Interface properties of magnetic tunnel junction ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}/{\text{SrTiO}}_{3}$ superlattices studied by standing-wave excited photoemission spectroscopy

Interface properties of magnetic tunnel junction ${La}_{0.7} {Sr}_{0.3} {MnO}_{3} / {SrTiO}_{3}$ superlattices studied by standing-wave excited photoemission spectroscopy

A. X. Gray, C. Papp, B. Balke, S.-H. Yang, M. Huijben, E. Rotenberg, A. Bostwick, S. Ueda, Y. Yamashita, K. Kobayashi, E. M. Gullikson, J. B. Kortright, F. M. F. de Groot, G. Rijnders, D. H. A. Blank, R. Ramesh, and C. S. Fadley

Phys. Rev. B 82, 205116 – Published 16 November 2010

Abstract

The chemical and electronic-structure profiles of magnetic tunnel junction (MTJ) ${La}_{0.7} {Sr}_{0.3} {MnO}_{3} / {SrTiO}_{3}$ (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 superlattice and its interfaces was quantitatively derived with angstrom precision. The multilayers were found to have a small $\sim 6 %$ change in periodicity from top to bottom. Interface compositional mixing or roughness over $\sim 6 Å$ 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 $Mn 3 p$ peak near the interface, which is not observed for $Mn 3 s$ . 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.

Received 29 September 2010

DOI:https://doi.org/10.1103/PhysRevB.82.205116

Authors & Affiliations

A. X. Gray^1,2, C. Papp^2,3, B. Balke^2,4, S.-H. Yang⁵, M. Huijben⁶, E. Rotenberg⁷, A. Bostwick⁷, S. Ueda⁸, Y. Yamashita⁸, K. Kobayashi⁸, E. M. Gullikson⁷, J. B. Kortright², F. M. F. de Groot⁹, G. Rijnders⁶, D. H. A. Blank⁶, R. Ramesh^2,10,11, and C. S. Fadley^1,2

¹Department of Physics, University of California–Davis, Davis, California 95616, USA
²Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
³Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
⁴Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität, 55099 Mainz, Germany
⁵IBM Almaden Research Center, San Jose, California 95120, USA
⁶Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands
⁷Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁸NIMS Beamline Station at SPring-8, National Institute for Materials Science, Hyogo 679-5148, Japan
⁹Department of Chemistry, Utrecht University, 3584 CA Utrecht, The Netherlands
¹⁰Department of Physics, University of California, Berkeley, California 94720, USA
¹¹Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA

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Vol. 82, Iss. 20 — 15 November 2010

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Figure 1
(Color online) (a) Schematic of the investigated multilayer structure consisting of 48 and 120 bilayers of LSMO, grown to be 4 units cell or $15.5 Å$ thick, and STO, also grown to be 4 units cells or $15.6 Å$ thick, grown epitaxially on a single-crystal STO substrate. Photon energies of 833.2 eV corresponding to the $La 3 d_{5 / 2}$ absorption edge, and 5956.4 eV were used for the photoemission experiments. (b) X-ray optical properties of LSMO exhibit a sharp peak in absorption due to the $La 3 d_{5 / 2}$ resonance effect, which, in turn, enhances the reflectivity of the sample, as shown in (c).Reuse & Permissions
Figure 2
(Color online) The best fits between the experimental and calculated rocking curves at 833.2 eV and 5956.4 eV for [(a) and (b)] $Sr 3 p_{3 / 2}$ , [(c) and (d)] $Ti 2 p_{3 / 2}$ , [(e) and (f)] $O 1 s$ , (g) $La 4 d_{5 / 2}$ , (h) $La 3 d_{5 / 2}$ , [(i) and (j)] $Mn 3 p$ , and [(k) and (l)] $C 1 s$ . Calculations are based on a specially written x-ray optical theoretical code (Ref. 29).Reuse & Permissions
Figure 3
(Color online) (a) The $1 Å$ smooth bottom-to-top thickness gradient for the thickness of the LSMO and STO layers in the superlattice that is required to fit the rocking curves in Fig. 2. (b) Model of the sample which self-consistently describes the behavior of the rocking curves for every constituent element in the structure for both soft and hard x-ray regimes.Reuse & Permissions
Figure 4
(Color online) (a) Modeling the superlattice thickness gradients. Effect of simple linear thickness gradients, as well as no gradient, on the behavior of the rocking curves for [(b) and (c)] $Ti 2 p_{3 / 2}$ and [(d) and (e)] $Mn 3 p$ for photon energies of 833.2 and 5956.4 eV.Reuse & Permissions
Figure 5
(Color online) Analysis of the $Mn 3 p$ , $Ti 2 p$ , and $Mn 3 s$ core levels. (a) A single photoelectron spectrum over the region of $Mn 3 p$ , $Ti 2 p$ , and $Mn 3 s$ (which exhibits a multiplet splitting) is shown with a contour plot of the photoemission intensity as a function of incidence angle and binding energy. At the grazing incidence angle of $13.7 °$ the $Mn 3 p$ core level undergoes a 0.25 eV chemical shift toward higher binding energy which persists, although getting somewhat smaller, at higher grazing incidence angles. (b) Binding energy of the $Mn 3 p$ peak as a function of grazing incidence angle. (c) Binding energy and multiplet splitting of the $Mn 3 s$ peak as a function of grazing incidence angle. (d) Intensity $(E^{2})$ of the standing-wave electric field inside the sample as a function of depth and incidence angle. The line cuts indicate that, for incidence angles $< 13.7 °$ , the standing-wave field emphasizes the bulk of the LSMO layer, but for angles $> 13.7 °$ the interface regions of the LSMO layer are highlighted. Therefore, we assign the $Mn 3 p$ peak shift to atoms near the interface.Reuse & Permissions
Figure 6
(Color online) Modeling of the binding-energy shift of the $Mn 3 p$ and $Mn 3 s$ core peaks. (a) Experimental $Mn 3 p$ and $Mn 3 s$ spectra. (b) Simulated spectra using the Anderson impurity model and with and without a Jahn-Teller distortion of the lattice. The $Mn 3 p$ spectrum exhibits a 0.25 eV shift toward higher binding energy for the spectrum affected by Jahn-Teller, but the $Mn 3 s$ spectrum remains virtually unaffected, again in agreement with experiment.Reuse & Permissions

Physical Review B

covering condensed matter and materials physics

Interface properties of magnetic tunnel junction ${La}_{0.7} {Sr}_{0.3} {MnO}_{3} / {SrTiO}_{3}$ superlattices studied by standing-wave excited photoemission spectroscopy

A. X. Gray, C. Papp, B. Balke, S.-H. Yang, M. Huijben, E. Rotenberg, A. Bostwick, S. Ueda, Y. Yamashita, K. Kobayashi, E. M. Gullikson, J. B. Kortright, F. M. F. de Groot, G. Rijnders, D. H. A. Blank, R. Ramesh, and C. S. Fadley

Phys. Rev. B 82, 205116 – Published 16 November 2010

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Physical Review B

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Interface properties of magnetic tunnel junction La0.7Sr0.3MnO3/SrTiO3 superlattices studied by standing-wave excited photoemission spectroscopy

A. X. Gray, C. Papp, B. Balke, S.-H. Yang, M. Huijben, E. Rotenberg, A. Bostwick, S. Ueda, Y. Yamashita, K. Kobayashi, E. M. Gullikson, J. B. Kortright, F. M. F. de Groot, G. Rijnders, D. H. A. Blank, R. Ramesh, and C. S. Fadley

Phys. Rev. B 82, 205116 – Published 16 November 2010

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Interface properties of magnetic tunnel junction ${La}_{0.7} {Sr}_{0.3} {MnO}_{3} / {SrTiO}_{3}$ superlattices studied by standing-wave excited photoemission spectroscopy