Pure nuclear Bragg reflection of a periodic ⁵⁶Fe/⁵⁷Fe multilayer

Grazing incidence nuclear multilayer diffraction of synchrotron radiation from a periodic stack of alternating ⁵⁶Fe and ⁵⁷Fe layers was observed. Resonant layer fraction, substrate size, flatness, and surface roughness limits were optimized by previous simulations. The isotopic multilayer (ML) sample of float glass/⁵⁷Fe(2.25nm)/[⁵⁶Fe(2.25nm)/ ⁵⁷Fe(2.25nm)]×15/Al(9.0nm) nominal composition was prepared by molecular beam epitaxy at room temperature. Purity structure and lateral homogenity of the isotopic ML film was characterized by magnetometry, Auger electron, Rutherford backscattering, and conversion electron Mössbauer spectroscopies. The isotopic ML structure was investigated by neutron and synchrotron Mössbauer reflectometry. Surface roughness of about 1 nm of the flat substrate (curvature radius >57 m) was measured by scanning tunneling microscopy and profilometry. A pure nuclear Bragg peak appeared in synchrotron Mössbauer reflectometry at the angle expected from neutron reflectometry while no electronic Bragg peak was found at the same position by x-ray reflectometry. The measured width of the Bragg peak is in accordance with theoretical expectations.