Probing buried interfaces with soft x-ray standing wave spectroscopy: application to the Fe/Cr interface

The nanometre-scale materials and devices that are under ever more intense study at present often depend for their unique properties on buried interfaces between two phases. Yet the number of experimental techniques that can specifically probe such interfaces, particularly with magnetic sensitivity, is limited. We here report a novel type of non-destructive method for spectroscopically studying buried nanometre-scale interfaces and other nanostructures with soft x-ray standing waves. Strong standing waves with a period of 4.0 nm and approximately 3:1 contrast ratios are created via Bragg reflection from a synthetic multilayer of form [B 4 C/W] 40 . By growing a wedge-shaped Fe/Cr bilayer on top of this multilayer, the mechanical translation of the bilayer exposed to a fixed and finely focused synchrotron radiation beam is converted into a translation of the standing wave through the interface. Analysing various core photoelectron intensities as a function of angle and beam position permits derivation of layer thicknesses and interface mixing/roughness scales. Magnetic circular dichroism in photoemission from the 2p and 3p levels of Fe and Cr further permits derivation of the positions and widths of regions with decreased (increased) ferromagnetic alignment for Fe (Cr), showing that normally antiferromagnetic Cr becomes ferromagnetic just below the centre of the interface but with antiparallel alignment with respect to Fe, and that the equal-concentration region in the centre of the interface strongly inhibits magnetic alignment for both species along the direction of net magnetizations that is probed. The magnetically altered regions in both metals are only 1-2 atomic layers in thickness. 3s spectra from Fe and Cr further indicate that the local spin moments on both atoms do not change on crossing the interface. This standing-wave-plus-wedge method should have a range of applications for the characterization of nanostructures and their interfaces.