  We theoretically consider magnetization dynamics in a ferromagnetic slab
induced by the magnetic field of a strong femtosecond laser pulse. The
longitudinal geometry, in which the initial magnetization lies in both the
plane of incidence and the sample plane, is studied. The magnetization
oscillations at the optical wave frequency are calculated with the use of the
Kapitza pendulum approach taking into account that the optical frequency is
much greater than the magnetization oscillation eigenfrequency. We study the
reflection of the electromagnetic wave from a ferromagnet %with such a dynamics
and show that this laser-induced low-frequency magnetization dynamics leads to
the appearance of the second-order nonlinearity in the Maxwell's equations,
which in turn gives rise to both the second harmonic generation (SHG) and
rectification effect. Although the amplitude of the magnetization oscillations
is small, the considered effect may be responsible for the SHG with the
efficiency comparable to that of nonmagnetic SHG from metal surfaces. Our
estimations show that the suggested mechanism may explain the recent
experiments on magnetization induced modulation of the SHG intensity in a
``forbidden'' $P_{in}P_{out}$ combination of incident and reflected waves in
cobalt/heavy metal systems, where it can be even more pronounced due to the
spin current flow through the ferromagnet/ heavy metal interface.
