  The Fe/MgO magnetic tunnel junction is a classic spintronic system, with
current importance technologically, and interest for future innovation. The key
magnetic properties are linked directly to the structure of hard-to-access
buried interfaces, and the Fe and MgO components near the surface are unstable
when exposed to air, making a deeper probing, non-destructive, in-situ
measurement ideal for this system. We have thus applied hard x-ray
photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few keV
energy range to probe the structure of an epitaxially-grown MgO/Fe
superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by
magnetron sputtering on an MgO (001) substrate, with a protective Al2O3 capping
layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and
ordered, with a period of 33 $\pm$ 4 angstrom, with minor presence of FeO at
the interfaces and a significantly distorted top bilayer with ca. 3 times the
oxidation of the lower layers at the top MgO/Fe interface. There is evidence of
asymmetrical oxidation on the top and bottom of the Fe layers. We find
agreement with dark-field scanning transmission electron microscope (STEM) and
x-ray reflectivity measurements. Through the STEM measurements we confirm an
overall epitaxial stack with dislocations and warping at the interfaces of ca.
5 angstrom. We also note a distinct difference in the top bilayer, especially
MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used
to probe deep buried interfaces of novel magnetic devices with few angstrom
precision.
