Observation of the Nonreciprocal Magnon Hanle Effect

Janine Gückelhorn, Sebastián de-la-Peña, Monika Scheufele, Matthias Grammer, Matthias Opel, Stephan Geprägs, Juan Carlos Cuevas, Rudolf Gross, Hans Huebl, Akashdeep Kamra, and Matthias Althammer

Phys. Rev. Lett. 130, 216703 – Published 24 May 2023

Abstract

The precession of magnon pseudospin about the equilibrium pseudofield, the latter capturing the nature of magnonic eigenexcitations in an antiferromagnet, gives rise to the magnon Hanle effect. Its realization via electrically injected and detected spin transport in an antiferromagnetic insulator demonstrates its high potential for devices and as a convenient probe for magnon eigenmodes and the underlying spin interactions in the antiferromagnet. Here, we observe a nonreciprocity in the Hanle signal measured in hematite using two spatially separated platinum electrodes as spin injector or detector. Interchanging their roles was found to alter the detected magnon spin signal. The recorded difference depends on the applied magnetic field and reverses sign when the signal passes its nominal maximum at the so-called compensation field. We explain these observations in terms of a spin transport direction-dependent pseudofield. The latter leads to a nonreciprocity, which is found to be controllable via the applied magnetic field. The observed nonreciprocal response in the readily available hematite films opens interesting opportunities for realizing exotic physics predicted so far only for antiferromagnets with special crystal structures.

Received 16 September 2022
Revised 14 December 2022
Accepted 18 April 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.216703

Physics Subject Headings (PhySH)

Magnetism Magnetotransport Magnons Spin Hall effect Spin current Spin diffusion Spin dynamics Spintronics

Antiferromagnets

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Janine Gückelhorn ^1,2,*, Sebastián de-la-Peña ³, Monika Scheufele^1,2, Matthias Grammer^1,2, Matthias Opel ¹, Stephan Geprägs ¹, Juan Carlos Cuevas ³, Rudolf Gross ^1,2,4, Hans Huebl ^1,2,4, Akashdeep Kamra ^3,†, and Matthias Althammer ^1,2,‡

¹Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, D-85748 Garching, Germany
²Technische Universität München, TUM School of Natural Sciences, Physik-Department, D-85748 Garching, Germany
³Condensed Matter Physics Center (IFIMAC) and Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
⁴Munich Center for Quantum Science and Technology (MCQST), D-80799 München, Germany

^*janine.gueckelhorn@wmi.badw.de
^†akashdeep.kamra@uam.es
^‡matthias.althammer@wmi.badw.de

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Vol. 130, Iss. 21 — 26 May 2023

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Images

Figure 1
Schematic depiction of magnon spin and pseudospin transport in an antiferromagnetic insulator (AFI) [30, 31], shown in blue. Two normal metal (NM) leads act as injector (dark gray) and detector (light gray) of magnonic spin current, which corresponds to the pseudospin $z$ component in the AFI. Because of easy-plane anisotropy in the studied AFI, the pseudofield is directed along $\hat{x}$ and the pseudospin precesses about this direction as the magnons propagate from injector to detector. Because of slightly different pseudofields in the forward (upper panels) and backward (lower panels) propagation directions, there is a difference $μ_{asym}$ in the observed magnon signal ( $\propto μ_{s z}$ ) which is (a) positive, (b) negative, and (c) zero for the corresponding average pseudofield $ω$ . This sign change in the difference is qualitatively understood from the different pseudofield precession rates as captured in the depiction here.
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Figure 2
(a) Sketch of the sample configuration for the forward (left panel) and backward (right panel) transport directions, the electrical wiring scheme, and the coordinate system with the in-plane rotation angle $φ$ of the applied magnetic field $μ_{0} H$ . The corresponding net magnetization $M_{net}$ is aligned along the applied magnetic field $μ_{0} H$ ( $M_{net} ∥ H$ ), while the Néel order parameter $n ⊥ H$ . (b) Angle dependence of the electrically induced magnon spin signal $R^{el} \propto μ_{sz}$ measured at $T = 250 K$ for a center-to-center distance of $d = 1.2 μ m$ and different magnetic field magnitudes for the 89 nm thick hematite film. The full and open circles, respectively, depict the measured signal in the forward and backward transport configurations. A constant offset arising from the experimental setup has been subtracted from the curves. (c) Symmetric part of the two measurement configurations for the same magnetic fields as in panel (b). The lines are fits to the expected $Δ R_{sym}^{el} \sin^{2} (φ)$ function [30, 32]. (d) Antisymmetric part of the respective curves in panel (b). The lines represent a $Δ R_{asym}^{el} \sin^{3} (φ)$ fit, indicating a $\sin φ$ dependence of the pseudofield nonreciprocity $δ ω$ as discussed in the text.
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Figure 3
Magnetic field dependence of the symmetric and antisymmetric magnon spin signal $Δ R_{sym}^{el} \propto μ_{sym}$ (black dots) and $Δ R_{asym}^{el} \propto μ_{asym}$ (red dots), extracted from the data shown in Figs. 2 and 2, respectively. The error bars have been extracted from the fits to the angle dependencies in Figs. 2 and 2. The dashed lines are fits to Eq. (2).
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