Orientational instability of the director in a nematic cell caused by electro-induced anchoring modification

We theoretically investigate the threshold for the director reorientation from the homeotropic state to the hybrid homeotropic-planar state and vice versa in a cell filled with a flexoelectric nematic liquid crystal (NLC) subjected to an electric field. The liquid crystal is doped by a CTAB-like substance, a part of molecules of which dissociates into positive and negative ions. The anchoring on one of the cell surfaces is assumed to be strong and homeotropic, while the other surface can adsorb positive ions which play the role of an orienting surfactant for NLC molecules on this surface. At certain voltages, the orientational transitions in the bulk of the NLC are possible due to the changing conditions for the director on the adsorbing surface. We calculate respective threshold voltages as functions of anchoring parameters. The existence of the critical values of these parameters, beyond which the orientational transitions do not take place, is established.

Magneto-orientation response of ferronematic with a tilted orientation of the director and magnetization

The article considers orientational transitions induced by a magnetic field in a highly dispersed suspension of ferromagnetic particles in a nematic liquid crystal – ferronematic. We research the case when, in the absence of an external magnetic field between the directions of the liquid crystal director and magnetization of the impurity particles, a constant pretilt angle is maintained, and the coupling of the liquid crystal molecules to the surface of the ferroparticles is assumed to be soft and planar. An example of such an impurity is ferromagnetic carbon nanotubes magnetized at a certain angle to their long axes. On the basis of the continuum theory, the equilibrium values of the orientation angles of the liquid crystal director and magnetization are calculated for different values of the pretilt angle. The results of calculations are compared with the previously considered case of planar coupling between the director and magnetization. Analytical expressions are obtained for determining the orientational and magnetic structure of the ferronematic in the case of weak magnetic fields. The magnetization of the ferronematic is studied for different values of the pretilt angle.

Inverse “guest–host” effect: ferroelectric nanoparticles mediated switching of nematic liquid crystals

We report an inverse guest–host effect: Ferroelectric nanoparticles (a guest) can fully control orientational transitions in a liquid crystal host.

Магнитные взаимодействия на границе оксидный ферромагнетик/ферромагнитный интерметаллид

The magnetic properties of heterostructures consisting of two films are investigated: the upper layer is rare-earth intermetallic superlattices consisting of exchange-coupled TbCo2 / FeCo layers, and the lower layer, either epitaxially grown La0.7Sr0.3MnO3 (LSMO) manganite optimally doped with strontium, or an epitaxial film of an iron-yttrium garnet Y3Fe5O12 (YFO). The material (TbCo2 / FeCo) n (TCFC) provides giant magnetostriction, a large value of the magnetomechanical coupling coefficient, controlled induced magnetic anisotropy, and spin-orientational transitions induced by a magnetic field or elastic stresses. Experimental studies have shown that the interlayer interaction of the heterostructure TCFC / LSMO has an antiferromagnetic character. An increase in the FMR line width in the structures was ob-served, caused by the spin current flowing through the interface between two films. In the TCFC/YFO heterostructure, an electric voltage induced in the TCFC intermetallic film was observed, caused by the reverse spin Hall effect under ferromagnetic resonance conditions.

Rotational remagnetization and orientational transitions in submicron YIG films

It is shown that for the quasinormal remagnetization of the submicron YIG films with the normal along the [111] axis, which are in the single-domain state, two types of orientation transitions (OT) due to the influence of easy axes can be distinguished. The nonreciprocity of the FMR spectrum for forward and reverse magnetization due to the influence of easy axes is shown.

Magnetic field induced orientational transitions in liquid crystals doped with carbon nanotubes

We propose a continuum theory of orientational phase transitions induced by an external magnetic field in a suspension of carbon nanotubes in a nematic liquid crystal. It is shown that in a magnetic field a non-uniform and two different uniform phases are possible in the suspension. The uniform phases of the suspension differ by the type of orientational coupling of nanotubes with the liquid crystal matrix (the planar type when the nanotubes are oriented along the matrix director, and the homeotropic type when the nanotubes are perpendicular to the director). The possibility of a redistribution of the nanotube concentration (segregation effect) is shown. The fields of orientational transitions between uniform and non-uniform phases of the suspension are found analytically. It is shown that, when the nanotubes are weakly coupled to the matrix, the magnetic field induces reentrant transitions (uniform planar phase–non-uniform phase–uniform homeotropic phase–non-uniform phase). These transitions can be of first or of second order depending on the carbon nanotubes segregation intensity.