RELAXATION MOSSBAUER-SPECTRA UNDER RF MAGNETIC-FIELD EXCITATION

In the present study the general equations which describe the absorpti on Mossbauer spectra under radiofrequency (rf) magnetic field excitati on with arbitrary frequency and field strength have been derived. With in our model chosen for a ferromagnet as a system of exchange-coupled (interacting) Stoner-Wohlfarth particles, the evolution of the magneti zation and the corresponding Mossbauer spectra as a function of temper ature and initial magnetization relaxation parameters are traced. It i s found that the collapse effect is of a pronounced threshold characte r with respect to the rf field strength and does not need strong rf fi elds 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 de veloped allows us to perform calculations of Mossbauer spectra under r f magnetic field excitation and the corresponding magnetization curves (also for multiphase systems such as modern nanostructured magnetic a lloys). The results are also useful in a situation when the hyperfine field at the nuclei does not follow the rf magnetic field. This circum stance determines the rather nontrivial transition from the collapsed line (in strong enough rf field) to the well-resolved hyperfine struct ure (in weak rf field) and contributes therefore in understanding the selective partial collapse effect.