# Hard x-ray standing-wave photoemission insights into the structure of an   epitaxial Fe/MgO multilayer magnetic tunnel junction

**Authors:** C.S. Conlon (1, 2), G. Conti (1, 2), S. Nem\v{s}\'ak (3), G., Palsson (4), R. Moubah (4), C.-T. Kuo (1, 2), M. Gehlmann (1, 2), J., Ciston (5), J. Rault (6), J.-P. Rueff (6, 7), F. Salmassi (8), W. Stolte, (3), A. Rattanachata (1, 2), S.-C. Lin (1, 2), A. Keqi (1, 2), A., Saw (1, 2), B. Hj\"orvarsson (4), C.S. Fadley (1, 2) ((1) Department, of Physics, University of California, Davis, Davis, CA, USA, (2) Materials, Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, (3) Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley,, CA, USA, (4) Department of Physics, Uppsala University, Uppsala, Sweden, (5), National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley, National Laboratory, Berkeley, CA, USA, (6) SOLEIL Synchrotron, Saint-Aubin,, France, (7) Sorbonne Universit\'e, CNRS, Laboratoire de Chimie, Physique-Mati\`ere et Rayonnement, Paris, France, (8) Center for X-ray, Optics, Lawrence Berkeley National Laboratory, Berkeley, CA, USA)

arXiv: 1904.04999 · 2020-01-29

## TL;DR

This study uses advanced hard x-ray standing-wave photoemission spectroscopy to non-destructively analyze the buried interface structure of an epitaxial Fe/MgO multilayer, revealing detailed layer composition, oxidation states, and interface quality.

## Contribution

It demonstrates the application of standing-wave HXPS for high-precision, in-situ analysis of buried interfaces in magnetic tunnel junctions, providing insights into their structure and oxidation.

## Key findings

- 8 of 9 superlattice repeats are similar and ordered
- Minor FeO presence at interfaces and distorted top bilayer
- Agreement with STEM and x-ray reflectivity measurements

## Abstract

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.

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Source: https://tomesphere.com/paper/1904.04999