  Propagation of the finite amplitude electromagnetic wave through the
partially spin-polarized degenerate plasmas leads to the instability. The
instability happens at the interaction of the electromagnetic wave with the
small frequency longitudinal spin-electron-acoustic waves. Strongest
instability happens in the high density degenerate plasmas with the Fermi
momentum close to $m_{e}c$, where $m_{e}$ is the mass of electron, and $c$ is
the speed of light. The increase of the increment of instability with the
growth of the spin polarization of plasmas is found.
