Anisotropy of the critical magnetic field in a ferroelectric liquid crystal

The angular dependence of the critical magnetic field for the unwinding of the 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 Sm-C*-Sm-C-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.