TRANSITION OF A FERROSMECTIC IN A VERY WEAK MAGNETIC-FIELD

Lamellar phases doped with magnetic particles, called ferrosmectic pha ses, undergo a reorientation transition when submitted to a very weak magnetic field. In contrast to the predictions for thermotropic smecti c phases, the development of that instability occurs through the appea rance of localized defects. The critical field corresponding to that t ransition is calculated and compared to our experimental results. We d iscuss the occurrence of defect formation, rather than the expected ho mogeneous instability, in the more general terms of a second-order pha se transition changed into first order by the presence of host particl es.