arXiv subjects

Kouta Kondou

Publications and source records attributed to Kouta Kondou.

At least 19 recordsLinked to original sources

arXiv2026

Significant modulation of acoustoelectric current associated with charge density wave transitions

We studied acoustoelectric (AE) currents in materials that undergo charge density wave (CDW) transitions, induced by a surface acoustic wave (SAW) on a piezoelectric substrate. The polarity and magnitude of the AE current in NbSe$_3$ and 2H-TaSe$_2$ were modulated due to their CDW transitions. We also found that the sign of the AE current depends on the SAW propagation direction with respect to the crystalline axis of the substrate.A phenomenological model assuming strain-modified conductivity can qualitatively account for the significant modulation of the AE current associated with the CDW transition, as well as the SAW propagation orientation dependence. The present results offer a powerful probe for exploring SAW-electron interactions in van der Waals materials, thereby highlighting their potential for advancing the emerging field of straintronics.

cond-mat.mes-hall↗

arXiv2026

Nonreciprocity reversal of magnetoacoustic attenuation in NiFe alloy thin films

Nonreciprocity, the asymmetry of transport, underlies technologies from the diode to the microwave isolator. In a ferromagnet, a surface acoustic wave generates an elliptical effective field with propagation-locked handedness, breaking the reciprocity of its propagation. Despite decades of study on this phenomenon, a method for controlling the sign of the nonreciprocity has remained elusive. Here we observe a sign reversal in Ni$_x$Fe$_{100-x}$ films. A 0.8 at.% change across Permalloy's zero-magnetostriction composition, where the magnetoelastic coefficient $b$ changes sign, reverses the handedness of the elliptical effective field and thereby the nonreciprocity, from 78.6% to -61.8%. Angle-dependent measurements and spin-wave-ellipticity modelling show that reversing the sign of $b$ reverses the handedness of the elliptically polarized effective field. Aided by cubic frequency scaling, we resolve the sign of $b$ down to -0.05 MPa in a 10-nm film, establishing nonreciprocity as a nanoscale probe of magnetoelastic coupling.

cond-mat.mtrl-sci↗

arXiv2026

Evidence for spin swapping from modulation of transverse resistance in magnetic heterostructures with Rashba interface

We investigate the transverse response under the out-of-plane magnetic field for magnetic heterostructures with Cu/Bi2O3 or Ag/Bi2O3 Rashba interfaces. We detect opposite contributions on the transverse resistance by the Cu/Bi2O3 and the Ag/Bi2O3 interfaces, which interestingly coincide well with the opposite signs of the spin/charge interconversion from the two interfaces. We suppose the opposite influences on the transverse resistance feature spin swapping occurrence of the converted spin current. The transverse spin flow emerges due to the spin swapping in both Cu and Ag layer, but the direction of the spin flow is opposed dependent on the spin direction of the converted spin current.

cond-mat.mes-hall↗

arXiv2026

Thickness dependence of diode efficiency in superconducting Fe(Se,Te)/FeTe thin-film heterostructure devices

The superconducting diode effect (SDE) is a nonreciprocal transport phenomenon, in which the superconducting critical current density depends on the polarity of the current. It has attracted recent attention because of its potential applications to a rectifier without energy dissipation. While SDE has been observed in a wide range of superconducting materials with broken inversion symmetry as well as thin-film heterostructures, the microscopic origin linking structural inversion asymmetry of electronic band, spin-orbit interaction, and vortex pinning remains to be clarified. In this study, we investigate SDE in Fe(Se,Te)/FeTe heterostructure devices as a function of the superconducting Fe(Se,Te) layer thickness tFST to elucidate the role of structural inversion asymmetry on the vortex-induced SDE. We find that the SDE efficiency monotonically increases with increasing tFST, which can be understood by considering that the band bending in the bulk Fe(Se,Te) layer induces the structural inversion asymmetry and thus, the Rashba spin-orbit interaction. In addition, we demonstrate almost 100% rectification for the Fe(Se,Te)/FeTe heterostructure devices in half- and full-wave oscillation configurations. Our findings point out the importance of structural architecture for realization of highly efficient SDE devices based on superconducting thin-film heterostructures.

cond-mat.supr-con↗

arXiv2026

Identifying the origin of out-of-plane spin polarization in the noncollinear antiferromagnet Mn$_3$Ge

The noncollinear antiferromagnets Mn$_3$Sn/Ge emerge as promising spin-current sources with both in-plane and out-of-plane spin polarizations, thereby enabling field-free magnetization switching. However, the microscopic origin of the out-of-plane spin polarization remains under debate, specifically whether it arises from the magnetic spin Hall effect (MSHE) or the spin swapping (SSW). Here, we comparatively evaluate the spin torques in single-crystal Mn$_3$Ge/Py bilayers with different crystallographic orientations using the ferromagnetic resonance technique. The distinct angular dependences of the measured spin-torque signals provide clear evidence for the bulk MSHE, which depends on antiferromagnetic order. In addition, we identify the antiferromagnetic-order independent component originating from the interfacial SSW. The coexisting MSHE and SSW, with comparable magnitudes, give rise to the out-of-plane spin polarization. Our study disentangles the origins of spin-torque generation in noncollinear antiferromagnets, providing valuable insights for their spintronic applications.

cond-mat.mes-hall↗

arXiv2026

Crossover between intrinsic and temperature-assisted regimes in spin-orbit torque switching of antiferromagnetic order

Intensive studies have been made on antiferromagnets as candidate materials for next generation memory bits due to their ultrafast dynamics reaching picosecond time scales. Recent demonstrations of electrical bidirectional switching of antiferromagnetic states have attracted significant attention. However, under the presence of significant Joule heating that destabilizes the magnetic order, the timescales associated with the switching can be limited to nanoseconds or longer. Here, we present the observation of a crossover in the switching behavior of the chiral antiferromagnet Mn3Sn by tuning the magnetic layer thickness. While Joule heating interferes with switching in thicker devices, we find clear signatures of an intrinsic spin-orbit torque mechanism as the thickness is reduced, avoiding the heating effect. The suppression of heating enables switching without significant attenuation of the readout signal using pulses shorter than those required by temperature-assisted mechanisms. The crossover into the spin-orbit torque switching behavior clarifies the potential for achieving ultrafast switching as expected from the picosecond spin dynamics of antiferromagnets. Our results lay the groundwork for designing antiferromagnetic memory devices that can operate at ultrafast timescales.

cond-mat.mtrl-sci↗

arXiv2026

Spin Hall effect in van der Waals ferromagnet Fe$_{5}$GeTe$_{2}$

We investigate the spin Hall effect (SHE) in a van der Waals (vdW) ferromagnet Fe$_{5}$GeTe$_{2}$ (FGT) with a Curie temperature $T_{\rm C}$ of 310 K utilizing the spin-torque ferromagnetic resonance method. In synchronization with the emergence of the ferromagnetic phase resulting in the anomalous Hall effect (AHE), a noticeable enhancement in the SHE was observed below $T_{\rm C}$. On the other hand, the SHE shows a different temperature dependence from the AHE: the effective spin Hall conductivity is clearly enhanced with decreasing temperature unlike the anomalous Hall conductivity, reflecting the variation of band-structure accompanied by the complicated magnetic ordering of the FGT. The results provide a deep understanding of the SHE in magnetic materials to open a new route for novel functionalities in vdW materials-based spintronic devices.

cond-mat.mes-hall↗

arXiv2025

Harmonic and Subharmonic Magnon Generation in a Surface Acoustic Wave Resonator

We experimentally observe the generation of magnon harmonics and subharmonics in an on-chip surface acoustic wave resonator incorporating a thin Co$_{20}$Fe$_{60}$B$_{20}$ film, using micro-focused Brillouin light scattering. In our devices, rotating the in-plane magnetic field allows continuous tuning of the magnon-phonon coupling from weak to strong within the same resonator. In the weak coupling regime, we only observe fundamental magnetoelastic wave signal at $f_{1}$. Conversely, in the strong coupling regime, in addition to the fundamental magnetoelastic wave, we observe subharmonic and harmonic signals at $3/2f_{1}$, $2f_{1}$, and $3f_{1}$, which are well reproduced by our analytical model. Our results establish phonons as a means to generate and control nonlinear magnons in the strong coupling regime, providing a new route for magnonic signal processing.

cond-mat.mes-hall↗

arXiv2025

Phenomenological model for the direct and inverse Edelstein effects

We have developed a phenomenological model that connects the direct and the inverse Edelstein effects. Our model implies a trade-off relation between the conversion coefficients for the direct and inverse effects. Thus, a large conversion coefficient for the inverse effect does not necessarily bring a large conversion coefficient for the direct effect. Instead of these coefficients, we propose a figure of merit of Edelstein effects which consists of two factors; one of them represents the magnitude of the spin-orbit coupling, and the other represents the strength of the hybridization between bulk and interface states. Both of them are quit important for the efficient conversion through Edelstein effects. To test our model, we measured the inverse and direct Edelstein effects at the Bi2O3/Cu interface using spin absorption method with a non-local spin valve structure and calculated the conversion coefficients. The effective spin Hall angle reaches ~0.09 in this system. This relatively large value is attributable to not only the large spin-orbit coupling but also the strong hybridization between the interface and bulk states at the Bi2O3/Cu interface.

cond-mat.mtrl-sci↗

arXiv2024

Observation of chiral domain walls in an octupole-ordered antiferromagnet

Spin chirality in antiferromagnets offers new opportunities for spintronics. The kagome antiferromagnet Mn3Sn is a paradigmatic material in which the antiferromagnetic order parameter can be detected and controlled by electrical means. However, direct investigation of the magnetic texture of Mn3Sn has been challenging because of the tiny moment hosted in its magnetic octupole, hindering further clarification of this unique material. Here, we address this issue by observing the stray magnetic field from Mn3Sn using a diamond quantum scanning magnetometer. The spatially-resolved intrinsic domains and domain walls in a high-quality single-crystalline Mn3Sn film quantitatively reveal the polarization angle of the magnetic octupole in the kagome plane, the domain's local magnetization, the domain wall's width and chirality, and the octupole order in domain walls. Our nanoscale investigation of Mn3Sn, a powerful complement to macroscopic measurements, paves the road for developing chiral antiferromagnetism and its potential for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Strongly Coupled Spin Waves and Surface Acoustic Waves at Room Temperature

Here, we report the observation of strong coupling between magnons and surface acoustic wave (SAW) phonons in a thin CoFeB film constructed in an on-chip SAW resonator by analyzing SAW phonon dispersion anticrossings. Our device design provides the tunability of the film thickness with a fixed phonon wavelength, which is a departure from the conventional approach in strong magnon--phonon coupling research. We detect a monotonic increase in the coupling strength by expanding the film thickness, which agrees with our theoretical model. Our work offers a significant way to advance fundamental research and the development of devices based on magnon--phonon hybrid quasiparticles.

cond-mat.mes-hall↗

arXiv2023

Spontaneous spin selectivity in chiral molecules at the interface

Chirality-induced spin selectivity (CISS) has been extensively studied over the past two decades. While current-induced spin polarization in chiral molecules is widely recognized as the fundamental principle of the CISS, only a few studies have been reported on bias-current-free CISS, where there is no bias electric current in chiral molecules. Recent studies on the chirality-induced exchange bias and current-in-plane magnetoresistance (CIP-MR) effects using chiral molecule/ferromagnet bilayer systems indicate that chiral molecules at the interface possess thermally driven broken-time-reversal symmetry, which induces bias-current-free CISS, i.e. a spontaneous effective magnetic field in the system. In this paper, we briefly review CISS-related phenomena in terms of the symmetry and discuss the mechanism of bias-current-free CISS. We also discuss the possibility of the linear magnetoelectric effect of chiral molecules, which arises from the spin polarization at the edges of molecules with metallic contacts, and its potential impact on the observed CISS phenomena.

cond-mat.mtrl-sci↗

arXiv2021

Current-in-plane magnetoresistance in chiral-molecule/ferromagnetic metal bilayer due to thermally induced spin polarization

We report chirality-induced current-in-plane magnetoresistance (CIP-MR) in chiral molecule/ferromagnetic metal bilayer at room temperature. The previously reported chiralityinduced current-perpendicular-to-plane magnetoresistance (CPP-MR) originates from the chiral induced spin-selectivity (CISS) effect that needs charge-current passing through the molecule. In contrast, the observed CIP-MR in the present study requires no bias charge current through the molecule. The temperature dependence of CIP-MR suggests thermally induced spin-polarization in the chiral molecules is the key for the observed MR.

cond-mat.mtrl-sci↗

arXiv2021

Chirality-induced effective magnetic field in a phthalocyanine molecule

Chirality in organic molecules has attracted considerable attention in the fields of chemistry, biology, and spintronics. This paper reports on perpendicular magnetization hysteresis loops of a multilayer consisting of ultrathin Fe (001), chiral phthalocyanine molecule ((P)- or (M)-PbPc-DTBPh), and MgO (001). We find a chirality-dependent shift of the hysteresis loop. Unlike the previously reported bias current induced phenomena, the result shows a chirality-induced effective magnetic field in the phthalocyanine molecule in the absence of a bias current in the system. This study opens up a new direction in the emerging field of chiral molecular spintronics.

cond-mat.mtrl-sci↗

arXiv2020

Electrical Manipulation of a Topological Antiferromagnetic State

Electrical manipulation of emergent phenomena due to nontrivial band topology is a key to realize next-generation technology using topological protection. A Weyl semimetal is a three-dimensional gapless system that hosts Weyl fermions as low-energy quasiparticles. It exhibits various exotic phenomena such as large anomalous Hall effect (AHE) and chiral anomaly, which have robust properties due to the topologically protected Weyl nodes. To manipulate such phenomena, the magnetic version of Weyl semimetals would be useful as a magnetic texture may provide a handle for controlling the locations of Weyl nodes in the Brillouin zone. Moreover, given the prospects of antiferromagnetic (AF) spintronics for realizing high-density devices with ultrafast operation, it would be ideal if one could electrically manipulate an AF Weyl metal. However, no report has appeared on the electrical manipulation of a Weyl metal. Here we demonstrate the electrical switching of a topological AF state and its detection by AHE at room temperature. In particular, we employ a polycrystalline thin film of the AF Weyl metal Mn$_3$Sn, which exhibits zero-field AHE. Using the bilayer device of Mn$_3$Sn and nonmagnetic metals (NMs), we find that an electrical current density of $\sim 10^{10}$-$10^{11}$ A/m$^2$ in NMs induces the magnetic switching with a large change in Hall voltage, and besides, the current polarity along a bias field and the sign of the spin Hall angle $θ_{\rm SH}$ of NMs [Pt ($θ_{\rm SH} > 0$), Cu($θ_{\rm SH} \sim 0$), W ($θ_{\rm SH} < 0$)] determines the sign of the Hall voltage. Notably, the electrical switching in the antiferromagnet is made using the same protocol as the one used for ferromagnetic metals. Our observation may well lead to another leap in science and technology for topological magnetism and AF spintronics.

cond-mat.mtrl-sci↗

arXiv2020

Enhancement of acoustic spin pumping by acoustic distributed Bragg reflector cavity

Surface acoustic waves (SAWs) in the GHz frequency range can inject spin currents dynamically into adjacent nonmagnetic layers via spin pumping effect associated with ferromagnetic resonance. Here, we demonstrate an enhancement of acoustic ferromagnetic resonance and spin current generation by a pair of SAW reflector gratings, which form an acoustic analogue of the distributed Bragg reflector cavity. In the experiment, we confirmed 2.04 $\pm$ 0.02 times larger SAW power absorption in a device with cavity than in case of no acoustic cavity. We confirmed up to 2.96 $\pm$ 0.02 times larger spin current generation by measuring electric voltages generated by the inverse Edelstein effect (IEE) at the interface between Cu and Bi$_2$O$_3$. The results suggest that acoustic cavities would be useful to enhance the conversion efficiency in SAW driven coupled magnon-phonon dynamics.

cond-mat.mes-hall↗

arXiv2020

Non-trivial charge-to-spin conversion in ferromagnetic metal/Cu/Al2O3 by orbital transport

Efficient spin/charge interconversion is desired to develop innovative spin-based devices. So far, the interconversion has been performed by using heavy atomic elements, strong spin-orbit interaction of which realizes the interconversion through the spin Hall effect and the Edelstein effect. We demonstrate highly efficient charge-to-spin conversion in a ferromagnetic metal/Cu/Al2O3 trilayers, which do not contain any heavy element. The resulting spin torque efficiency is higher than those of conventional spin Hall and Rashba systems consisting of heavy elements such as Pt and Bi. Our experimental results qualitatively deviate from typical behaviors arising from spin transport. However, they are surprisingly consistent with the behaviors arising from the orbital transport. Our results thus demonstrate a new direction for efficient charge-to-spin conversion through the orbital transport.

cond-mat.mes-hall↗

arXiv2018

Inverse Edelstein effect induced by magnon - phonon coupling

We demonstrate a spin to charge current conversion via magnon-phonon coupling and inverse Edelstein effect on the hybrid device Ni/Cu(Ag)/Bi$_{2}$O$_{3}$. The generation of spin current ($J_{s}\approx 10^{8}A/m^{2}$) due to magnon - phonon coupling reveals the viability of acoustic spin pumping as mechanism for the development of spintronic devices. A full in-plane magnetic field angle dependence of the power absorption and a combination of longitudinal and transverse voltage detection reveals the symmetric and asymmetric components of the inverse Edelstein effect voltage induced by Rayleigh type surface acoustic waves. While the symmetric components are well studied, asymmetric components are widely unexplored. We assign the asymmetric contributions to the interference between longitudinal and shear waves and an anisotropic charge distribution in our hybrid device.

cond-mat.mes-hall↗