Nanoscale physics

Magnetic field-controlled molecular and magnetic order parameters in ferronematic liquid crystals
Krupa M.M., Korostil A.A
Department of Physics of Meso- and Nanocrystalline Magnetic Structures, Institute of Magnetism NAS and MES  of Ukraine.  Vernadsky av., 36b, Kyiv, Ukraine. 
E-mail: amand@rambler.ru

Ferronematic liquid crystals (FNLC) represent novel type of the nematic liquid crystals in which the strong sensitivity of the molecular orientation to the external magnetic field is realized via the anchoring of ferromagnetic nanoparticle to the molecular ordering [1]. Corresponding change of optical anisotropy provides the low field and low consumption magneto-optic effect of the polarization twisting of  the linearly polarized monochromatic beam transmitting through the NLC. These properties determine perspectives of utilizing such FNLCs for a variety of technological developments. Specially, it is light-driven modulators in optical schemes of the adaptive optics, electric and magnetic-controlled switches and attenuators   of  optical signals. 
Main problems of FNLCs involve their structural stable with respect to  ferromagnetic nanoparticle aggregation  and speedup of magneto-optic dynamics together with the respond and relaxation under low field magnetic impact. Solving the first problem is related to the equilibrium between interparticle dispersion and magnetic attractive and repulsive Van-der-Waals interactions that are determined by the interphase interaction on the nanoparticle surfaces. The magneto-optic dynamics is determined  by the energy functional of the FNLC and is described by the system of coupled equations for the molecular ordering parameter (so-called director) and ferromagnetic ordering parameter, magnetization, following from  solving  of the corresponding variation problem.  In terms of the tensor operators [2] of molecular ordering, representing moments of spherical harmonics of different orders, it is obtained the self-consistent description of the magnetization and molecular ordering responses on the magnetic field impact.

1.	Mertelj A., Lisjak D.  Ferromagnetic nematic liquid crystals // Liquid  crystals reviews.-2017.- 5, N 1.- P. 1-13.
2.	Mei S., Zhang P.  On molecular based Q-tensor model liquid crystals with density variation  // Multiscale Modeling. Simulation.-2015.-13. -P.977-1000.