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Gain-driven magnon-polariton dynamics in the ultrastrong coupling regime: Effective circuit approach for coherence versus nonlinearity

Ryunosuke Suzuki1,2,*, Takahiro Chiba1,2,†, and Hiroaki Matsueda1,3

  • 1Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan
  • 2Department of Information Science and Technology, Graduate School of Science and Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan
  • 3Center for Science and Innovation in Spintronics, Tohoku University, Sendai 980-8577, Japan

  • *Contact author: suzuki.ryunosuke.r4@dc.tohoku.ac.jp
  • †Contact author: t.chiba@yz.yamagata-u.ac.jp
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Phys. Rev. B 113, 024412 – Published 13 January, 2026

DOI: https://doi.org/10.1103/q418-bxr6

Abstract

We theoretically study the dynamics of gain-driven magnon-polaritons (MPs), which characterizes auto-oscillation of MPs, across the strong coupling (SC) and ultrastrong coupling (USC) regimes. Taking into account the magnon dynamics via the magnetic flux, we present an effective circuit model of gain-driven MPs, which allows us to manipulate the coupling strength of MPs by tuning the size of a ferromagnet and incorporates the self-Kerr nonlinearity of magnons due to the shape magnetic anisotropy. In the SC regime, we find that the self-Kerr nonlinearity generates a frequency shift and reduces the coherent magnon-photon coupling. In contrast, in the USC regime, we find that the coherent magnon-photon coupling not only overcomes the self-Kerr nonlinearity but also effectively couples to gain via the imaginary part of complex eigenfrequencies, resulting in magnonlike auto-oscillations. Subsequently, the USC enables one to widely tune the auto-oscillation frequency by means of an external magnetic field. These findings indicate that there is a trade-off relation between the coupling strength of MPs and the self-Kerr nonlinearity of magnons. This work contributes to the understanding of the interplay between gain-loss and USC in nonlinear polariton dynamics, offering a novel principle for frequency-tunable maserlike devices based on gain-driven MPs.

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Phys. Rev. B 113, 024412– Published 13 January, 2026
  • Received 28 July 2025
  • Revised 12 December 2025
  • Accepted 18 December 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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