crystals, we revealed threshold effects, which are unstipulated by the orientational elasticity of a liquid crystal, in contrast to the conventional Fréedericksz transition.     

Tricritical phenomena in ferronematic liquid crystals

Orientational transitions induced by an external magnetic field in a ferronematic (a suspension of single-domain magnetic particles in a nematic liquid crystal) are considered in the framework of the continuum theory. The surface potential of the interaction between the ferronematic and the bounding plates is used taking into account the fourth-order anisotropy (the modified Rapini potential). It is shown that the ferronematic can be in one of three phases that correspond to three types of orientational ordering: homogeneous ordering, inhomogeneous ordering, and saturation state. The influence of the segregation effect of magnetic particles on the nature of orientational transitions of the ferronematic in an external field is studied. It is established that the transitions between these phases can be of the first and second order depending on the values of the anchoring parameters and the segregation parameter. Tricritical values of the parameter of anchoring anisotropy and the segregation parameter are determined.     

Spatial distortions of the orientational structure of a ferronematic in the presence of external fields

Spatial distortions of the director field and magnetization of a ferronematic (suspension of magnetic nanoparticles in a nematic liquid crystal) that are induced by simultaneous action of electric and magnetic fields are studied with allowance for the flexoelectric polarization of the liquid-crystalline matrix. Soft coupling of liquid crystal and magnetic particles and layer boundaries is considered. The dependence of the phase lag of the transmitted light on the external magnetic field is analyzed.     

Equilibrium structures of planar nematic and cholesteric films in electric fields

Liquid crystal layers subjected to an electric or a magnetic field can have several types of instabilities. This paper reviews recent theoretical studies concerning equilibrium structures of planar layers.

Using the Oseen-Frank elasticity theory, the Freedericksz transition and the transition to static periodic domains of planar nematic and cholesteric films are reconsidered. A perturbation treatment of nonlinear torque balance equations for the director is suitable to derive amplitude equations for the film distortions under the action of a field and to predict the topological features of phase diagrams for equilibrium states. The competition between the Freedericksz effect and the formation of periodic distortions is studied varying material and geometrical parameters. Some results are useful to optimize cholesteric mixtures for application in display devices in such a manner, that the occurrence of periodic domains is avoided.     

Orientational transitions in a ferronematic layer with bistable anchoring at the boundary

A possibility of the first-order transition, as well as reentrant transitions, induced by an external magnetic field between the homeotropic phase and the hybrid homeotropically planar phase in a ferronematic liquid crystal (ferronematic) with bistable anchoring at the layer boundary is demonstrated in the framework of a continuum theory. The critical values of the material parameters of the ferronematic, the anchoring energy, the thickness of the layer, and the magnetic field strength, for which this transition is possible, are determined. The cases of positive and negative diamagnetic anisotropy of the ferronematic are considered.     

Ferronematics with soft particle anchoring in magnetic and laser fields

The influence of a magnetic field and an optical laser field on the orientational behavior of a ferronematic with positive magnetic and dielectric anisotropy is investigated. A slow transition from the homeotropic to the distorted alignment of the nematic director is found when the field strengths exceed certain threshold values. A correlation between the threshold values of the two fields is obtained and a phase diagram for this transition is determined. It is also found that in the presence of the two fields the distorted arrangement is more stable than the homeotropic one (its free energy per unit area is smaller).     

Study of light-induced reorientation of the nematic liquid crystal director by birefringence dynamics

It is proposed to apply the birefringence method to measure the threshold of the light-induced Freedericksz transition and the nonlinearity enhancement factor of nematic liquid crystals by director relaxation dynamics in the light field. A birefringence change is recorded by interference of ordinary and extraordinary components of the light beamaffecting the liquid crystal director orientation. The Freedericksz transition thresholds and nonlinearity enhancement factors are measured for the samples doped with comb-shaped polymers with various degrees of polymerization. The determined thresholds are in agreement with the results obtained by the aberrational self-action method.     

Fluctuations in liquid crystals in the presence of a flexoelectric effect

We examine director fluctuations in nematic liquid crystals in an electric field in the presence of a flexoelectric effect. For the planar and homeotropic orientations we calculate the correlation functions and analyze these functions near the Freedericksz transition and the flexoelectric instability threshold. For both geometries we calculate the angular dependence of the intensity of light scattered by director fluctuations and examine its behavior at electric field strengths close to critical. Zh. éksp. Teor. Fiz. 112, 1675–1693 (November 1997)     

The effect of magnetic fields and boundary conditions on the Couette flow of nematics

The paper discusses the theory of Couette flow of a nematic liquid crystal. The apparent viscosity, orientation and velocity profiles are computed forp-azoxyanisole as functions of shear rate and magnetic field for symmetric and asymmetric molecular alignments at the boundaries and for different relative radii of the cylinders. For symmetric homeotropic boundary condition an azimuthal field exhibits a threshold analogous to a Freedericksz transition. An expression is also derived for the Freedericksz threshold in the hydrostatic case.     

Reentrant phases in compensated ferrocholesterics

Influence of magnetic field on orientation and magnetic properties of a compensated ferrocholesteric, a suspension of needle-like ferromagnetic particles in a cholesteric liquid crystal, was studied theoretically. A phase transition from a ferrocholesteric to a ferronematic state in a magnetic field oriented normally to the axis of the helical structure was considered. The dependences of the transition field to the ferronematic phase on the material parameters of the suspension and of the helical structure pitch and magnetization on the field strength were investigated. A possibility of existence of a reentrant ferrocholesteric phase was shown.

     

Behaviour of nematic liquid crystals doped with ferroelectric nanoparticles in the presence of an electric field

A planar nematic liquid crystal cell (NLC) doped with spherical ferroelectric nanoparticles is considered. Polarisation of the nanoparticles are assumed to be along the NLC molecules parallel and antiparallel to the director with equal probability. The NLC molecules anchoring to the cell walls are considered to be strong, while soft anchoring at the nanoparticles surface is supposed. Behaviour of the NLC molecules and nanoparticles in the presence of a perpendicular electric field to the NLC cell is theoretically investigated. The electric field of the nanoparticles is taken into account in the calculations. Freedericksz transition (FT) threshold field in the presence of nanoparticles is found. Then, the director and particles reorientations for the electric fields larger than the threshold field are studied. Measuring the onset of the nanoparticles reorientation is proposed as a new method for the FT threshold measurement.     

Optical bistability of the director field of the dendrimer-doped nematic liquid crystal

It was found that the second-generation carbosilane dendrimers with statistically distributed terminal aliphatic and azobenzene fragments induce negative orientational nonlinearity in nematic liquid crystals (NLC) (the NLC director turns perpendicular to the light field, decreasing the refractive index of an extraordinary wave). The light-induced Freedericksz transition in planar oriented NLCs is the first-order orientational phase transition characterized by a wide bistability region.     

Magnetic-field induced orientational transition in a helicoidal liquid-crystalline antiferromagnet

The magnetic-field induced orientational transition in helicoidal liquid-crystalline antiferromagnets representing compensated suspensions of magnetic nanoparticles in cholesteric liquid crystals is theoretically studied. The untwisting of a helicoidal structure and the behavior of mean magnetization as a function of the field strength and material parameters are investigated. It is shown that the magnetic subsystems in the field-untwisted ferronematic phase are not completely compensated, and the ferronematic phase is ferrimagnetic.