2 Magnetic Properties of Liquid Crystals

In 1888, Friedrich Reinitzer [1] first observed an anomalous turbid fluid phase in cholesteryl benzoate. This phase interposed itself between a crystal and a normal clear fluid phase. This fluid was later to be identified as the first liquid crystal. After some years of study, it became clear that the turbidity was not due to density inhomogeneity, as would normally be supposed in such a situation, but rather due to inhomogeneities in the principal optical axis in an optically anisotropic fluid. Thus a key problem in studying what were at that time often labeled as Lehmann's liquid crystals was to obtain a uniform optical alignment. In 1911, Charles Mauguin [1], working with azoxyanisole and azoxyphenetole, found that magnetic fields aligned liquid crystals in situations when prepared boundaries alone were unable to do so. Mauguin's fields up to about 0.75 Teven competed with boundary alignment in such a way that the principal optic axis could sometimes be tilted with respect to the cell. Further investigations by Ornstein and coworkers in the Netherlands in the 1920s showed that magnetic fields (in modern language) reduced the orientational correlation lengths, although these results were originally interpreted in terms of an incorrect theory. More extensive investigations by Frederiks in the Soviet Union in the 1920s [1 , 2] , working with magnetic fields up to 2.5 T, showed that a given magnetic field was able to align a thick liquid crystalline cell, but failed to align a thinner cell. This is the familiar Frederiks effect. Zocher [1 ], following an elastic theory originally introduced by Oseen [1 ], introduced the term coupling the magnetic field to what later became known as the nematic director. By doing so, he was able to fit Frederiks's results to Oseen's theory, thereby establishing the elastic theory as the liquid crystal theory of choice. Nowadays, the aligning field is usually electric, and the field, rather than the cell thickness, is varied. The physics is the same, but the response to easily accessible electric fields is much greater than to magnetic fields. The question of increasing the magnetic sensitivity has become a subject of interest in recent years because of its device implications, and we return to this problem later in this chapter.