Magnetic field effects in nematic and smectic liquid crystals probed by time resolved observation of orientation relaxation of the spin probe

The kinetics of reorientation of the liquid crystal HOPOOB in nematic and smectic phases in the magnetic field was studied by the spin probe technique. The time evolution of the EPR spectrum shape in the course of reorientation in the nematic phase indicates that the mechanism of alignment comprises redistribution between domains of different orientations rather than the turn of the orientation distribution function. A microscopic model of domain structure rearrangement is provided to qualitatively describe the experimental data. In the smectic C phase, partial reorientation is observed upon the action of the magnetic field. A possible mechanism of this process is the reorientation of tilt direction within the smectic layers. It is shown that this process is constrained, and the value of the corresponding elastic constant is estimated.