Depth-resolved resonant inelastic x-ray scattering at a superconductor/half-metallic-ferromagnet interface through standing wave excitation

Cheng-Tai Kuo, Shih-Chieh Lin, Giacomo Ghiringhelli, Yingying Peng, Gabriella Maria De Luca, Daniele Di Castro, Davide Betto, Mathias Gehlmann, Tom Wijnands, Mark Huijben, Julia Meyer-Ilse, Eric Gullikson, Jeffrey B. Kortright, Arturas Vailionis, Nicolas Gauquelin, Johan Verbeeck, Timm Gerber, Giuseppe Balestrino, Nicholas B. Brookes, Lucio Braicovich, and Charles S. Fadley

Phys. Rev. B 98, 235146 – Published 21 December 2018

Abstract

We demonstrate that combining standing wave (SW) excitation with resonant inelastic x-ray scattering (RIXS) can lead to depth resolution and interface sensitivity for studying orbital and magnetic excitations in correlated oxide heterostructures. SW-RIXS has been applied to multilayer heterostructures consisting of a superconductor $L a_{1.85} S r_{0.15} Cu O_{4}$ (LSCO) and a half-metallic ferromagnet $L a_{0.67} S r_{0.33} Mn O_{3}$ (LSMO). Easily observable SW effects on the RIXS excitations were found in these LSCO/LSMO multilayers. In addition, we observe different depth distribution of the RIXS excitations. The magnetic excitations are found to arise from the LSCO/LSMO interfaces, and there is also a suggestion that one of the $d d$ excitations comes from the interfaces. SW-RIXS measurements of correlated-oxide and other multilayer heterostructures should provide unique layer-resolved insights concerning their orbital and magnetic excitations, as well as a challenge for RIXS theory to specifically deal with interface effects.

Received 27 August 2018
Revised 20 November 2018

DOI:https://doi.org/10.1103/PhysRevB.98.235146

Physics Subject Headings (PhySH)

Heterostructures Interfaces Oxides

Resonant inelastic x-ray scattering X-ray standing waves

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Cheng-Tai Kuo^1,2,*, Shih-Chieh Lin^1,2, Giacomo Ghiringhelli³, Yingying Peng^3,†, Gabriella Maria De Luca⁴, Daniele Di Castro⁵, Davide Betto⁶, Mathias Gehlmann^1,2, Tom Wijnands⁷, Mark Huijben⁷, Julia Meyer-Ilse², Eric Gullikson², Jeffrey B. Kortright², Arturas Vailionis⁸, Nicolas Gauquelin^7,9, Johan Verbeeck⁹, Timm Gerber^1,2,10, Giuseppe Balestrino⁵, Nicholas B. Brookes⁶, Lucio Braicovich³, and Charles 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
³CNR-SPIN and Dipartimento di Fisica Politecnico di Milano, Piazza Leonardo da Vinci 32, Milano I-20133, Italy
⁴Dipartimento di Fisica “E. Pancini” Università di Napoli ‘Federico II‘ and CNR-SPIN, Complesso Universitario di Monte Sant'Angelo, via Cinthia, Napoli I-80126, Italy
⁵CNR-SPIN and Dipartimento di Ingegneria Civile e Ingegneria Informatica, Università di Roma Tor Vergata, Via del Politecnico 1, I-00133 Roma, Italy
⁶European Synchrotron Radiation Facility, 71 Avenue des Martyrs, CS40220, F-38043 Grenoble Cedex 9, France
⁷Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, Enschede 7500 AE, The Netherlands
⁸Stanford Nano Shared Facilities, Stanford University, Stanford, California 94305, USA
⁹Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
¹⁰Peter Grünberg Institut PGI-6, Research Center Jülich, 52425 Jülich, Germany

^*Corresponding author: chengtaikuo@lbl.gov
^†Present address: Department of Physics and Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA.
^‡Corresponding author: fadley@lbl.gov

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Vol. 98, Iss. 23 — 15 December 2018

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Images

Figure 1
Schematic illustrations of the standing wave (SW) excited resonant inelastic x-ray scattering (RIXS) measurement. (a) Diagram of the multilayer sample with bilayer period $d_{M L}$ , including the geometry of the exciting x-ray beam, the scattered photons, and the standing wave indicated. The multilayer samples consist of 20 bilayers of 2 unit cells of $L a_{1.85} S r_{0.15} Cu O_{4}$ (LSCO) and 7 unit cells of $L a_{0.67} S r_{0.33} Mn O_{3}$ (LSMO), grown epitaxially on an SrO-terminated STO substrate. The dimensions shown are nominal, based on bulk lattice parameters. (b) The SW-RIXS experimental geometry in real space and momentum space. (c) A typical RIXS spectrum, from the SrO-terminated growth, that exhibits quasielastic, magnetic, and $d d$ excitations. (d),(e) The STEM-HAADF and EELS results near the initial growth on STO for both the SrO-terminated growth and the less regular $Ti O_{2}$ -terminated growth. In the RBG images, Ti is orange, Mn is green, Cu is blue, Sr is turquoise, and La is green.
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Figure 2
SW-RIXS of various excitations for growth on SrO-terminated STO. RIXS spectra of (a) $d d$ excitations and (b) quasielastic and magnetic excitations. The $d d$ excitations have three components: $d_{x y}, d_{x z} / d_{y z}$ , and $d_{z 2}$ . The quasielastic intensity includes three components: the zero-loss or elastic line and phonon excitations, with these three components being summed to give the rocking curve (RC). The magnetic spectra are fit with two components (magnon and bimagnon), whose intensities are summed to yield the magnetic RC. (c) The experimental $d d$ -excitation RCs (data points) together with YXRO calculations (lines). (d) The experimental RCs for the magnetic and quasielastic excitations (data points) together with YXRO calculations (lines). (e) The summed weighting factors from Eq. (2) for the magnetic excitations. (f) The summed weighting factors from Eq. (2) for the quasielastic excitations.
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Figure 3
(a)–(c) Results of x-ray optical simulations of the SW effects and the depth-resolved RCs, for growth on SrO-terminated STO. (a) The depth and incidence-angle dependence of the SW electric-field intensity | $E (z, θ_{inc}) |^{2}$ . Note in particular the movement of the SW through the top two LSCO-top/LSMO-bottom interfaces. (b) The model sample profile of LSCO that is used to simulate the RCs resulting from 0 to 22.5 Å, with “delta-layer” thickness of 2.5 Å. (c) The calculated RCs for various $Δ z$ values. (d) Experimental (black curve) and calculated (blue and red curves) soft x-ray reflectivity. (e) Experimental (black dots) and calculated second derivative of the reflectivity data. The blue curves in (d) and (e) are for the STEM-determined sample configuration, allowing fully for the nonuniformity of some bilayer thicknesses, and the red curve in (d) is for the ideal sample configuration. (see Supplemental Material [18]).
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