  The nonlinear interaction of DC current flowing in a thin metal film with an
external low-frequency AC electromagnetic field is studied theoretically. The
nonlinearity is related to the influence of the magnetic field of the DC
current and the magnetic field of the wave on the form of electron
trajectories. This magnetodynamic mechanism of nonlinearity is the most typical
for pure metals at low temperatures. We find that such interaction causes sharp
kinks in the temporal dependence of the AC electric field of the wave on
surface of the sample. The phenomenon of amplification of the electromagnetic
signal on the metal surface is predicted. We also calculate the nonlinear
surface impedance and show that it turns out to be imaginary-valued and its
modulus decreases drastically with the increase of the wave amplitude.
