Emission of electromagnetic radiation due to spin-flip transitions in a ferromagnet

Abstract

We theoretically analyze the probability of electromagnetic wave emission due to electron transitions between spin subbands in a ferromagnet. Different mechanisms of such spin-flip transitions are considered. One mechanism is electron transitions caused by the magnetic field of the wave. Another mechanism is due to the Rashba spin–orbit interaction. While the two mentioned mechanisms contribute in a homogeneously magnetized ferromagnet, there are two other mechanisms that occur for a non-collinearly magnetized medium. The first mechanism is known and is due to the dependence of the exchange interaction constant on the quasimomentum of the conduction electrons. The second one is due to the minimal coupling. It follows from the connection of spin and spatial degrees of freedom in any non-collinearly magnetized medium. We study these mechanisms in a non-collinear ferromagnet with a helicoidal magnetization distribution. Estimations of the probabilities of electron transitions due to different mechanisms are made for realistic parameters, and we compare the mechanisms. We also estimate the radiation power and the threshold current in a simple model in which spin is injected into the ferromagnet by a spin-polarized electric current through a tunnel barrier.