Magnonics is a research field complementary to spintronics, in which the quanta of spin waves (magnons) replace electrons as information carriers, promising lower dissipation1,2,3. The development of ultrafast, nanoscale magnonic logic circuits calls for new tools and materials to generate coherent spin waves with frequencies as high and wavelengths as short as possible4,5. Antiferromagnets can host spin waves at terahertz frequencies and are therefore seen as a future platform for the fastest and least dissipative transfer of information6,7,8,9,10,11. However, the generation of short-wavelength coherent propagating magnons in antiferromagnets has so far remained elusive. Here we report the efficient emission and detection of a nanometre-scale wavepacket of coherent propagating magnons in the antiferromagnetic oxide dysprosium orthoferrite using ultrashort pulses of light. The subwavelength confinement of the laser field due to large absorption creates a strongly non-uniform spin excitation profile, enabling the propagation of a broadband continuum of coherent terahertz spin waves. The wavepacket contains magnons with a shortest detected wavelength of 125 nm that propagate into the material with supersonic velocities of more than 13 km s–1. This source of coherent short-wavelength spin carriers opens up new prospects for terahertz antiferromagnetic magnonics and coherence-mediated logic devices at terahertz frequencies.
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Source data for figures are publicly available at https://doi.org/10.5281/zenodo.4716539. All other data that support the findings of this paper are available from the corresponding authors upon request.
The code used to simulate the magnon dynamics is available upon reasonable request.
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We thank V. V. Kruglyak and R. Rejali for critically reading the manuscript and T. C. van Thiel and E. Demler for useful discussions. This work was supported by the EU through the European Research Council grant no. 677458 (AlterMateria), the Netherlands Organization for Scientific Research (NWO/OCW) as part of the Frontiers of Nanoscience programme (NanoFront), and the Veni, Vidi, Vici programme. R.V.M. and R.L. acknowledge support from the European Research Council grant no. 852050 (MAGSHAKE). R.C. acknowledges support by the project Quantox grant no. 731473, QuantERA-NET Cofund in Quantum Technologies, implemented within the EU-H2020 programme. B.A.I. acknowledges support from the National Scientific Foundation of Ukraine under grant no. 2020.02/0261.
The authors declare no competing interests.
Peer review information Nature Physics thanks Xin Fan, Markus Münzenberg and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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RR: gold retroreflector mounted on a motorized mechanical precision delay stage, OPA: optical parametric amplifier, BBO: β-barium borate crystal, WP: Wollaston Prism, D1, D2: a pair of balanced silicon photodetectors.
a,b, Time resolved polarization rotation in the transmission (a) and reflection geometry (b) following excitation at hν = 3.1 eV for different temperatures. The probe incidence angle is near-normal, with λ0 = 700 nm.
Real part n of the refractive index, as extracted using spectroscopic ellipsometry measurements.
Real-space distribution of the magnon spin deflection at different times t, after optical excitation at hν = 3.1 eV with a penetration depth of 50 nm, as determined by equation (3).
Time-resolved polarization rotation originating from a propagating magnon, as obtained in the reflection geometry. The solid line represents a best fit of a damped sine, giving a lifetime of about 85 ps. With the largest estimated group velocities vg of the measured magnons of about 13 km/s, this gives a propagation distance lc = vgτ = 1.1 μm.
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Hortensius, J.R., Afanasiev, D., Matthiesen, M. et al. Coherent spin-wave transport in an antiferromagnet. Nat. Phys. 17, 1001–1006 (2021). https://doi.org/10.1038/s41567-021-01290-4
Nature Physics (2021)