Anisotropy of the Critical Magnetic Field in a Ferroelectric Liquid Crystal

Abstract

The angular dependence of the critical magnetic field for the unwinding of the $\mathrm{Sm}\ensuremath{-}{C}^{*}$ helix of a ferroelectric liquid crystal has been measured by a deuteron NMR sample tilting technique. A minimum at intermediate angles, resulting in a reentrant $\mathrm{Sm}\ensuremath{-}{C}^{*}--\mathrm{Sm}\ensuremath{-}C--\mathrm{Sm}\ensuremath{-}{C}^{*}$ behavior, has been observed. The solitonlike deformation of the helix has a $2\ensuremath{\pi}$ symmetry at a general orientation, whereas it has a $\ensuremath{\pi}$ symmetry for the magnetic field parallel to the smectic layers. The results can be interpreted in terms of the Landau theory with the inclusion of a nonparallel magnetic field energy term.