  We report a theoretical study of the second harmonic generation in a
noncollinearly magnetized conductive medium with equilibrium spin current. The
hydrodynamic model is used to unravel the mechanism of a novel effect of the
double frequency signal generation due to the spin current. According to our
calculations, this second harmonic response appears due to the "non-adiabatic"
spin polarization of the conduction electrons induced by the oscillations in
the non-uniform magnetization forced by the electric field of the
electromagnetic wave. Together with the linear velocity response this leads to
the generation of the double frequency spin current. This spin current is
converted to the electric current via the inverse Spin Hall effect, and the
double-frequency electric current emits the second harmonic radiation.
