Evidence for spin-triplet superconducting correlations in metal-oxide heterostructures with noncollinear magnetization

Yu. N. Khaydukov, G. A. Ovsyannikov, A. E. Sheyerman, K. Y. Constantinian, L. Mustafa, T. Keller, M. A. Uribe-Laverde, Yu. V. Kislinskii, A. V. Shadrin, A. Kalaboukhov, B. Keimer, and D. Winkler

Phys. Rev. B 90, 035130 – Published 22 July 2014

Abstract

Heterostructures composed of ferromagnetic ${La}_{0.7} {Sr}_{0.3}$ ${MnO}_{3}$ , ferromagnetic ${SrRuO}_{3}$ , and superconducting ${YBa}_{2}$ ${Cu}_{3} O_{6 + x}$ were studied experimentally. Structures of composition $Au / {La}_{0.7} {Sr}_{0.3}$ ${MnO}_{3} / {SrRuO}_{3} / {YBa}_{2}$ ${Cu}_{3} O_{6 + x}$ were prepared by pulsed laser deposition, and their high quality was confirmed by x-ray diffraction and reflectometry. A noncollinear magnetic state of the heterostructures was revealed by means of superconducting quantum interference device magnetometry and polarized neutron reflectometry. We have further observed superconducting currents in mesa structures fabricated by deposition of a second superconducting Nb layer on top of the heterostructure, followed by patterning with photolithography and ion-beam etching. Josephson effects observed in these mesa structures can be explained by the penetration of a triplet component of the superconducting order parameter into the magnetic layers.

Received 20 June 2014

DOI:https://doi.org/10.1103/PhysRevB.90.035130

Authors & Affiliations

Yu. N. Khaydukov^1,2,3, G. A. Ovsyannikov^4,5, A. E. Sheyerman⁴, K. Y. Constantinian⁴, L. Mustafa¹, T. Keller^1,2, M. A. Uribe-Laverde⁶, Yu. V. Kislinskii⁴, A. V. Shadrin^4,5, A. Kalaboukhov^3,5, B. Keimer¹, and D. Winkler⁵

¹Max-Planck Institute for Solid State Research, D-70569, Stuttgart, Germany
²Max Planck Society Outstation at FRM-II, D-85747 Garching, Germany
³Skobeltsyn Institute of Nuclear Physics, Moscow State University, 119991 Moscow, Russia
⁴Kotel'nikov Institute of Radio Engineering and Electronics of Russian Academy of Sciences, 125009, Moscow, Russia
⁵Chalmers University of Technology, Department of Microtechnology and Nanoscience, Gothenburg, S-41296, Sweden
⁶University of Fribourg, Department of Physics, and Fribourg Centre for Nanomaterials, CH-1700 Fribourg, Switzerland

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Vol. 90, Iss. 3 — 15 July 2014

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Images

Figure 1
(a) X-ray 2θ–ω scan for the heterostructure Au/LSMO/SRO/YBCO deposited on a (001) ${LaAlO}_{3}$ substrate. (b) X-ray experimental (dots) and model (solid line) reflectivities for the same sample. Inset: X-ray SLD profile corresponding to the model curve.
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Figure 2
(a) Temperature dependence of the magnetic moment along magnetic field of Au/LSMO/SRO/YBCO/LAO heterostructure at magnetic field $H$ = 100 Oe applied parallel to the sample surface. Solid line shows the theoretical dependence for the LSMO layer within the mean-field approximation. (b) The in-plane hysteresis loop for the two temperatures. (c) Family of the $m_{⊥}$ $(T)$ temperature dependencies measured at different magnetic fields applied normal to the sample surface. (d) The hysteresis loop measured at $T$ = 100 K for magnetic field applied normal to the sample surface.
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Figure 3
(a) Experimental (dots) reflectivity curves measured at $T$ = 80 K and $H$ = 30 Oe. Model reflectivity curves are shown by solid lines. The vertical arrow shows the center of the waveguide peak. Inset: The SLD depth profiles correspondent to the model reflectivities (solid lines). Dashed lines show the density of spin-up $η^{+}$ and spin-down $η^{-}$ neutrons at the waveguide mode calculated for the correspondent SLD profiles. Note that $η^{-}$ is multiplied by factor of 4. (b) Integral of the waveguide spin-flip peak as a function of magnetic field measured at $T$ = 10 K. Inset: Sketch of the vector magnetic profile of the LSMO/SRO magnetic subsystem. Vector of magnetization of the LSMO layer lies in plane and subtends the angle $α_{LSMO}$ with the external field. Vector of magnetization of the SRO layer is inclined by the angle γ with respect to the sample plane. In-plane component $M_{SRO}$ is tilted by the angle $α_{SRO}$ with respect to the external field. (d) The field dependence of the $M_{LSMO}$ (black) and $M_{SRO}$ (red). (e) The field dependence of $α_{LSMO}$ (black), $α_{SRO}$ (red), and γ (green).
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Figure 4
(a) Temperature dependence of resistance $R$ $(T)$ of mesa structure with $d_{SRO}$ = 10 nm and $d_{LSMO}$ = 9 nm. Inset: $R$ $(T)$ dependence in the vicinity of transition to superconducting state of Nb-Au electrode. (b) Temperature dependence of critical current for the same sample.
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Figure 5
Dependence of the critical current density $j_{C}$ measured at $T$ = 4.2 K on $d_{LSMO}$ at $d_{SRO}$ = 8.2 nm (black dots) and $d_{SRO}$ = 5.0 nm (red circles). For each point the data were obtained by averaging over several samples with different surface areas.
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Figure 6
The critical current (circles) and amplitudes of the first (triangles) Shapiro steps as functions of the normalized amplitude of microwave signal for the sample with $d_{SRO}$ = 10 nm and $d_{LSMO}$ = 9 nm. A fit (lines) to an expression calculated within the modified RSJ model yields 25% as the fraction of the second harmonic in the current-phase relation.
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