The main focus of this Master’s thesis are ferromagnetic nematic liquid crystals. These relatively new materials are prepared as suspensions of ferromagnetic nanoplatelets in nematic liquid crystalline hosts.
We rely on the electric Fréedericksz transition to experimentally demonstrate that their fundamental static parameters (elastic constants K1 and K3, birefringence $\Delta n $, and dielectric anisotropy $\Delta \epsilon$) do not differ significantly from those in pure liquid crystal. We present the extended Frank elastic theory for a macroscopic description of ferromagnetic nematics by introducing the coupling constant γ between magnetization and director. The coupling constant was measured by observing the magnetic Fréedericksz transition.
We describe thermally excited fluctuation modes of director orientation in nematics (splay-bend and twist-bend) and outline an extended theory for describing such fluctuations in ferromagnetic nematics.
Using a relatively new experimental method, the cross-differential dynamic microscopy (c-DDM), which enables studying significantly faster dynamics than conventional DDM, along with appropriate selection of polarizer and analyzer orientations and ranges of investigated wave vectors, we can isolate and study fluctuation modes and their constituent deformation components and measure their relaxation rates. The relaxation rates measured at different external magnetic fields match the theoretically predicted values in several aspects. The bend component of the splay-bend mode is shown to be the most problematic for comparison with the theory.
Some further limitations of the described approach are pointed out at the end.
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