Spin current from sub-terahertz-generated antiferromagnetic magnons

Abstract

Spin dynamics in antiferromagnets has much shorter timescales than in ferromagnets, offering attractive properties for potential applications in ultrafast devices1,2,3. However, spin-current generation via antiferromagnetic resonance and simultaneous electrical detection by the inverse spin Hall effect in heavy metals have not yet been explicitly demonstrated4,5,6. Here we report sub-terahertz spin pumping in heterostructures of a uniaxial antiferromagnetic Cr2O3 crystal and a heavy metal (Pt or Ta in its β phase). At 0.240 terahertz, the antiferromagnetic resonance in Cr2O3 occurs at about 2.7 tesla, which excites only right-handed magnons. In the spin-canting state, another resonance occurs at 10.5 tesla from the precession of induced magnetic moments. Both resonances generate pure spin currents in the heterostructures, which are detected by the heavy metal as peaks or dips in the open-circuit voltage. The pure-spin-current nature of the electrically detected signals is unambiguously confirmed by the reversal of the voltage polarity observed under two conditions: when switching the detector metal from Pt to Ta, reversing the sign of the spin Hall angle7,8,9, and when flipping the magnetic-field direction, reversing the magnon chirality4,5. The temperature dependence of the electrical signals at both resonances suggests that the spin current contains both coherent and incoherent magnon contributions, which is further confirmed by measurements of the spin Seebeck effect and is well described by a phenomenological theory. These findings reveal the unique characteristics of magnon excitations in antiferromagnets and their distinctive roles in spin–charge conversion in the high-frequency regime.

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Fig. 1: AFM spin dynamics and pure spin current in an AFM/heavy-metal heterostructure.
Fig. 2: ISHE signals at the AFMR and the QFMR.
Fig. 3: Temperature dependence of ISHE signals.
Fig. 4: SSE from incoherent AFM magnons.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We acknowledge discussions with S. Zhang, W. Han, I. Barsukov, Y. Liu, T. Su and Y. Liu. Work at University of California Riverside was supported through Spins and Heat in Nanoscale Electronic Systems, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under award number SC0012670 (J.L., M.L., W.Y., M.A. and J.S.). The 0.240-THz measurements were performed at the Institute for Terahertz Science and Technology’s (ITST) Terahertz Facilities at the University of California, Santa Barbara, which have been upgraded under NSF award number DMR-1126894. Work by C.B.W., M.K. and M.S.S. was supported by NSF MCB 1617025.

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Authors

Contributions

J.S. conceived the experiments and supervised the project. J.L. and M.L. fabricated the devices for both the AFMR and SSE experiments with the help of W.Y. and M.A. J.L. and C.B.W. performed the AFMR experiments with the technical assistance of M.K. and N.A., under the supervision of M.S.S. R.C. developed the theoretical model and performed the data analysis with J.L. and P.W. All authors contributed to the writing of the manuscript.

Corresponding author

Correspondence to Jing Shi.

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The authors declare no competing interests.

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Peer review information Nature thanks Chiara Ciccarelli, Aurelien Manchon and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Crystal structure and surface morphology characterization.

a, Crystal structure of Cr2O3. The symbols and arrows indicate the Cr atoms and the spins associated with them, respectively. The coloured plane is the \((10\bar{1}0)\) plane. b, X-ray diffraction results of the Cr2O3 \((10\bar{1}0)\) single crystal. The inset shows the X-ray diffraction results of Cr2O3 \((10\bar{1}0)\) over a wide 2θ range. c, Atomic-force microscopy image of the polished surface of the Cr2O3 \((10\bar{1}0)\) single crystal.

Extended Data Fig. 2 Measurement geometry of sub-terahertz spin-pumping experiments.

a, Pt channel only: only Pt strips are wire-bonded in series. b, Ta channel only: only Ta strips are wire-bonded in series. c, Pt–Ta hybrid channel. In ac, black lines indicate conductive wires that connect the ends of the strips. H0 is an external magnetic field; h and k are the magnetic component and wavevector of the 0.240-THz microwaves, respectively. VISHE is the open-circuit voltage. The white arrows denote the c axis of Cr2O3 \((10\bar{1}0)\).

Extended Data Fig. 3 Linear microwave power dependence of ISHE signals at 10 K.

a, Field dependence of the ISHE signal at the AFMR for different microwave powers. b, Field dependence of the ISHE signal at the QFMR for different microwave powers. c, Microwave power dependence of the ISHE signal magnitude at both the AFMR and the QFMR. ΔVISHE is defined in a and b.

Extended Data Fig. 4 ISHE signal at the AFMR under negative external magnetic fields H0.

a, ISHE signal as a function of the negative magnetic field H0 at different temperatures. H0 is along the easy axis of the Cr2O3 \((10\bar{1}0)\) crystal. b, Temperature dependence of the magnitude of the ISHE signal under positive and negative magnetic fields. Inset, ISHE signal above 30 K.

Extended Data Fig. 5 ISHE signal from Pt- and Ta-only channels at 60 K.

a, ISHE signal at the AFMR for Pt (top) and Ta (bottom) channels. b, ISHE signal at the QFMR for Pt (top) and Ta (bottom) channels. The red curves are smoothed ISHE signals. At the AFMR, the ISHE signals of the Pt and Ta channels at 60 K have opposite signs to that at 5 K (Fig. 2a for Pt and Fig. 2c for Ta). By contrast, the ISHE signal for Pt and Ta at the QFMR maintains the same sign between 60 K and 5 K (Fig. 2b for Pt and Fig. 2d for Ta), which is expected because both coherent and incoherent magnons have the same chirality in the QFMR mode. At and above 60 K, the QFMR voltage signal shows a single Lorentzian peak with a slightly larger linewidth than that of the AFMR peak.

Extended Data Fig. 6 SSE signal at 9.9 K in Cr2O3 (100 nm)/Pt and Cr2O3(100 nm)/Ta heterostructures.

a, Cr2O3 (100 nm)/Pt heterostructure. b, Cr2O3(100 nm)/Ta heterostructure. The Cr2O3 is a \((11\bar{2}0)\)-oriented epitaxial thin film deposited on an Al2O3 \((11\bar{2}0)\) substrate. The magnetic field is applied along the c axis of Cr2O3. The SSE signal changes sign across the spin-flop transition, which further confirms that LH magnons (dominating the SSE below the spin-flop transition) and RH magnons (dominating the SSE above the spin-flop transition) carry opposite angular momenta.

Extended Data Fig. 7 SSE signal in bulk Cr2O3\({\boldsymbol{(}}{\bf{10}}\bar{{\bf{1}}}{\bf{0}}{\boldsymbol{)}}\)/Pt.

a, b, Results are shown for bulk Cr2O3\((10\bar{1}0)\)/Pt with untreated (a) and etched (b) interfaces. For the untreated sample, we anneal the crystal in air at 600 °C for 2 h using a tube furnace before the deposition of the Pt layer. For the etched sample, we first bombard the surface of the Cr2O3 crystal with argon ions using inductively coupled plasma, and then anneal it in air at 600 °C for 2 h using a tube furnace before we deposit the Pt layer. The etching process does not affect the sign of the SSE signal above the spin-flop transition; however, it changes its sign below the spin-flop transition. A possible reason is that the etching process may produce some uncompensated magnetic moments at the interface owing to the different sputtering yields of Cr and O atoms, and these uncompensated magnetic moments also contribute to the SSE signal by modifying the interfacial spin-mixing conductance or directly generating additional spin current. In addition, the etched sample generates a much lower SSE signal than the untreated sample under the same measurement conditions.

Extended Data Fig. 8 Schematic illustration of device used for theoretical modelling and numerical results of ξ(T).

a, Schematic device geometry used to solve the spin diffusion equation of non-equilibrium incoherent magnons (equation (S9) in Supplementary Information). The bilayer structure is represented by an AFM layer and a non-magnetic (NM) metal layer of thickness d and dN, respectively. b, Numerical plot and fittings of ξ(T). Black dots are numerical calculations based on equation (S13) in Supplementary Information. Red and blue dashed lines are power-law fittings for T > 2.3 K and T < 2.3 K, respectively.

Supplementary information

Supplementary Information

This file contains Supplementary Notes I–VI, including Supplementary Figures 1–9, Supplementary Table 1 and Supplementary References.

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Li, J., Wilson, C.B., Cheng, R. et al. Spin current from sub-terahertz-generated antiferromagnetic magnons. Nature 578, 70–74 (2020). https://doi.org/10.1038/s41586-020-1950-4

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