Transition of laser-induced terahertz spin currents from torque- to conduction-electron-mediated transport

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Transition of laser-induced terahertz spin currents from torque- to conduction-electron-mediated transport

Pilar Jiménez-Cavero, Oliver Gueckstock, Lukáš Nádvorník, Irene Lucas, Tom S. Seifert, Martin Wolf, Reza Rouzegar, Piet W. Brouwer, Sven Becker, Gerhard Jakob, Mathias Kläui, Chenyang Guo, Caihua Wan, Xiufeng Han, Zuanming Jin, Hui Zhao, Di Wu, Luis Morellón, and Tobias Kampfrath

Phys. Rev. B 105, 184408 – Published 12 May 2022

Abstract

Spin transport is crucial for future spintronic devices operating at bandwidths up to the terahertz range. In F|N thin-film stacks made of a ferromagnetic/ferrimagnetic layer $F$ and a normal-metal layer $N$ , spin transport is mediated by (1) spin-polarized conduction electrons and/or (2) torque between electron spins. To identify a crossover from (1) to (2), we study laser-driven spin currents in $F$ |Pt stacks where $F$ consists of model materials with different degrees of electrical conductivity. For the magnetic insulators yttrium iron garnet, gadolinium iron garnet (GIG) and $γ − {Fe}_{2} O_{3}$ , identical dynamics is observed. It arises from the terahertz interfacial spin Seebeck effect (SSE), is fully determined by the relaxation of the electrons in the metal layer, and provides a rough estimate of the spin-mixing conductance of the GIG/Pt and $γ − {Fe}_{2} O_{3} / Pt$ interfaces. Remarkably, in the half-metallic ferrimagnet ${Fe}_{3} O_{4}$ (magnetite), our measurements reveal two spin-current components with opposite direction. The slower, positive component exhibits SSE dynamics and is assigned to torque-type magnon excitation of the A- and B-spin sublattices of ${Fe}_{3} O_{4}$ . The faster, negative component arises from the pyrospintronic effect and can consistently be assigned to ultrafast demagnetization of minority-spin hopping electrons. This observation supports the magneto-electronic model of ${Fe}_{3} O_{4}$ . In general, our results provide a route to the contact-free separation of torque- and conduction-electron-mediated spin currents.

Received 20 November 2021
Revised 17 March 2022
Accepted 25 April 2022

DOI:https://doi.org/10.1103/PhysRevB.105.184408

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Magnetism Spin caloritronics Spin current Spintronics

Terahertz spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Pilar Jiménez-Cavero ^1,2,3,4,*, Oliver Gueckstock ^1,2,*, Lukáš Nádvorník ^1,2,5,†, Irene Lucas^3,4, Tom S. Seifert ^1,2, Martin Wolf², Reza Rouzegar^1,2, Piet W. Brouwer¹, Sven Becker ⁶, Gerhard Jakob ⁶, Mathias Kläui⁶, Chenyang Guo^7,8, Caihua Wan⁷, Xiufeng Han^7,8, Zuanming Jin^9,10, Hui Zhao¹¹, Di Wu¹¹, Luis Morellón^3,4, and Tobias Kampfrath^1,2,‡

¹Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
²Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4–6, 14195 Berlin, Germany
³Instituto de Nanociencia y Materiales de Aragón (INMA), Universidad de Zaragoza-CSIC, Mariano Esquillor, Edificio I+D, 50018 Zaragoza, Spain
⁴Departamento Física de la Materia Condensada, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
⁵Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Prague, Czech Republic
⁶Institut für Physik, Johannes Gutenberg-Universität Mainz, 55128 Mainz, Germany
⁷Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, University of Chinese Academy of Sciences, Beijing 100190, China
⁸Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
⁹Shanghai Key Lab of Modern Optical Systems, University of Shanghai for Science and Technology, Shanghai 200093, China
¹⁰Department of Physics, Shanghai University, Shanghai 200444, China
¹¹Department of Physics and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

^*These authors contributed equally to this work.
^†nadvornik@karlov.mff.cuni.cz
^‡tobias.kampfrath@fu-berlin.de

Article Text

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Supplemental Material

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References

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Issue

Vol. 105, Iss. 18 — 1 May 2022

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Images

Figure 1
(a) Schematic of photoinduced spin transport in an $F$ |Pt stack, where Pt is platinum, and $F$ is a magnetic layer with equilibrium magnetization $M_{0}$ . An ultrashort laser pulse excites the sample and induces an ultrafast spin current with density $j_{s}$ from $F$ to Pt along the $z$ axis. In the Pt layer, $j_{s}$ is converted into a transverse charge current with density $j_{c}$ that gives rise to the emission of a terahertz electromagnetic pulse. Schematics (1) and (2) indicate spin transfer across the $F$ /Pt interface that is mediated by (1) spin-polarized conduction electrons and (2) spin torque, the latter coupling to magnons in $F$ . Both flavors (1) and (2) can be driven by gradients of temperature and spin accumulation. (b) Simplified schematic of the single-electron density of states vs electron energy $ε$ of the tetrahedral A and octahedral B sites of the ferrimagnetic half-metal ${Fe}_{3} O_{4}$ . The DC conductivity predominantly arises from the B-site minority-type hopping electrons of the e-sublattice. (c) In our interpretation, optical excitation of the ${Fe}_{3} O_{4} | Pt$ stack triggers spin transfer through both the spin Seebeck effect (SSE) and pyrospintronic effect (PSE). While the SSE current is mediated by torque between Pt electrons and ${Fe}_{3} O_{4}$ electron spins far below the Fermi level $μ_{0}$ [(2) in panel (a)], the PSE transiently increases the chemical potential of the B-site minority-spin electrons around $μ_{0}$ and, thus, induces a conduction-electron spin current [(1) in panel (a)].
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Figure 2
Terahertz emission from $F$ |Pt bilayers as a function of $F$ conductivity. (a) Electrical conductivities of the studied $F$ materials on a logarithmic scale. (b) Electro-optic signals of terahertz pulses emitted from various $F$ |Pt bilayers with $F$ = YIG (thick and thin), GIG, $γ − {Fe}_{2} O_{3}, {Fe}_{3} O_{4}$ , and Fe. Note the different amplitude scalings. The time-axis origin is the same for all signals and was determined by the signal from Fe|Pt reference stacks (Fig. S1 in the Supplemental Material [76]). The dashed vertical line marks the minimum signal for the insulating $F$ materials YIG, GIG, and $γ − {Fe}_{2} O_{3}$ , and the two black arrows label a sharp feature in the traces for $F = {Fe}_{3} O_{4}$ and Fe. (c) Fourier amplitude spectra of the signals of panel (b), normalized to peak height 1. Dashed lines show two duplicates of the spectrum of $γ − {Fe}_{2} O_{3} | Pt$ . Curves in (b) and (c) are vertically offset for clarity.
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Figure 3
Ultrafast photoinduced spin transport in $F$ |Pt stacks. (a) Curves show the spin-current density $j_{s} (t)$ in magnetic-insulator|Pt and Fe|Pt stacks, i.e., YIG(3 μm)|Pt(5 nm), GIG(58 nm)|Pt(2 nm), $γ − {Fe}_{2} O_{3}$ (10 nm)|Pt(2.5 nm), and Fe(2.5 nm)|Pt(2.5 nm), as extracted from the terahertz emission signals of Fig. 2. Each signal is normalized by the pump-excitation density inside the Pt layer and by the indicated factor. (b) Spin current $j_{s} (t)$ in ${Fe}_{3} O_{4} (10 nm) | Pt (2.5 nm)$ along with scaled spin currents in $γ − {Fe}_{2} O_{3} | Pt$ and Fe|Pt. The violet arrows F1 and F2 mark characteristic features of the curves. Note that $j_{s} (t)$ in ${Fe}_{3} O_{4} | Pt$ can be well described as a linear combination of the other two spin currents (light violet curve). (c) Same as in panel (b), but for the terahertz-emission raw signals.
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