Magnon instability in ferromagnetic semiconductors under strong fields

The problem of magnon instability in an electron-magnon system is studied in the presence of a static uniform magnetic field. It is found that the magnons may become unstable when certain threshold values for the drift velocity and the magnetic field are exceeded. However, even after the thresholds are exceeded, it is predicted that there exist alternate bands of the magnetic field, in which the magnons are unstable in one band, stable in the next band, unstable again in the following band, etc. These alternate bands have their origin in the discreteness of the Landau levels of the electrons.