Observation of Antiferromagnetic Magnon Pseudospin Dynamics and the Hanle Effect

T. Wimmer, A. Kamra, J. Gückelhorn, M. Opel, S. Geprägs, R. Gross, H. Huebl, and M. Althammer

Phys. Rev. Lett. 125, 247204 – Published 10 December 2020

Abstract

We report on experiments demonstrating coherent control of magnon spin transport and pseudospin dynamics in a thin film of the antiferromagnetic insulator hematite utilizing two Pt strips for all-electrical magnon injection and detection. The measured magnon spin signal at the detector reveals an oscillation of its polarity as a function of the externally applied magnetic field. We quantitatively explain our experiments in terms of diffusive magnon transport and a coherent precession of the magnon pseudospin caused by the easy-plane anisotropy and the Dzyaloshinskii-Moriya interaction. This experimental observation can be viewed as the magnonic analog of the electronic Hanle effect and the Datta-Das transistor, unlocking the high potential of antiferromagnetic magnonics toward the realization of rich electronics-inspired phenomena.

Received 31 July 2020
Revised 2 October 2020
Accepted 9 November 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.247204

Physics Subject Headings (PhySH)

Antiferromagnetism Dzyaloshinskii-Moriya interaction Electrical generation of spin carriers Electrical spin injection Magnetism Magnetotransport Magnons Spin Hall effect Spin waves Spintronics

Antiferromagnets Thin films Ultrathin films

Transport techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

T. Wimmer^1,2,*, A. Kamra³, J. Gückelhorn^1,2, M. Opel ¹, S. Geprägs ¹, R. Gross ^1,2,4, H. Huebl ^1,2,4, and M. Althammer ^1,2,†

¹Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
²Physik-Department, Technische Universität München, 85748 Garching, Germany
³Center for Quantum Spintronics, Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
⁴Munich Center for Quantum Science and Technology (MCQST), Schellingstrasse 4, D-80799 München, Germany

^*tobias.wimmer@wmi.badw.de
^†matthias.althammer@wmi.badw.de

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Vol. 125, Iss. 24 — 11 December 2020

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Images

Figure 1
(a) Pseudospin $S$ description of magnonic excitations obtained by linear superpositions of spin-up and -down antiferromagnetic magnons that correspond to right- and left-circular precessions of the Néel vector $n$ , respectively. A pseudospin collinear with the $z$ axis corresponds to spin-up or -down magnons carrying spin $\pm 1$ . As the pseudospin rotates away from the $z$ axis, the precession of the Néel vector becomes increasingly elliptical merging into a linear oscillation for $S ∥ \hat{x}$ , corresponding to zero-spin excitations. The $z$ component of pseudospin $S_{z}$ determines the actual magnonic spin which is probed in our measurements. (b)–(d) Magnonic spin along $\hat{z} ∥ n$ is injected and detected, respectively, by the left and right HM electrodes deposited on an AFI. The pseudospin precesses with a frequency controlled by the applied magnetic field while diffusing from the injector to the detector. As a result, positive (b), zero (c), or negative (d) magnon spin is detected giving rise to an analogous behavior of the measured spin signal between the two electrodes as shown in (e). The white curves depict the theoretical model fit [Eq. (3)] to the experimental data shown via black and blue circles for devices with an injector-detector distance of $d = 950 nm$ and $d = 750 nm$ , respectively, at $T = 200 K$ .
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Figure 2
(a) Sketch of the device geometry, the electrical wiring, and the coordinate system. The canting of the magnetic sublattices $m_{1}$ and $m_{2}$ and the corresponding net moment $m_{net}$ as well as the Néel order parameter $n$ are illustrated. Upon applying a charge current $I_{inj}$ to the injector, a spin current $I_{s}$ with spin polarization $s$ is generated via the SHE and injected into the hematite ( $α − {Fe}_{2} O_{3}$ ) with thickness $t$ . The emerging antiferromagnetic magnon current is then detected via the inverse SHE-induced current at the detector by measuring the electrical voltage drop $V_{\det}^{el}$ . (b) Angle-dependent magnon spin signals $R_{\det}^{el} \propto V_{\det}^{el} / I_{inj}$ for electrically excited magnons measured at the detector for $T = 200 K$ with a center-to-center distance of $d = 750 nm$ . The white solid lines are fits to a $\sin^{2} (φ)$ -type function.
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Figure 3
(a) Electrically excited magnon spin signals $Δ R_{\det}^{el} \propto V_{\det}^{el} / I_{inj}$ for a structure with strip distance $d = 750 nm$ plotted as a function of the magnetic field for different temperatures. Light colored solid lines are fits to Eq. (3). (b) Compensation field $μ_{0} H_{c}$ versus temperature extracted from experiments with devices of varying $d$ . The temperature dependence of $μ_{0} H_{c}$ follows the temperature trend of the uniaxial anisotropy of hematite. (c) Spin diffusion length $λ_{s}$ as a function of the temperature extracted from experimental data from different devices with varying $d$ . $λ_{s}$ increases with increasing temperature for all investigated structures.
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