Mössbauer Sidebands by rf Excitation of Magnetic Materials

Abstract

The phenomenon of frequency-modulation (F M) sidebands induced with rf fields in the Mössbauer spectra of ferromagnetic materials is considered in detail. The general expression for the Mössbauer transition probability in the presence of rf acoustic vibrations is given and is derived on a quantum basis. The experimental properties of the sideband effect are reviewed and new experimental results are given. Experiments show that F M sidebands are caused by the generation of rf acoustic vibrations in the sample. The sideband effect is shown to vanish in an FeB${\mathrm{O}}_3$ sample if it is heated above the Curie temperature or in an $\alpha$-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$ sample if it is cooled below the Morin transition. The dependences of sideband formation on rf driving frequency, on rf skin depth, and on the application of additional magnetic fields are also discussed. Several previously proposed mechanisms for the generation of rf sidebands are now ruled out by these new experimental data. One mechanism which is supported by the new data is the magnetoacoustic coupling of the sample to the rf field by rf magnetostriction. This model which was suggested in our original paper and also by others is discussed in some detail. In the past the single important difficulty with the magnetostriction hypothesis has been that the rf acoustic vibrations that one calculated on a static basis from it did not have sufficient amplitude to account for the observed sideband intensities. We point out here that this difficulty with numerical magnitudes may be overcome by postulating that the primary rf magnetostrictive strain is induced in the sample plane rather than perpendicular to it as had previously been assumed. The rf acoustic vibrations resulting from this time-varying strain are then presumed to scatter within the sample so that some vibrational amplitudes come to have components along the $\gamma$-ray axis. Both the in-plane-rf-strain assumption and the acoustic-scattering assumption find support in the experimental data.