Interface properties of magnetic tunnel junction Lao_(0.7)Sr_(0.3)MnO_3/ SrTiO_3 superlattices studied by standing-wave excited photoemission spectroscopy

摘要

The chemical and electronic-structure profiles of magnetic tunnel junction (MTJ) (LSMO/STO) superlattices have been quantitatively determined via soft and hard x-ray standing-wave excited photoemission, x-ray absorption and x-ray reflectivity, in conjunction with x-ray optical and core-hole multiplet theoretical modeling. Epitaxial superlattice samples consisting of 48 and 120 bilayers of LSMO and STO, each nominally four unit cells thick, and still exhibiting LSMO ferromagnetism, were studied. By varying the incidence angle around the superlattice Bragg condition, the standing wave was moved vertically through the interfaces. By comparing experiment to x-ray optical calculations, the detailed chemical profile of the super-lattice and its interfaces was quantitatively derived with angstrom precision. The multilayers were found to have a small -6% change in periodicity from top to bottom. Interface compositional mixing or roughness over ~6 A was also found, as well as a significant change in the soft x-ray optical coefficients of LSMO near the interface. The soft x-ray photoemission data exhibit a shift in the position of the Mn 3p peak near the interface, which is not observed for Mn 3s. Combined with core-hole multiplet theory incorporating Jahn-Teller distortion, these results indicate a change in the Mn bonding state near the LSMO/STO interface. Our results thus further clarify the reduced (MTJ) performance of LSMO/STO compared to ideal theoretical expectations.