Absolute instability of magnetostatic surface waves interacting with drifting carriers in closed ferrite-semiconductor structure

The dispersion relation of magnetostatic surface waves (MSSW) in a semiconductor-ferrite-semiconductor system between two metal plates is analyzed. The coupling between a propagating mode and an evanescent mode occurs and splits both spectra of these modes that degenerated in the absence of the semiconductors in the vicinity of the point at which the group velocity changes its sign due to the effect of metal. When the drift velocity of carriers exeeds the phase velocity of the waves in the splitting portion of the dispersion relation, the function has a saddle point for complex frequency. On this condition, the two waves propagating direction of which are opposite to each other near the saddle point frequency are amplifying in each propagating direction, while at the saddle point these two waves degenerate and are amplifying without propagating since the group velocity at this point, that is, an absolute instability occurs.