Focusing and defocusing of electromagnetic waves in a ferromagnet

We show that the modulation of an electromagnetic wave in a ferromagnet in the presence of an external magnetic field is governed by the nonlinear Schrodinger equation. We characterize the existence of an oscillatory instability of the Benjamin-Feir type for electromagnetic propagation in a saturated ferrite. Depending on the physical parameters of the system, we establish the regions in which focusing or defocusing of the initial carrier envelope occurs. We show that one of the two plane waves, circularly polarized, allowed to propagate into a ferrite, is bunching into solitons and the other is modulated by a dark soliton.