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1. Acoustic attenuation in magnetic insulator films: effects of magnon polaron formation

   https://doi.org/10.1088/1361-6463/acae30  

   Zhuang, Shihao; Hu, Jia-Mian (January 2023, Journal of Physics D: Applied Physics)

   Abstract A magnon and a phonon are the quanta of spin wave and lattice wave, respectively, and they can hybridize into a magnon polaron when their frequencies and wavenumbers match close enough the values at the exceptional point. Guided by an analytically calculated magnon polaron dispersion, dynamical phase-field simulations are performed to investigate the effects of magnon polaron formation on the attenuation of a bulk acoustic wave in a magnetic insulator film. It is shown that a stronger magnon–phonon coupling leads to a larger attenuation. The simulations also demonstrate the existence of a minimum magnon–phonon interaction time required for the magnon polaron formation, which is found to decrease with the magnetoelastic coupling coefficient but increase with the magnetic damping coefficient. These results deepen the understanding of the mechanisms of acoustic attenuation in magnetic crystals and provide insights into the design of new-concept spin interconnects that operate based on acoustically driven magnon propagation. 

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2. Observation of the Magnon Hall Magnetoresistance Effect

   https://doi.org/10.1103/gkdb-1j21  

   Mai, Shuan-Cheng; Wu, Po-Hsun; Chen, Chao-Wei; Huang, Ssu-Yen; Fan, Xin; Qu, Danru (March 2026, Physical Review Letters)

   Interconversion between charge and spin currents via spin-orbit coupling underpins spin orbitronics. Magnons, which are the quanta of spin waves, can exchange angular momentum with conduction-electron spins through spin-flip scattering, suggesting a direct route for charge-to-magnon conversion. Here, we predict that in single-layer ferromagnets, an applied electric current induces a transverse magnon current, producing electrical magnon Hall and inverse magnon Hall effects that share the symmetry of the spin Hall and inverse spin Hall effects. This effect gives rise to a magnon Hall magnetoresistance in CoFeB and NiFe, with an efficiency comparable to the spin Hall effect and a characteristic decay length on the order of micrometers, far exceeding typical electron spin diffusion lengths. By enabling the direct generation and detection of long-range magnon currents, our findings open new pathways for low-loss, on-chip spin-based logic and energy-harvesting devices. 

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3. Magnonic superradiant phase transition

   https://doi.org/10.1038/s42005-021-00785-z  

   Bamba, Motoaki; Li, Xinwei; Marquez Peraca, Nicolas; Kono, Junichiro (December 2022, Communications Physics)

   Abstract In the superradiant phase transition (SRPT), coherent light and matter fields are expected to appear spontaneously in a coupled light–matter system in thermal equilibrium. However, such an equilibrium SRPT is forbidden in the case of charge-based light–matter coupling, known as no-go theorems. Here, we show that the low-temperature phase transition of ErFeO₃at a critical temperature of approximately 4 K is an equilibrium SRPT achieved through coupling between Fe³⁺magnons and Er³⁺spins. By verifying the efficacy of our spin model using realistic parameters evaluated via terahertz magnetospectroscopy and magnetization experiments, we demonstrate that the cooperative, ultrastrong magnon–spin coupling causes the phase transition. In contrast to prior studies on laser-driven non-equilibrium SRPTs in atomic systems, the magnonic SRPT in ErFeO₃occurs in thermal equilibrium in accordance with the originally envisioned SRPT, thereby yielding a unique ground state of a hybrid system in the ultrastrong coupling regime. 

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4. Formation of binary magnon polaron in a two-dimensional artificial magneto-elastic crystal

   https://doi.org/10.1038/s41427-023-00499-4  

   Majumder, Sudip; Drobitch, J L; Bandyopadhyay, Supriyo; Barman, Anjan (December 2023, NPG Asia Materials)

   Abstract We observed strong tripartite magnon-phonon-magnon coupling in a two-dimensional periodic array of magnetostrictive nanomagnets deposited on a piezoelectric substrate, forming a 2D magnetoelastic “crystal”; the coupling occurred between two Kittel-type spin wave (magnon) modes and a (non-Kittel) magnetoelastic spin wave mode caused by a surface acoustic wave (SAW) (phonons). The strongest coupling occurred when the frequencies and wavevectors of the three modes matched, leading to perfect phase matching. We achieved this condition by carefully engineering the frequency of the SAW, the nanomagnet dimensions and the bias magnetic field that determined the frequencies of the two Kittel-type modes. The strong coupling (cooperativity factor exceeding unity) led to the formation of a new quasi-particle, called a binary magnon-polaron, accompanied by nearly complete (~100%) transfer of energy from the magnetoelastic mode to the two Kittel-type modes. This coupling phenomenon exhibited significant anisotropy since the array did not have rotational symmetry in space. The experimental observations were in good agreement with the theoretical simulations. 

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5. Dilute magnetic impurity-induced effective phonon magnetic moment in Fe-doped monolayer MoS ₂

   https://doi.org/10.1088/2053-1583/ae04fc  

   Mustafa, Hussam; Ye, Gaihua; Nnokwe, Cynthia; Fang, Mengqi; Kandil, Mohamed; Al-Mahboob, Abdullah; Wu, Kai; Stollenwerk, Andrew James; Kidd, Timothy E; Shand, Paul M; et al (September 2025, 2D Materials)

   Abstract Realization of large effective phonon magnetic moment in monolayer MoS₂has established an important route for exploring intriguing magnetic phenomena in a nonmagnetic material. The sizable coupling between the orbital transition and the circularly polarized phonon results in the large effective phonon magnetic moment. In this work, using magneto-Raman spectroscopy, we investigate substitutional doping of magnetic atoms as a tuning knob of the electronic and phononic properties of MoS₂. We show that Fe-doping polarizes the spin of the conduction bands and introduces a localized Fe band underneath the conduction band. As a result, an additional orbital transition between the Mo 4dand Fe 3dstates emerges, producing an orbital-phonon hybridized mode at 283 cm⁻¹. Our magnetic field dependent measurements demonstrate that this new mode carries 2.8 ${\mu }_{\mathrm{B}}$effective phonon magnetic moment, which is comparable to that of the undoped MoS₂. Moreover, even though a long-range magnetic order is absent in Fe-doped MoS₂, the local magnetic moment of Fe modifies the nature of the spin fluctuation, producing monotonically increasing quasielastic scattering spectral weight as temperature decreases. Our results highlight two-dimensional dilute magnetic semiconductors synthesized by substitutional doping as a promising material platform to manipulate the phonon magnetic moment through orbital-phonon coupling. 

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