arXiv subjects

Hiroki Matsumoto

Publications and source records attributed to Hiroki Matsumoto.

At least 19 recordsLinked to original sources

arXiv2026

Model of the magnon Kerr effect in a highly anisotropic ferromagnet

The magnon Kerr effect is a nonlinear phenomenon that occurs universally in any ferromagnet with finite magnetic anisotropy. It manifests itself as a magnon population-dependent change in the magnon dispersion relation. Here, we derive the Hamiltonian for the magnon Kerr effect for a thin film with uniaxial anisotropy. We consider the external field, parallel, perpendicular, and at an intermediate angle to the anisotropy axis, and determine the Kerr coefficient in each case. We show how the nonlinearity scales for different magnetic materials as a function of magnon frequency and magnetic sample volume. Moreover, we derive the equations of motion for a hybrid cavity-magnon system with such thin-film magnon Kerr nonlinearity, to obtain populations of photons and magnons, and scattering matrix terms for the microwave transmission of the cavity, relevant for experimental observations.

cond-mat.mes-hall↗

arXiv2026

Disambiguating electrical detection of magnetization dynamics in magnetic insulators

Electrical detection of magnetization dynamics in magnetic insulators underpins both fundamental studies of magnon transport and the development of low-loss magnonic devices. In heavy-metal/magnetic-insulator heterostructures, spin pumping and spin-torque ferromagnetic resonance (ST-FMR) are widely used for this purpose and are often treated separately in different measurement geometries. In practice, the competition between these two effects gives rise to electrical voltage signals of opposite signs, which can lead to ambiguous interpretations of the underlying physics. Here, we show how to disambiguate their respective contributions and provide a framework for interpreting experiments involving microwave excitation of magnetic insulators and detection of magnetization dynamics via spin-charge conversion in heavy metals. We systematically investigate spin pumping and ST-FMR in nonlocal and local devices using Pt-capped thin films of thulium and bismuth-yttrium iron garnets. We show how spin-wave character, magnetic dissipation, magnetic field orientation and device geometry govern the sign and magnitude of the resulting signals. In several cases, the voltage generated by microwave excitation changes sign between an out-of-plane or in-plane magnetic configuration. We disentangle a contribution due to spin pumping, induced by exponentially decaying propagating spin waves, and a weakly distance-dependent contribution from ST-FMR, remotely induced by inductive coupling. We show that both the spin wave excitation profile across the film thickness and magnetic damping largely determine which of the two contributions dominates. Hence, the sign of the electrical signal cannot be uniquely assigned to the chirality of the magnon modes.

cond-mat.mes-hall↗

arXiv2026

Electromagnetic evanescent field associated with surface acoustic wave: Response of metallic thin films

Surface acoustic waves (SAWs), coherent vibrational modes localized at solid surfaces, have been employed to manipulate and detect electronic and magnetic states in condensed-matter systems via strain. SAWs are commonly excited in a piezoelectric material, often the substrate. In such systems, SAWs not only generate strain but also electric field at the surface. Conventional analysis of the electric field accompanying the SAW invokes the electrostatic approximation, which may fall short in fully capturing its essential characteristics by neglecting the effect of the magnetic field. Here we study the electric and magnetic fields associated with SAWs without introducing the electrostatic approximation. The plane wave solution takes the form of an evanescent field that decays along the surface normal with a phase velocity equal to the speed of sound. If a metallic film is placed on the piezoelectric substrate, a time- and space-varying electric field permeates into the film with a decay length along the film normal defined by the skin depth and the SAW wavelength. For films with high conductivity, the phase of the electric field varies along the film normal. The emergence of the evanescent field is a direct consequence of dropping the electrostatic approximation, providing a simple but critical physical interpretation of the SAW-induced electromagnetic field.

cond-mat.mes-hall↗

arXiv2026

Identifiability in Blind Source Separation through Stabilizer Shrinkage: Unifying Non-Gaussianity and Observation Diversity

Identifiability is a central issue in blind source separation (BSS), determining whether latent sources can be uniquely recovered from observed mixtures. Classical approaches address identifiability either by exploiting source non-Gaussianity via higher-order statistics (HOS) or by enriching the observation structure through temporal, spatial, or multi-channel diversity using second-order statistics (SOS), and these routes are often regarded as fundamentally different. In this paper, we revisit identifiability in BSS from a structural perspective, interpreting it as constraint-induced reduction of residual ambiguity in the mixing model. Within this framework, the observation mechanism is viewed broadly to include both input-side statistical constraints and output-side observation structures. HOS-based and SOS-based approaches are then unified as mechanisms of stabilizer shrinkage, in which observation-induced constraints reduce an initially continuous ambiguity to a finite residual one. To connect this structural viewpoint with finite-sample regimes, we introduce a Jacobian-based sensitivity probe as a numerical diagnostic of local identifiability. Numerical experiments show that increasing non-Gaussianity or observation diversity suppresses the same residual symmetry, revealing a structural trade-off between source statistics and observation design. These results provide a unified interpretation of classical BSS methods and clarify how observation constraints govern identifiability.

eess.SP↗

arXiv2026

Unfolded Laplacian Spectral Embedding: A Theoretically Grounded Approach to Dynamic Network Representation

Dynamic relational data arise in many machine learning applications, yet their evolving structure poses challenges for learning representations that remain consistent and interpretable over time. A common approach is to learn time varying node embeddings, whose usefulness depends on well defined stability properties across nodes and across time. We introduce Unfolded Laplacian Spectral Embedding (ULSE), a principled extension of unfolded adjacency spectral embedding to normalized Laplacian operators, a setting where stability guarantees have remained out of reach. We prove that ULSE satisfies both cross-sectional and longitudinal stability under a dynamic stochastic block model. Moreover, the Laplacian formulation yields a dynamic Cheeger-type inequality linking the spectrum of the unfolded normalized Laplacian to worst case conductance over time, providing structural insight into the embeddings. Empirical results on synthetic and real world dynamic networks validate the theory.

stat.ML↗

arXiv2026

Magnon Kerr effect in a magnetic thin film strongly coupled to a microwave resonator

Cavity magnonics investigates hybrid systems where magnons interact coherently with photons, providing a platform to harness light-matter interaction in magnetic materials. Progress in this field hinges on achieving stronger and tunable nonlinear effects, which are essential for controlling magnon dynamics and frequency conversion. Here, we demonstrate the magnon Kerr effect in an anisotropic magnonic system comprising a 200~nm-thick yttrium iron garnet film strongly coupled to a three-dimensional microwave resonator. The strong shape anisotropy significantly enhances the magnon Kerr effect compared to a sphere of equivalent volume, while the cavity enables sensitive probing of magnetization dynamics. We demonstrate continuous tunability of the magnitude and sign of the Kerr shift by controlling the static orientation of the magnetization. Input-output modeling of the magnon-photon interaction provides a consistent description of our system and Kerr coefficients matching the experimental results. Our findings demonstrate a scalable approach to enhancing Kerr anharmonicity in hybrid magnon-photon systems while preserving strong coupling.

cond-mat.mes-hall↗

arXiv2026

Ultrathin bismuth-yttrium iron garnet films with tunable magnetic anisotropy

We report on the epitaxial growth of nm-thick films of bismuth-substituted yttrium iron garnet (BiYIG) by high-temperature off-axis radio-frequency magnetron sputtering. We demonstrate accurate control of the magnetic properties by tuning of the sputtering parameters and epitaxial strain on various (111)-oriented garnet substrates. BiYIG films with up to -0.80\% lattice mismatch with the substrate remain fully strained up to 60~nm-thick, maintaining a high crystalline quality. Transmission electron microscopy and energy-dispersive X-ray spectroscopy confirm coherent epitaxial growth, the absence of defects, and limited interdiffusion at the BiYIG/substrate interface. Varying the tensile or compressive strain between -0.80\% and +0.56\% in BiYIG allows for accurate compensation of the total magnetic anisotropy through magneto-elastic coupling. The effective magnetic anisotropy of sputtered BiYIG films can be further tuned via the off-axis deposition angle and the oxygen flow during growth, which determine the cation stoichiometry. Under optimized growth conditions, a ferromagnetic resonance (FMR) linewidth of 1~mT at 10~GHz is reliably obtained even for thicknesses as low as 10~nm. We also report small FMR linewidths in ultrathin (2-5~nm) BiYIG films grown on diamagnetic substrate yttrium scandium gallium garnet. These findings highlight the promise of low-damping, strain-engineered nm-thick BiYIG films for implementing advanced functionalities in spin-orbitronic and magnonic devices. Specifically, the magnetic-anisotropy compensation and low damping enable large cone-angle magnetization dynamics immune to magnon-magnon nonlinear scattering.

cond-mat.mtrl-sci↗

arXiv2025

Acoustic orbital Hall effect and orbital pumping in light-metal-ferromagnet bilayers

Orbital currents provide a new degree of freedom for controlling magnetism, yet their interaction with lattice dynamics remains largely unexplored. Here we report a systematic investigation of the acoustic orbital Hall effect in light metals such as Ti and Cr, where surface acoustic waves generate orbital currents through phonon-orbital coupling. The acoustic orbital current in Ti exhibits higher efficiency and longer diffusion length compared to the acoustic spin current in Pt. The sign and magnitude of the rectified acoustic voltages in nonmagnetic (Ti, Cr)/ferromagnetic (Ni, Co, Fe$_x$Co$_{1-x}$) bilayers are determined by the product of orbital-to-spin conversion and magnetoelastic coupling efficiencies of the ferromagnet. Additionally, we find evidence for acoustic orbital pumping, whereby the excitation of ferromagnetic resonance by surface acoustic waves injects an orbital current from the ferromagnet into the nonmagnet. These results establish lattice dynamics as an efficient driver of orbital transport, opening opportunities for low-dissipation orbitronic devices that harness and sense phonons.

cond-mat.mes-hall↗

arXiv2025

Model calculations of the strains associated with surface acoustic waves

Magnon-phonon coupling has garnered increasing interest in condensed matter physics due to its fertile physics and potential applications in devices with novel functionalities. Surface acoustic waves (SAWs) are commonly employed as a source of coherent acoustic phonons. The strain associated with SAWs couples to magnetization of magnetic materials via magnetoelastic coupling and/or spin-rotation coupling. A typical SAW device is formed on a piezoelectric substrate with anisotropic crystal structure. Since the form of strain depends on the material parameters and structure of the SAW device, it is of vital importance to understand its character. In this paper, we present a comprehensive methodology to numerically calculate the SAW velocity, SAW excitation efficiency, lattice displacement and all strain components associated with SAW. LiNbO$_3$ is used as a prototypical material system. All quantities depend on the SAW propagation direction with respect to the crystalline axis and on the electrical boundary conditions. In contrast to non-piezoelectric isotropic media, we find that all shear strain components can be induced in LiNbO$_3$, with their amplitude and relative phase (with respect to the longitudinal strain) dependent on the propagation direction and the boundary conditions at the LiNbO$_3$ surface. These results offer a robust foundation for analyzing strain-driven magnon-phonon coupling mechanisms and contribute to designing strain-engineered functional magnonic and phononic devices.

cond-mat.mtrl-sci↗

arXiv2025

Néel vector rotation driven by spin-orbit torque in amorphous ferrimagnetic GdCo

Spin superfluidity, a phenomenon enabling low dissipative spin transport analogous to superfluidity in liquid helium and superconductivity in electronic systems, has remained a theoretical concept. To realize the spin superfluidity in an antiferromagnet, it is necessary to excite a Néel vector rotation within the magnetic easy-plane, which has been elusive so far. In this study, we demonstrated spin-orbit torque-driven Néel vector rotation in amorphous ferrimagnetic GdCo. A pseudo-magnetic easy-plane is formed in a spin-flop state under an external magnetic field at the vicinity of the magnetization compensation temperature, and we observed stochastic binary switching in the anomalous Hall resistance, directly attributed to Néel vector rotation. Furthermore, homodyne detection confirmed rotation frequencies in the GHz range as expected from atomic spin simulations, providing evidence of dynamic Néel vector behavior. These findings represent a crucial advance towards the realization of spin superfluidity.

cond-mat.mtrl-sci↗

arXiv2024

Spin current generation by acousto-electric evanescent wave

We experimentally demonstrate that a spin current can be induced by the acousto-electric evanescent wave, an electric field associated with surface acoustic waves (SAWs) that decay along the surface normal. A previous study showed that a magnetic-field-dependent dc voltage (acoustic voltage) emerges in heavy metal (HM)/ferromagnet (FM) bilayers under excitation of SAWs. The effect, referred to as the acoustic spin Hall effect, was understood by assuming a SAW-induced ac spin current rectified by the oscillation of the FM layer magnetization and the inverse spin Hall effect. However, the mechanism of the spin current generation remained unidentified. Here we measure the acoustic voltage as a function of the SAW propagation direction relative to the crystalline orientation of a LiNbO$_3$ substrate. We find that the magnetic field angle dependence of the acoustic voltage exhibits a phase shift depending on the SAW propagation direction. The result is consistently explained in terms of the acousto-electric evanescent wave generating the spin current in HM layer via the spin Hall effect, thus clarifies the origin of the acoustic spin Hall effect.

cond-mat.mes-hall↗

arXiv2024

Hierarchical Narrative Analysis: Unraveling Perceptions of Generative AI

Written texts reflect an author's perspective, making the thorough analysis of literature a key research method in fields such as the humanities and social sciences. However, conventional text mining techniques like sentiment analysis and topic modeling are limited in their ability to capture the hierarchical narrative structures that reveal deeper argumentative patterns. To address this gap, we propose a method that leverages large language models (LLMs) to extract and organize these structures into a hierarchical framework. We validate this approach by analyzing public opinions on generative AI collected by Japan's Agency for Cultural Affairs, comparing the narratives of supporters and critics. Our analysis provides clearer visualization of the factors influencing divergent opinions on generative AI, offering deeper insights into the structures of agreement and disagreement.

cs.CL↗

arXiv2024

Hypergraph Change Point Detection using Adapted Cardinality-Based Gadgets: Applications in Dynamic Legal Structures

Hypergraphs provide a robust framework for modeling complex systems with higher-order interactions. However, analyzing them in dynamic settings presents significant computational challenges. To address this, we introduce a novel method that adapts the cardinality-based gadget to convert hypergraphs into strongly connected weighted directed graphs, complemented by a symmetrized combinatorial Laplacian. We demonstrate that the harmonic mean of the conductance and edge expansion of the original hypergraph can be upper-bounded by the conductance of the transformed directed graph, effectively preserving crucial cut information. Additionally, we analyze how the resulting Laplacian relates to that derived from the star expansion. Our approach was validated through change point detection experiments on both synthetic and real datasets, showing superior performance over clique and star expansions in maintaining spectral information in dynamic settings. Finally, we applied our method to analyze a dynamic legal hypergraph constructed from extensive United States court opinion data.

cs.SI↗

arXiv2024

Quantitative evaluation method for magnetoelastic coupling between surface acoustic waves and spin waves using electrical and optical measurements

Coupling and hybridization of different elementary excitations leads to new functionalities. In phononics and spintronics, magnetoelastic coupling between Rayleigh-type surface acoustic wave (SAW) and spin wave (SW) has recently attracted much attention. Quantitatively evaluating and comparing the coupled system are essential to develop the study of the magnetoelastic SAW-SW coupling. So far, previous studies of SAW-SW coupling have employed a quantity called coupling strength. However, it is still challenging to compare the coupling strength values among studies fairly because the quantity depends on the device geometry and the applied magnetic field angle, which are not unified among the previous studies. Here, we focus on a practical constant composed of a magnetoelastic constant and a strain amplitude that depends only on the material properties. We demonstrate a versatile evaluation technique to evaluate the practical constant by combining electrical measurements and optical imaging. An essential part of the technique is an analysis that can be used under off-resonance conditions where SAW and SW resonance frequencies do not match. Existing analysis can only handle the case under on-resonance conditions. Our analysis makes it possible to observe the magnetoelastic couplings between SAW with resonance frequencies that can be imaged optically and SW with resonance frequencies in the gigahertz range. Our demonstrated technique, which uses electrical and optical measurements under off-resonance conditions, can significantly advance research on SAW-SW coupled systems.

cond-mat.mes-hall↗

arXiv2024

Acoustoelectric non-local spin wave power detector for studying magnon-phonon coupling

We have developed a simple detection scheme to study spin waves excited by surface acoustic wave (SAW) in ferromagnetic thin films. Metallic antennas made of Ta and a ferromagnetic element are placed along the SAW propagation path. The SAW excites spin waves in the ferromagnetic element and induces acoustoelectric current in the antennas, which are detected as a DC voltage. The DC voltage takes an extremum at the spin wave resonance condition, which demonstrates that the antenna acts as a non-local spin wave detector. The antennas placed before and after the ferromagnetic element along the SAW propagation path can probe spin wave excitation from reflected and transmitted SAWs, respectively. Interestingly, we find characteristics of spin wave excitations that are different for the reflected and transmitted SAWs: the former excites spin waves with larger frequency with broader linewidth compared to the latter. The results show that the antennas act as a non-local spin wave power detector and can be used to map out the spin wave spectra in a unique way, providing insights into the magnon-phonon coupling in magnetic nanostructures fabricated on phononic SAW devices.

cond-mat.mes-hall↗

arXiv2023

Magnon-phonon coupling of synthetic antiferromagnets in a surface acoustic wave cavity resonator

We use a surface acoustic wave (SAW) cavity resonator to study the coupling of acoustic magnons in a synthetic antiferromagnet (SAF) and the phonons carried by SAWs. The SAF is composed of a CoFeB/Ru/CoFeB trilayer and the scattering matrix of the SAW resonator is studied to assess the coupling. We find that the spectral linewidth of the SAW resonator is modulated when the frequency of the excited magnons approaches the SAW resonance frequency. Moreover, the linewidth modulation varies with the magnitude and orientation of the external magnetic field. Such change in the spectral linewidth can be well reproduced using macrospin-like model calculations. From the model analyses, we estimate the magnon-phonon coupling strength to be $\sim$15.6 MHz at a SAW resonance frequency of 1.8 GHz: the corresponding magnomechanical cooperativity is $\sim$0.66. As the spectral shape hardly changes in a CoFeB single layer reference sample under the same experimental condition, these results show that SAF provides an ideal platform to study magnon-phonon coupling in a SAW cavity resonator.

cond-mat.mes-hall↗

arXiv2023

Helicity resolved Raman spectroscopy of mono- and a few-layers thick PtSe$_2$

We studied helicity resolved Raman scattering in PtSe$_2$ flakes with different thicknesses. The peak amplitude of helicity-switched Raman scattering is significantly larger than that of helicity-conserved scattering for the in-plane $E_{g}$ mode, consistent with the Raman tensor analyses and conservation law of angular momentum. The peak amplitude of the helicity-switched $E_{g}$ mode is larger for the thinner flakes. In addition, we find Raman peaks near the energy levels of infrared (IR)-active $E_u$ and $A_{2u}$ modes, only for monolayer and a few-layers thick flakes. Interestingly, these peaks manifest themselves only for helicity-switched Raman scattering; they are nearly absent for helicity-conserved scattering.

cond-mat.mes-hall↗

arXiv2023

Cavity magnomechanical coupling with coupled magnon modes in a synthetic antiferromagnet

On-chip cavity magnomechanics is an emerging field exploring acoustic and magnonic functionalities of various ferromagnetic materials and structures using strongly confined phonons. It is expected that such cavity magnomechanics can be extended to multilayer ferromagnets, especially synthetic antiferromagnets (SAFs) that exhibit zero net magnetization through interlayer exchange coupling. However, the conventional theoretical framework for a single ferromagnet cannot be used directly because of the antiferromagnetic magnetization dynamics associated with the interlayer exchange coupling. In this paper, we theoretically investigate phonon-magnon coupling with a three-layer SAF. Our formulation of the phonon-magnon coupling constants reveals that the acoustic (optical) magnon mode dominantly couples to the cavity phonon when the magnetization angles in the two ferromagnetic layers are antiparallel (orthogonal). Moreover, numerical calculations including the effects of dipole-dipole interactions and in-plane uniaxial magnetic anisotropy allow us to predict phonon frequency shifts and linewidth broadening that can be detected in experiments. These theoretical insights would greatly help us to make a strategy for bringing the system into the strong coupling regime and to devise novel control protocols in analogy to cavity quantum electrodynamics and cavity optomechanics.

cond-mat.mes-hall↗