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Hallmarks of Ballistic Terahertz Magnon Currents in an Antiferromagnetic Insulator

Hongsong Qiu1,2,3,§, Oliver Franke2,§, Yuanzhe Tian3,§, Zdeněk Kašpar2, Reza Rouzegar2, Oliver Gueckstock2, Ji Wu1, Maguang Zhu4, Biaobing Jin5 et al.

Yongbing Xu1, Tom S. Seifert2, Di Wu3,*, Piet W. Brouwer2,†, and Tobias Kampfrath2,‡

  • 1State Key Laboratory of Spintronics Devices and Technologies, School of Integrated Circuits, Nanjing University, Suzhou 215163, People’s Republic of China
  • 2Department of Physics, Freie Universität Berlin, Berlin 14195, Germany
  • 3National Laboratory of Solid-State Microstructures, Jiangsu Provincial Key Laboratory for Nanotechnology, Collaborative Innovation Center of Advanced Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China
  • 4School of Integrated Circuits, Nanjing University, Suzhou 215163, People’s Republic of China
  • 5Research Institute of Superconductor Electronics, School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, People’s Republic of China

  • *Contact author: dwu@nju.edu.cn
  • †Contact author: brouwer@physik.fu-berlin.de
  • ‡Contact author: tobias.kampfrath@fu-berlin.de
  • §These authors contributed equally to this work.
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Phys. Rev. Lett. 135, 176703 – Published 22 October, 2025

DOI: https://doi.org/10.1103/mk8h-yzkj

Abstract

Efficient transport of spin angular momentum is expected to play a crucial role in future spintronic devices, which will potentially operate at frequencies reaching the terahertz range. On the other hand, antiferromagnetic insulators exhibit significant potential for facilitating ultrafast pure spin currents by terahertz magnons. Consequently, we here use femtosecond laser pulses to trigger ultrafast spin currents across thin films of the model antiferromagnet NiO in Py|NiO|Pt stacks, where permalloy (Py) and Pt serve as spin-current source and detector, respectively. We find that the spin-current pulses traversing NiO reach a velocity up to 40  nm/ps and experience increasing attenuation, delay, and broadening as the NiO thickness is increased. We can consistently explain our observations by ballistic transport of incoherent magnons. The relaxation length of 20 nm is 1 order of magnitude larger than for ballistic conduction-electron spin transport in metals and, thus, relevant for potential applications. Methodologically, our all-optical and contact-free spin-conductance approach has large potential to characterize terahertz magnon transport in insulators with any kind of magnetic order.

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Phys. Rev. Lett. 135, 176703– Published 22 October, 2025
  • Received 25 May 2025
  • Revised 1 October 2025
  • Accepted 3 October 2025
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