Cholesteric-nematic transition induced by a magnetic field in the strong-anchoring model.

We investigate the cholesteric-nematic transition induced by an external bulk field in a sample of finite thickness ℓ. The analysis is performed by considering a tilted magnetic field with respect to the easy direction imposed by rigid boundary conditions inducing planar orientation. In the case of parallel orientation between the magnetic field and of the easy direction, in the limit of ℓ→∞ we reobtain the results of de Gennes where the effective pitch of the cholesteric is a continuous function of the magnetic field diverging at the critical field related to the cholesteric-nematic transition. For finite ℓ we obtain a cascade of transitions, where the bulk expels a half-pitch at a time to avoid divergences in the elastic energy, in a similar manner as solids expel defects in the presence of strong deformation. In the case of oblique orientation between the magnetic field and the easy direction, only the completely untwisted state depends on the tilt angle. Therefore, only the cholesteric-nematic transition depends on the tilt angle while all the other magnetic transition values are unchanged.