Interface properties of magnetic tunnel junctionLa0.7Sr0.3MnO3/SrTiO

Abstract

The chemical and electronic-structure profiles of magnetic tunnel junction ͑MTJ͒ La 0.7 Sr 0.3 MnO 3 / SrTiO 3 ͑LSMO/STO͒ 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 superlattice and its interfaces was quantitatively derived with angstrom precision. The multilayers were found to have a small ϳ6% change in periodicity from top to bottom. Interface compositional mixing or roughness over ϳ6 Å 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 ͑MTJ͒ performance of LSMO/STO compared to ideal theoretical expectations.

Making use of x-ray optical effects in photoelectron-, Auger electron-, and x-ray emission spectroscopies: Total reflection, standing-wave excitation, and resonant effects

“…18,[20][21][22][23][24][25][26][27][28][29][30][31][32] A. Semi-infinite sample, non-resonant excitation…”

“…We will illustrate the utility of these properties of standing waves for depth-resolved emission experiments below. In addition, we note that additional reflections in a given multilayer structure of total thickness D can give rise to further standing-wave effects that are often termed Kiessig fringes 29 or Fresnel fringes, with a similar equation describing the maxima in reflectivity associated with these fringes…”

“…At the outset, we note that this program has been tested extensively and used to successfully model a variety of sets of experimental data, to which we refer the reader. 18,[20][21][22][23][24][25][26][27][28][29][30][31][32] We here thus focus primarily on the theoretical modeling of various x-ray optical effects, pointing out the actual and potential utility of them in future studies of buried layers and interfaces.…”

“…The reduction of reflection and refraction caused by the interdiffusion at the interfaces are also taken into account. 29 Proceeding by complete analogy from the semi-infinite layer, the differential photoemission intensity originating from k-atom orbital of A-atom at depth z in the ith layer is given by dI i…”

“…13 Beyond these total reflection studies, x-ray optical effects have not been rigorously accounted for in the analysis of most photoemission or x-ray emission data, with the assumption being made that the x-rays penetrate much more deeply, and essentially without attenuation or modulation, into the sample, as compared to the depths from which the photoelectrons or fluorescent x-rays are emitted. Yet x-ray optical effects can play a particularly significant role for measurements done at grazing incidence angles, 9-13 near absorption-edge resonances, 14 in Bragg-like reflection from single-crystal planes [15][16][17] and synthetic multilayers, 8,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] and with variable polarization so as to produce magnetic and other dichroic effects. 19,21,30 In beginning this discussion, it is worthwhile to discuss in more detail the specific types of x-ray optical effects that need to be considered: first are optical interference effects that modify the total electric field intensity vs. depth into the sample.…”