Freedericksz transition under electric and rotating magnetic field: application to nematics with negative dielectric and magnetic anisotropies

We study the action of a rotating magnetic field on the Freedericksz instability under electric field of a homeotropic nematic sample sandwiched between two parallel electrodes. The liquid crystal (LC) is of negative dielectric and magnetic anisotropies and the magnetic field is parallel to the electrodes used to apply the electric field. We show that the sample destabilises above a critical voltage V-c that depends on the magnetic field B and its angular rotation velocity omega. The relation V-c(B, omega) is calculated analytically in the synchronous regime, where the director rotates at the same angular velocity as the magnetic field. These predictions are compared to the experiment performed with the LC CCN-37. From this experiment, the values of the bend constant K-3, of the rotational viscosity gamma(1) and of the magnetic anisotropy chi(a) are deduced. [GRAPHICS] .