  We investigate theoretically and experimentally fluctuations of high spin
(F>1/2) beyond the linear response regime and demonstrate dramatic
modifications of the spin noise spectra in the high power density probe field.
Several effects related to an interplay of high spin and perturbation are
predicted theoretically and revealed experimentally, including strong
sensitivity of the spin noise spectra to the mutual orientation of the probe
polarization plane and magnetic field direction, appearance of high harmonics
of the Larmor frequency in the spin noise and the fine structure of the Larmor
peaks. We demonstrate the ability of the spin-noise spectroscopy to access the
nonlinear effects related to the renormalization of the spin states by strong
electromagnetic fields.
