  We theoretically analyze a possibility of electromagnetic wave emission due
to electron transitions between spin subbands in a ferromagnet. Different
mechanisms of such spin-flip transitions are cousidered. One mechanism is the
electron transitions caused by magnetic field of the wave. Another mechanism is
due to Rashba spin-orbit interaction. While two mentioned mechanisms exist in a
homogeneously magnetized ferromagnet, there are two other mechanisms that exist
in non-collinearly magnetized medium. First mechanism is known and is due to
the dependence of exchange interaction constant on the quasimomentum of
conduction electrons. Second one exists in any non-collinearly magnetized
medium. We study these mechanisms in a non-collinear ferromagnet with
helicoidal magnetization distribution. The estimations of 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 threshold current in a simple model in which spin is injected into the
ferromagnet by a spin-polarized electric current through a tunnel barrier.
