Moessbauer forward scattering spectra of ferromagnets in radio-frequency magnetic field

The transmission of Moessbauer radiation through a thick ferromagnetic crystal, subjected to the radio-frequency (rf) magnetic field, is studied. A quantum-mechanical dynamical scattering theory is developed, taking into account both the periodical reversals of the magnetic field at the nuclei and their coherent vibrations. The Moessbauer forward scattering (FS) spectra of the weak ferromagnet FeBO3 exposed to the rf field are measured. It is discovered that the coherent gamma wave in the crystal, interacting with Moessbauer nuclei, absorbs or emits only couples of the rf photons. As a result, the FS spectra consist of equidistant lines spaced by twice the frequency of the rf field in contrast to the absorption spectra. Our experimental data and calculations well agree if we assume that the hyperfine field at the nuclei in FeBO3 periodically reverses and there are no coherent vibrations.