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.