Anisotropy of the Critical Magnetic Field in a Ferroelectric Liquid Crystal

The angular dependence of the critical magnetic field for the unwinding of the $\mathrm{Sm}-C^{*}$ 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 $\mathrm{Sm}-C^{*}–\mathrm{Sm}-C–\mathrm{Sm}-C^{*}$ behavior, has been observed. The solitonlike deformation of the helix has a $2\pi$ symmetry at a general orientation, whereas it has a $\pi$ 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.