Ferroelectric Control of Interface Spin Filtering in Multiferroic Tunnel Junctions

J. Tornos, F. Gallego, S. Valencia, Y. H. Liu, V. Rouco, V. Lauter, R. Abrudan, C. Luo, H. Ryll, Q. Wang, D. Hernandez-Martin, G. Orfila, M. Cabero, F. Cuellar, D. Arias, F. J. Mompean, M. Garcia-Hernandez, F. Radu, T. R. Charlton, A. Rivera-Calzada, Z. Sefrioui, S. G. E. te Velthuis, C. Leon, and J. Santamaria

Phys. Rev. Lett. 122, 037601 – Published 22 January 2019

Abstract

The electronic reconstruction occurring at oxide interfaces may be the source of interesting device concepts for future oxide electronics. Among oxide devices, multiferroic tunnel junctions are being actively investigated as they offer the possibility to modulate the junction current by independently controlling the switching of the magnetization of the electrodes and of the ferroelectric polarization of the barrier. In this Letter, we show that the spin reconstruction at the interfaces of a ${La}_{0.7} {Sr}_{0.3} {MnO}_{3} / {BaTiO}_{3} / {La}_{0.7} {Sr}_{0.3} {MnO}_{3}$ multiferroic tunnel junction is the origin of a spin filtering functionality that can be turned on and off by reversing the ferroelectric polarization. The ferroelectrically controlled interface spin filter enables a giant electrical modulation of the tunneling magnetoresistance between values of 10% and 1000%, which could inspire device concepts in oxides-based low dissipation spintronics.

Received 30 March 2018
Revised 27 July 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.037601

Physics Subject Headings (PhySH)

Exchange interaction Ferroelectricity Spin filtering Surface & interfacial phenomena

Complex oxides

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

J. Tornos^1,*, F. Gallego^1,*, S. Valencia², Y. H. Liu^3,4, V. Rouco⁵, V. Lauter³, R. Abrudan^2,6, C. Luo^2,7, H. Ryll², Q. Wang⁴, D. Hernandez-Martin¹, G. Orfila¹, M. Cabero¹, F. Cuellar¹, D. Arias^1,†, F. J. Mompean^8,9, M. Garcia-Hernandez^8,9, F. Radu², T. R. Charlton¹⁰, A. Rivera-Calzada^1,9, Z. Sefrioui^1,9,11, S. G. E. te Velthuis⁴, C. Leon^1,9,11, and J. Santamaria^1,9,11

¹GFMC, Universidad Complutense de Madrid, 28040 Madrid, Spain
²Hemholtz-Zentrum Berlin für Materialen und Energie, Albert-Einstein-Strasse 15, 12489 Berlin, Germany
³Oak Ridge National Laboratory, Neutron Scattering Division, Oak Ridge, Tennessee 37831, USA
⁴Argonne National Laboratory, Materials Science Division, Argonne, Illinois 60439, USA
⁵Unité Mixte de Physique, CNRS, Thales, Université Paris-Saclay, 91767 Palaiseau, France
⁶Institut für Experimentalphysik (Festkörperphysik), Ruhr-Universität Bochum, 44780 Bochum, Germany
⁷University of Regensburg, Universitätsstrasse 31, 93053 Regensburg, Germany
⁸2D-Foundry Group, Instituto de Ciencia de Materiales de Madrid ICMM-CSIC, 28049 Madrid, Spain
⁹Laboratorio de Heteroestructuras con aplicación en spintrónica, Unidad Asociada UCM/CSIC, 28049 Madrid, Spain
¹⁰ISIS, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, United Kingdom
¹¹GFMC, Instituto de Magnetismo Aplicado, Universidad Complutense de Madrid, 28040 Madrid, Spain

^*These authors contributed equally to this work.
^†On leave from Universidad del Quindio, Armenia, Colombia.

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Vol. 122, Iss. 3 — 25 January 2019

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Images

Figure 1
(Main) Hysteretic I–V curves of a $LSMO / BTO / LSMO$ tunnel junction measured at 120 K and up to $\pm 5 V$ bias voltages to ensure the switching of the ferroelectric polarization [marked with black (up) and red (down) arrows]. The sketch illustrates the sequence and thicknesses of the individual layers of the device. (Bottom inset) Electroresistance loops measured at 10 mV after exciting with the continuous voltages displayed in the $x$ axis.
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Figure 2
(a) Layer sequence of the samples used for the x-ray absorption and neutron reflectometry experiment. (b) XRMS hysteresis loops measured at Mn (641.4 eV) and Ti (464.6 eV) selected energies at the $L_{2, 3}$ edge of a LSMOtop $(3 nm) / BTO (4 nm) / LSMObot$ (10 nm) sample. Both loops were measured at 10 K with magnetic field applied in the [100] direction. (c) Polarized neutron reflectometry spectra at 10 K and an applied field of $1 T$ of a LSMOtop (8 nm)/BTO (4 nm)/LSMObot (25 nm) sample. (d) The depth profile of the nuclear scattering length density (nuc. sld) and magnetization (Mx) that corresponds to the fit (lines) of the data (markers). No intentional polarization state was set, although samples displayed a preferred polarization down state.
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Figure 3
Resistance ( $R$ ) vs magnetic field ( $H$ ), $R (H)$ loops measured at 14 K with ferroelectric polarization pointing down (a) and ferroelectric polarization pointing up (b). Notice the semilogarithmic scale used due to the large values of the TMR for ferroelectric polarization pointing up. Bias voltages are 0.1 (red), 0.3 (green), 0.5 (blue), 0.8 (magenta) and 1 V (olive) in (a) and 0.05 (black), 0.1 (red), 0.3 (green), and 0.5 V (blue) in (b). (Insets) Dependence of the TMR with voltage for both orientations of the ferroelectric polarization.
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Figure 4
Temperature dependence of the tunneling magnetoresistance TMR measured at 100 mV voltage for ferroelectric polarization pointing up (black symbols) and down (red symbols). Circles and triangles correspond to two different samples. Lines are fits to the spin filtering model (see text). Small panels display the bias dependence of the TMR at selected temperatures as obtained from I–V curves measured in parallel and antiparallel states. Notice the monotonic bias dependence for polarization pointing up and the nonmonotonic dependence for polarization pointing down.
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Figure 5
(a) Orbitals scheme of the manganite-titanate bonding at the interface. We do not show electrons in the $t 2 g$ orbitals of the manganite. For clarity we have only illustrated bonding at the right interface. The unpolarized $d x z$ and $d y z$ bands hybridize with spin up and down $t 2 g$ band splitted by the exchange interaction of the manganite. The thick gray line shows the profile of the spin up conduction band in the $z$ direction relevant for the tunneling transport in the device. Sketches illustrating the alternative electron doping of bottom (b) and top interfaces (d) due to the simultaneous switching of ferroelectric polarization and oxygen vacancies. The Ti magnetism occurring preferentially at the bottom interface has a negative spin filtering functionality (c) not occurring at the top interface (e).
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