Ferromagnetic and antiferromagnetic liquid crystal suspensions: Experiment and theory

In this paper, the complete theoretical and experimental studies of electrically and magnetically induced orientational transitions in suspensions of goethite nanorods in a nematic liquid crystal 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT), known as ferronematics are presented. Two types of samples with different magnetic properties are prepared and observed. A compensated ferronematic with zero initial magnetization is obtained after cooling of the sample from isotropic to nematic phase in the absence of a magnetic field. A magnetized ferronematic with nonzero initial magnetization is prepared during application of magnetic field. The magnetically induced transitions demonstrated two different types of orientational behavior of the samples. The Fréedericksz transition in the compensated ferronematic occurred in higher magnetic field in comparison with pure nematic while the magnetized ferronematic shown well measurable response in capacitance to the applied magnetic field, even much below the Fréedericksz threshold in usual 6CHBT. Full theoretical descriptions of such behaviours of compensated and magnetized ferronematic samples are presented and all observed dependencies of the cells capacitance on magnetic field are numerically calculated. A comparative analysis shows very well qualitative and quantitative agreement between theoretical results and experimental data. Due to this, the material parameters of the investigated ferronematic system with goethite nanoparticles are estimated.