Using Standing-wave X-ray Photoemission Spectroscopy to Determine Interfacial Composition In-situ in a Multi-layer Magnetic Tunnel Junction

Book excerpt: The Fe/MgO magnetic tunnel junction (MTJ) 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. The physical structures that are known from literature to contribute to the tunneling magneto-resistance (TMR) or the interlayer magnetic exchange coupling of this system include: stoichiometry of Fe, Mg, and O at the interface; interface roughness; MgO and Fe layer thicknesses; Fe oxidation at the interface; and symmetry of Fe on MgO and MgO on Fe interfaces. An in-depth understanding of the interface of this system is critical to developing an understanding of the magnetic properties. Fe/MgO/Fe MTJs grown with even small variations in the growth environment or by different procedures result in significant differences in these interface structures. Along with a deep probing and non-destructive technique for characterizing the buried interfaces, a measurement with the future possibility of simultaneous determinations of the buried chemical, physical, and electronic structure, valence band dispersion, and magnetism with depth specificity is of interest. We have 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. HXPS is non-destructive, deep probing, and can determine these properties of interest. The SW technique allows for specific sample interfaces to be measured and compared to the bulk layer, and for structure determination with few-angstrom precision. We compare soft x-ray photoemission spectroscopy (SXPS) SW measurements to clearly demonstrate the advantages of the hard/tender x-ray excitation energy for this sample, and other similar superlattice samples. The superlattice sample 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 ± 4 Å, with minor presence of FeO at the interfaces and a significantly distorted top bilayer with c.a. 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 c.a. 5 Å. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We discuss the impacts of this distorted top bilayer on some measurements of interest, including x-ray photoelectron diffraction and angle-resolved photoemission spectroscopy. We demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few-angstrom precision. This supports crucial understanding of the interface structure of this system, which has direct implications for the interlayer magnetic exchange coupling. SW-HXPS shows promise for future directions with combined structural and magnetic measurements on a complex, nanolayered system with sensitive material components.