Relaxation Mössbauer spectra under rf magnetic field excitation

Abstract

In the present study the general equations which describe the absorption M\"ossbauer spectra under radio-frequency (rf) magnetic field excitation with arbitrary frequency and field strength have been derived. Within our model chosen for a ferromagnet as a system of exchange-coupled (interacting) Stoner-Wohlfarth particles, the evolution of the magnetization and the corresponding M\"ossbauer spectra as a function of temperature and initial magnetization relaxation parameters are traced. It is found that the collapse effect is of a pronounced threshold character with respect to the rf field strength and does not need strong rf fields for its realization. The necessary condition for the observation of a collapse effect is only a rf amplitude causing the corresponding magnetization curves to be symmetrical in time reversal. The theory developed allows us to perform calculations of M\"ossbauer spectra under rf magnetic field excitation and the corresponding magnetization curves (also for multiphase systems such as modern nanostructured magnetic alloys). The results are also useful in a situation when the hyperfine field at the nuclei does not follow the rf magnetic field. This circumstance determines the rather nontrivial transition from the collapsed line (in strong enough rf field) to the well-resolved hyperfine structure (in weak rf field) and contributes therefore in understanding the selective partial collapse effect.