Lamellar phases doped with magnetic particles, called ferrosmectic phases, undergo a reorientation transition when submitted to a very weak magnetic field. In contrast to the predictions for thermotropic smectic phases, the development of that instability occurs through the appearance of localized defects. The critical field corresponding to that transition is calculated and compared to our experimental results. We discuss the occurrence of defect formation, rather than the expected homogeneous instability, in the more general terms of a second-order phase transition changed into first order by the presence of host particles.