Exchange-Induced Generation of Electromagnetic Radiation in a Helical Magnetic Structure

It has been predicted that electromagnetic radiation can be generated owing to the transition of conduction electrons between spin subbands in a noncollinear ferromagnet with a helical magnetic structure. The frequency dependence of the power density of spontaneous radiation has been obtained from the calculation of the probability of such transitions in the electric dipole approximation. For experimentally known parameters of holmium, the threshold current of generation and radiation power have been estimated for a helical structure placed in a resonator. Two cases have been considered. In the first case, the frequency of the resonator corresponds to the resonance frequency of the system (it is determined by the splitting energy of spin subbands in holmium and is approximately 89 THz). In the second case, the frequency of the resonator is strongly shifted from the resonance frequency by about 1 THz. The results indicate that the effect can possibly be detected in experiment.