Observation of Magnon Polarons in a Uniaxial Antiferromagnetic Insulator

Junxue Li, Haakon T. Simensen, Derek Reitz, Qiyang Sun, Wei Yuan, Chen Li, Yaroslav Tserkovnyak, Arne Brataas, and Jing Shi

Phys. Rev. Lett. 125, 217201 – Published 17 November 2020

Abstract

Magnon polarons, a type of hybridized excitations between magnons and phonons, were first reported in yttrium iron garnet as anomalies in the spin Seebeck effect responses. Here, we report an observation of antiferromagnetic (AFM) magnon polarons in a uniaxial AFM insulator ${Cr}_{2} O_{3}$ . Despite the relatively higher energy of magnon than that of the acoustic phonons, near the spin-flop transition of $\sim 6 T$ , the left-handed magnon spectrum shifts downward to hybridize with the acoustic phonons to form AFM magnon polarons, which can also be probed by the spin Seebeck effect. The spin Seebeck signal is founded to be enhanced due to the magnon polarons at low temperatures.

Received 17 May 2020
Revised 2 August 2020
Accepted 16 October 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.217201

Physics Subject Headings (PhySH)

Magnons Spin Seebeck effect Spin current Spintronics

Antiferromagnets

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Junxue Li¹, Haakon T. Simensen ², Derek Reitz ³, Qiyang Sun ⁴, Wei Yuan¹, Chen Li⁴, Yaroslav Tserkovnyak³, Arne Brataas², and Jing Shi ¹

¹Department of Physics and Astronomy, University of California, Riverside, California 92521, USA
²Center for Quantum Spintronics, Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
³Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA
⁴Materials Science and Engineering/Department of Mechanical Engineering, University of California, Riverside, California 92521, USA

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Vol. 125, Iss. 21 — 20 November 2020

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Images

Figure 1
Magnon-phonon hybridization in a uniaxial antiferromagnet. (a) Magnon energy of ${Cr}_{2} O_{3}$ at $k = 0$ vs magnetic field along the easy axis of ${Cr}_{2} O_{3}$ . SF denotes the spin-flop transition. $m - i$ ( $i = 1$ , 2,3,4) indicates AFM magnon mode $i$ . (b) Four different AFM magnon modes under magnetic field $μ_{0} H$ . $m_{1}$ and $m_{2}$ are the magnetizations of the two spin sublattices. modes 1 and 3 are the precessing modes with the right-hand (RH) chirality, whereas mode 2 is with the left-hand (LH) chirality, and mode 4 is linearly polarized. $σ_{RH}$ and $σ_{LH}$ are the spin polarizations associated with the RH and LH chiralities, respectively. (c) The magnon and acoustic phonon dispersions of ${Cr}_{2} O_{3}$ before SF. $k$ is perpendicular to the ( $10 \bar{1} 0$ ) plane. The black curve is the magnon dispersion at zero magnetic field, the red (blue) solid curve is the dispersion of magnon mode 1 (mode 2) at 5 T. Magnon mode 2 starts to hybridize with the acoustic phonons at high magnetic fields, as indicated by the thick yellow line segment. (d) Schematic real-space diagram of magnon-polaron in a uniaxial antiferromagnet.
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Figure 2
Magnon-polaron SSE signals in ${Cr}_{2} O_{3}$ –heavy-metal heterostructures at low temperatures. (a) and (b) Device schematics for SSE measurements in ${Cr}_{2} O_{3} (10 \bar{1} 0) / Ta (5 nm)$ and ${Cr}_{2} O_{3} (10 \bar{1} 0) / Pt (5 nm)$ , respectively. External field $μ_{0} H$ is along the $c$ axis. (c) and (d) SSE signals in ${Cr}_{2} O_{3} / Ta$ (at 2.2 K) and ${Cr}_{2} O_{3} / Pt$ (at 2.4 K), respectively. The SSE signal is normalized to heating power $P$ . The red curves in (c) and (d) are fitting results using Eq. (S2a) in Supplemental Material, Note 6 [41]. (e) Magnon-polaron signals $V_{MP} / P$ in ${Cr}_{2} O_{3} / Ta$ (at 2.2 K) and ${Cr}_{2} O_{3} / Pt$ (at 2.4 K) obtained by subtracting the background from the original SSE signals in (c) and (d). The blue (red) arrows indicate the maximum hybridization between magnon and LA (TA) phonon. (f) Enlarged dispersions under magnetic fields of 0 T, $μ_{0} H_{m, LA}$ , $μ_{0} H_{m, TA}$ . The magnon dispersion under $μ_{0} H_{m, LA}$ ( $μ_{0} H_{m, TA}$ ) becomes tangential to the dispersion of LA (TA) phonon.
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Figure 3
Field dependence of magnon-polaron signals in ${Cr}_{2} O_{3} / Ta$ heterostructure at different temperatures. $V_{MP} / P$ curves are obtained as discussed in Supplemental Material, Note 6 [41]. The blue and red dashed lines denote the magnetic fields $\pm μ_{0} H_{m, LA}$ and $\pm μ_{0} H_{m, TA}$ [as defined in Figs. 2 and 2], respectively. The blue and red arrows in the 2.5 K panel indicate the magnitude of magnon-polaron signals for LA and TA phonon, respectively.
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Figure 4
Temperature dependence of magnon-polaron signals in ${Cr}_{2} O_{3} / Ta$ heterostructure. (a) Magnon-polaron (MP) signal $V_{MP} / P$ and SSE background signal $V_{SSE}^{bg} / P$ at $+ μ_{0} H_{m, LA}$ and $+ μ_{0} H_{m, LA}$ vs temperature. $V_{SSE}^{bg} / P$ is evaluated from the fitting results and contains no magnon-polaron contributions. (b) Temperature dependence of both LA MP and TA MP signals $V_{MP}$ normalized by the SSE background $V_{SSE}^{bg}$ . The normalization eliminates the effect of the thermal conductivity on the temperature gradient which is common to both $V_{MP} / P$ and $V_{SSE}^{bg} / P$ .
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