Ka Shen

Non-Hermitian systems exhibit unique properties, with exceptional points (EPs) serving as a prominent example of singular spectral degeneracy that has attracted broad interest. This interest now encompasses magnon-polariton platforms. Here, we disclose a fundamentally distinct picture in a strongly driven magnon-polariton Floquet system: a pseudo-exceptional point (PEP). Analytical derivation and macrospin simulation reveal that both the mode's frequency and its decay rate remain finite on both sides of the PEP---a definitive departure from the characteristic eigenvalue splitting of a conventional EP. Crucially, we demonstrate that the key spectral features, particularly in the weak-driving limit, are governed by the damping coefficient, confirming the non-Hermitian origin of the phenomenon. This picture originates from the Floquet dynamics of a single driven mode and provides a natural mechanism for key nonlinear phenomena, such as frequency combs. Our work clarifies the underlying physics of non-Hermitian critical behavior in driven-dissipative spin systems and establishes the PEP as a framework, offering a robust and designable route to generating nonlinear phenomena in Floquet-engineered systems.

A recent study reporting interferencelike patterns in nonlocal spin-wave measurements on hematite [Sheng et al., Control of spin currents by magnon interference in a canted antiferromagnet, Nat. Phys. 21, 740 (2025).] suggested the existence of a hidden ultrafast spin-wave mode at tens of gigahertz. This hidden mode lies beyond the scope of conventional spin-wave theory in hematite, and its origin remains unexplained. In this Letter, we show that two mechanisms---crosstalk-induced spin-orbit torque and the antenna-generated Oersted field---quantitatively account for all observed interferencelike features, eliminating the need for a hypothetical hidden mode. We find that the spin-orbit torque generates both out-of-plane local magnetization dynamics and in-plane polarized propagating spin waves. While the former, combined with the antenna-generated propagating spin waves, leads to a frequency-dependent sign in the detected spin pumping signal, the latter gives rise to a finite Brillouin light scattering signal at twice the microwave frequency. Meanwhile, the Oersted field, which possesses both in-plane and out-of-plane components, produces the spatial oscillation profile previously observed in Brillouin light scattering at the excitation frequency. Such a spatial modulation is found to persist even in the absence of the crosstalk-induced spin-orbit torque, indicating that it arises from a conventional excitation effect. Our results not only elucidate the roles of spin-orbit torque and Oersted field in the magnetization dynamics of easy-plane antiferromagnets but also highlight the significance of crosstalk effects in nonlocal spin-wave measurements using microwave excitation and heavy-metal detectors.

Magnons with momentum-dependent chirality are a key signature of altermagnets. We identify bicircular light as a smoking-gun optical probe for chiral altermagnetic magnons, selectively targeting their quantum geometry induced by an alternation of magnonic chirality. We show that in d -wave altermagnets, under a canting magnetic field, the altermagnetic magnons realize a nontrivial quantum geometry, resulting in an enhancement of the nonlinear second-order light-magnon interactions. We find that the scattering of bicircular pulses probes the present magnonic quantum geometry, even if the magnonic topology is trivial. Hence, our findings establish bicircular Raman response as an optical effect of choice to identify altermagnetic magnons. As such, we propose a universal experimental protocol to distinguish altermagnets from antiferromagnets by detecting their magnon chirality patterns with light, independently of the underlying magnon topology.

The hybridization between ferrimagnetic magnons of different chiralities holds potentials for applications in the design of magnonic devices. Whereas the nutation mode observed in the ferromagnetic film also possesses opposite chirality to precessional magnon mode, in the present work, we study the interplay of the nutation modes and chiral magnons in ferrimagnets with both parallel and antiparallel magnetic configurations based on the inertial Landau-Lifshitz-Gilbert equation. While the two configurations show some general features, such as the individual excitation of the nutation modes and the repulsion between nutation modes and conventional modes, the antiparallel configuration also exhibits a hybridization between two nutation modes and distinct dissipation behaviors, compared to the parallel configuration. A theoretical mode analysis was applied based on a perturbation approach, which nicely reproduces the observations in the simulation and clarifies the key role of the magnetic anisotropy in mode hybridization.

Symmetry engineering is an effective approach for generating emergent phases and quantum phenomena. In magnetic systems, the Dzyaloshinskii-Moriya (DM) interaction is essential for stabilizing chiral spin textures. The symmetry manipulation of DM vectors, described in three dimensions, could provide a strategy toward creating abundant topologically magnetic phases. Here, we have achieved breaking the rotational and mirror symmetries of the three-dimensional DM vectors in a strongly correlated ferromagnet, which were directly measured through the nonreciprocal spin-wave propagations in both in-plane and out-of-plane magnetic field geometries. Combining cryogenic magnetic force microscopy and micromagnetic simulations, we discover a bimeron phase that emerges between the spin spiral and skyrmion phases under an applied magnetic field. Such an artificially manipulated DM interaction is shown to play a critical role in the formation and evolution of the large-area bimeron lattice, a phenomenon that could be realized across a broad range of materials. Our findings demonstrate that symmetry engineering of the DM vectors can be practically achieved through epitaxial strain, paving the way for the creation of diverse spin topologies and the exploration of their emergent functionalities.

Magnetic skyrmion, one of the intriguing topological textures in magnetic materials, exhibits various excellent properties for fundamental research and realistic application. The particle-like nature of skyrmion also offers a suitable platform to study many-body effect, which is however largely unexplored. In the present work, we demonstrate interlayer skyrmion drag in partially driven magnetic multilayers through micromagnetic simulations. While a small driving current leads to a conventional drag with passive skyrmion motion following the active skyrmions, a large current is found to give rise to an anomalous drag with distinct dynamics of passive and active skyrmions. The emergence and explicit behaviors of these features are well interpreted by the specific interlayer attractive interaction between the skyrmions through long-range dipolar fields. Our results reveal the skyrmion drag dynamics in magnetic multilayers and disclose the physics behind, which should be useful for designing skyrmion-based devices.