M A T T E R A T E X T R E M E S
S C I E N T I F I C H I G H L I G H T S
3 0 H I G H L I G H T S 2 0 2 2 I
The most essential difference in the spectrum shape, measured without and with p→s` polarisation selection, takes place at the critical angle. The p→s Mössbauer spectra are free from the non-resonant electronic scattering. Therefore, the resonant lines in the total reflection region look like dips in an ordinary reflectivity spectrum without polarisation analysis and show peaks in the p→s` reflectivity spectrum. At the Bragg angle, the asymmetry of the tails , originating from the interference of the Mössbauer and electronic scattering, and non-zero background distinguish the reflectivity spectrum without polarisation selection from that with the p→s selection.
The reflectivity spectra measured without the polarisation selection at the critical angle are much more complicated. The additional resonant components are presented in this spectrum. At least one line at ~7.7 mm/s (red arrow at Figure 21a) of the additional sextet (corresponding to Bhf ~ 45 T) is clearly seen. Its absence in the p→s
PRINCIPAL PUBLICATION AND AUTHORS
Unique surface sensitivity to ferro- and antiferromagnetic phases by polarization analysis in synchrotron Mössbauer reflectivity, R.A. Baulin (a,b), M.A. Andreeva (a), L. Häggström (c), V.E. Asadchikov (d), B.S. Roshchin (d), A.I. Chumakov (e,b), D. Bessas (e), R. Rüffer (e), Surf. Interfaces 27, 101521 (2021); https:/doi.org/10.1016/j.surfin.2021.101521 (a) Faculty of Physics, Lomonosov Moscow State University, Moscow (Russia) (b) National Research Centre Kurchatov Institute , Moscow (Russia) (c) Department of Physics, Uppsala University, Uppsala (Sweden) (d) Shubnikov Institute of Crystallography, Moscow (Russia) (e) ESRF
 M.A. Andreeva et al., J. Synchrotron Radiat. 25, 473-483 (2018).  M.A. Andreeva et al., Phys.Rev. B 97(2), 024417 (2018).  M.A. Andreeva et al., Condens. Matter 4(1), 8 (2019).
spectra means that it belongs to the phase with zero net magnetisation, i.e. antiferromagnetically ordered, which does not change the polarisation of radiation during reflection. The p→s spectra are formed only by the scattering from the nuclei in ferromagnetic phases magnetised along the X-ray beam direction.
The simultaneous fit of all four spectra gives several aniferromagnetically ordered multiplets for the 57Fe layer in the top iron layer; meanwhile, the other deeper iron layers in the [57Fe/V]20 multilayer are ferromagnetically ordered in 1 T external field as displayed in Figures 21c and d. It is clear that the contribution to the reflectivity spectrum from the top layer is negligible at the Bragg angle, but this contribution prevails at the critical angle.
These findings show the potential of polarisation analysis in Mössbauer reflectivity for the characterisation of magnetic phases of surfaces, nanolayers and multilayers with sub-nanometre depth resolution.
Fig. 21: Mössbauer reflectivity spectra measured (a) near
the critical angle and (b) at the Bragg angle
without and with p→s` polarisation
selection. c) Schematic picture of the sample
and (d) the set of hyperfine multiplets obtained by the fit, which characterises
the 57Fe nuclei in the top layer and in
the other periodic part of the [57Fe/V]20