Behaviour of ferrocholesterics under external magnetic fields

The influence of an external magnetic field on the orientational behaviour of a ferrocholesteric with a positive magnetic anisotropy is investigated. Both the phenomena arising when the field was switched on or switched off are considered. It is found that the field needed for a ferrocholesteric-ferronematic transition B(FC)up arrow is higher when compared to that obtained for the pure cholesteric (B(C)up arrow). A similar result was obtained when estimating the critical field for the homeotropic ferronematic-ferrocholesteric (focal conic) transition, occurring when the magnetic field was decreased or switched off. We found that B-FC down arrow > B(C)down arrow. These results are explained when considering that the magnetic moments of the magnetic powder are not oriented parallel to the liquid crystal molecular directors, therefore hindering their orientation under a magnetic field. (C) 2001 Published by Elsevier Science B.V.