Nuclear resonant spectroscopy at Bragg reflections from periodic multilayers: Basic effects and applications

A systematic study of the Bragg nuclear resonant reflectivity from periodic multilayers in the energy and time domains is presented. Using the kinematical approach of the general reflectivity theory we describe the basic features of the time evolution of the reflected wave after a pulsed excitation of resonant multilayers by synchrotron radiation. Effects of the collective excitation have been examined such as the shift of quantum beat phases, the interplay between electronic and nuclear subsystem excitations depending on their relative position in a multilayer, the energy and time evolution of standing waves inside a resonant multilayer, and their influence on the reflectivity spectra. The exact expression for the reflectivity by a thin resonant layer placed inside a multilayer structure has been derived. The observed shift of the delayed reflectivity Bragg peak relative to the prompt peak is explained by the developed formalism. Experimental applications are discussed with examples of magnetic profile determinations in Fe/Cr, Fe/V, and Fe/Co multilayers measured at ESRF