The evolution of spin system with the Fermi character of the relevant
excitations under the strong magnetic field pulse is presented. The no
nlinear dynamic equations describing interaction of the system and ext
ernal magnetic field are found. It is shown that under certain conditi
ons the initial state of the resonantly absorbing system does not chan
ge after passing of the field pulse (similar to optical self-induced t
ransparency in two-level molecular gas).