Structural changes in liquid crystals doped with spindle magnetic particles

Abstract

A new composite ferronematic system based on the nematic liquid crystal 6CB doped with hematite spindle-like magnetic particles characterized with magnetic moments oriented perpendicularly to their main axes was prepared. The surface acoustic waves (SAWs) were used to study the effect of magnetic particles on liquid crystals' structural changes in a magnetic field using measurements of the attenuation of SAW propagating along the ferronematic liquid crystal surface. For a study of new composite materials' orientational properties, the ferronematic samples were prepared with three different lengths of the particles and with the volume concentrations of particles φ1 = 10−3, φ2 = 10−4, and φ3 = 10−5. SAW attenuation response to the external magnetic field was investigated experimentally for two different directions of the magnetic field regarding the cell position, perpendicular and/or parallel to the LC cell. Experimental results validated a decrease of the threshold field in the ferronematic samples in comparison with the pure 6CB for the field in a perpendicular direction and an increase of the threshold shift with a growth of both the volume fraction of solid impurity and the length of particles for magnetic field applied perpendicularly to the cell plane. The effect of particle concentration on the magnitude of structural changes and also on the large residual attenuation at decreasing the magnetic field was observed in this arrangement. The parallel magnetic field direction led to the similar behavior of investigated ferronematic systems in early studied magnetic particles and provided switching processes.