The program REFTIM calculates and fits the time spectra of the nuclear resonant reflectivity for given grazing angles, the prompt and delayed reflectivity curves for any multilayer structure containing 57 Fe, 149 Sm, 151 Eu or another Möss-bauer nuclei with M1 transitions. The program is effective if you have a set of different kind of measured data for simultaneous fit.Keywords Nuclear resonance scattering · X-ray reflectivity · Ultrathin films · Multilayers Nuclear resonant reflectivity (NRR) gives valuable information about the structure and magnetic properties of ultrathin films, monolayers and periodic multilayers, containing Mössbauer isotope. However, even the qualitative interpretation of the experimental time spectra as well as of the delayed reflectivity curves is not simple. For the correct interpretation of experimental data and for the modeling of different effects in the NRR a special REFTIM program package has been created by Marina Andreeva, Bengt Lindgren and Vitalii Panchuk. It is free available at [1], where an extended description is attached.Theoretical background of the calculations is described in the refs. [2,3]. The 4 × 4-propagation matrices are used for calculations of the reflectivity from anisotropic layers, but the usage of an alternative method of the generalized (for the anisotropic case) Parratt recurrent equations is possible by choice. The model can be created by a dialog or can be taken as a ready file. The set of varied parameters is determined by user.
We present the depth-resolved nuclear resonance reflectivity (NRR) studies of the magnetization evolution in [ 57 Fe(3 nm)/Cr(1.2 nm)] 10 multilayer under the applied external field. The measurements have been performed at the station BL09XU of SPring-8 at different values of the external field (0 -1500 Oe). We apply the joint fit of the delayed reflectivity curves and the time spectra of the nuclear resonance reflectivity measured at different grazing angles for enhancement of the depth resolution and reliability of the results. For the first time we show that the azimuth angle, which is used in all papers devoted to the magnetization profile determination, has more complicated physical sense due to the partially coherent averaging of the scattering amplitudes from magnetic lateral domains. We describe the way how to select the true azimuth angle from the determined "effective azimuth angle". Finally we obtain the noncollinear twisted magnetization depth-profiles where the spin flop state appears sequentially in different 57 Fe layers at increasing applied field.
Mössbauer reflectivity spectra and nuclear resonance reflectivity (NRR) curves have been measured using the Synchrotron Mössbauer Source (SMS) for a [Fe/Cr] periodic multilayer, characterized by the antiferromagnetic interlayer coupling between adjacent Fe layers. Specific features of the Mössbauer reflectivity spectra measured with π-polarized radiation of the SMS near the critical angle and at the `magnetic' maximum on the NRR curve are analyzed. The variation of the ratio of lines in the Mössbauer reflectivity spectra and the change of the intensity of the `magnetic' maximum under an applied external field has been used to reveal the transformation of the magnetic alignment in the investigated multilayer.
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