  We probe the spin dynamics in a thin magnetic film at ferromagnetic resonance
by nuclear resonant scattering of synchrotron radiation at the 14.4 keV
resonance of $^{57}$Fe. The precession of the magnetization leads to an
apparent reduction of the magnetic hyperfine field acting at the $^{57}$Fe
nuclei. The spin dynamics is described in a stochastic relaxation model adapted
to the ferromagnetic resonance theory by Smit and Beljers to model the decay of
the excited nuclear state. From the fits of the measured data the shape of the
precession cone of the spins is determined. Our results open a new perspective
to determine magnetization dynamics in layered structures with very high depth
resolution by employing ultrathin isotopic probe layers.
