arXiv subjects

Kohei Ueda

Publications and source records attributed to Kohei Ueda.

17 recordsLinked to original sources

arXiv2026

Repeater-Assisted Massive MIMO Downlink Performance with Calibration Errors

Reciprocity-based downlink beamforming is imperative for a scalable time-division duplex massive multiple-input multiple-output~(MIMO) deployment. Specifically, for a dual-antenna repeater-assisted massive MIMO system, a mismatch between forward and reverse path gains at the repeater can exacerbate the overall calibration error between the user equipments (UEs) and the base station (BS), which potentially also contains calibration errors of their individual radio-frequency chains. This paper models the effects of such calibration errors, underpins the relations between the uplink and downlink channels for repeater-assisted systems with calibration errors clubbed with the over-the-air channel estimation errors, and derives analytical expressions of the downlink spectral efficiency. The presented results can then be simplified to several special cases, underscoring situations wherein such errors can become pronounced.

eess.SP↗

arXiv2026

On Optimal Strategies for Joint Reciprocity Calibration in Distributed MIMO

This paper investigates the impact of reciprocity calibration errors on the downlink spectral efficiency (SE) of multi-user large antenna systems. Specifically, we consider two calibration approaches: (a) global calibration, in which all antennas (can be distributed access-points (APs)) in the system cooperatively perform calibration, and (b) local calibration, wherein only a subset of antennas involved in downlink beamforming performs calibration. We derive the downlink SE considering the use-and-then-forget bound and side-information bound, and then demonstrate that, when downlink pilots are employed (in the case of side-information bound), the global calibration outperforms local calibration for arbitrary calibration topologies.

eess.SP↗

arXiv2025

Longitudinal spin current absorption in bilayers composed of ferromagnetic and highly-resistive non-magnetic layers

Spin Hall magnetoresistance (SMR) is an intriguing spin-dependent transport phenomenon in bilayers consisting of non-magnetic and magnetic layers. Here, we report on the influence of longitudinal spin current absorption by the magnetic layer on SMR in bilayers composed of Co$_{20}$Fe$_{60}$B$_{20}$ (CoFeB) and epitaxial SrIrO$_{3}$, where SrIrO$_{3}$ is used as a highly-resistive spin current source. We observed a clear SMR signal and an enhancement in the SMR ratio with increasing CoFeB layer thickness, in qualitative agreement with an SMR model that incorporates the spin current absorption. The effective spin Hall angle is corrected from 0.07 to 0.12 with consideration of the spin current absorption, corresponding to a relative correction of ~71%. Our findings highlight the pronounced impact of the spin current absorption by the magnetic layer on the SMR mechanism when employing highly-resistive non-magnetic layer such as SrIrO$_{3}$, as well as other emerging quantum materials.

cond-mat.mes-hall↗

arXiv2025

Effect of crystallinity on spin-orbit torque in 5$\textit{d}$ iridium oxide IrO$_{2}$

The 5$\textit{d}$ transition-metal oxides provide an intriguing platform for generating an efficient spin current due to a unique electronic structure dominated by 5d electrons with strong spin-orbit coupling. Here, we report on the effect of crystallinity on current-driven spin-orbit torque (SOT) in binary 5$\textit{d}$ iridium oxide IrO$_{2}$ thin films by controlling amorphous, polycrystalline, and epitaxial states. By conducting harmonic Hall measurement in bilayers composed of ferromagnetic Co$_{20}$Fe$_{60}$B$_{20}$ and IrO$_{2}$, we find that dampinglike (DL) SOT is larger than fieldlike SOT for all the samples. We also demonstrate that both electrical resistivity and the DL SOT efficiency increase in order of epitaxial, polycrystalline, and amorphous IrO$_{2}$. Despite their different electrical conductivities, spin Hall conductivities of the three states of the IrO$_{2}$ layer are found to be nearly constant, which is consistent with the intrinsic regime of the spin Hall effect scaling relation. Our results highlight the important role that crystallinity plays in the spin-current generation, leading to the potential technological development of spintronic devices based on the 5$\textit{d}$ transition-metal oxides.

cond-mat.mtrl-sci↗

arXiv2025

Quantitative imaging of nonlinear spin-wave propagation using diamond quantum sensors

Spin waves propagating in magnetic materials exhibit nonlinear behavior at large amplitudes due to the competition between excitation and relaxation, providing an attractive platform for exploring nonlinear wave dynamics. In particular, spin waves with a non-zero wavenumber that carry momentum undergo nonlinear relaxation and experience wavenumber modulation in the nonlinear regime. This nonlinearity has been observed experimentally, for example in S. R. Lake et al., Phys. Rev. Appl. 17, 034010 (2022), but a quantitative comparison with theory has not yet been carried out. Here, We image nonlinear spin-wave propagation in two yttrium iron garnet thin films with distinct spin-wave decay rates using a wide-field quantum diamond microscope. We obtain quantitative distributions of spin-wave amplitude and phase as a function of the excitation microwave strength. As a result, we observe a threshold in the spin-wave amplitude beyond which nonlinear effects become evident and confirm that this threshold is consistent with theoretical predictions based on four-magnon scattering for both samples. Moreover, as the amplitude of the spin waves increases, we observe modulation of the wavenumber across the field of view. We attribute this modulation primarily to a reduction in the saturation magnetization caused by incoherent spin waves generated by multi-magnon scattering. Our quantitative measurements provide a pathway for visualizing nonlinear spin-wave dynamics and are crucial for deepening our understanding of the underlying mechanisms.

cond-mat.mes-hall↗

arXiv2025

Suppression of ferromagnetism in rippled La$_{2/3}$Sr$_{1/3}$MnO$_3$ membrane with process-induced strain prepared by epitaxial lift-off technique

Transition metal oxides are a platform for exploring strain-engineered intriguing physical properties and developing spintronic or flexible electronic functionalities owing to strong coupling of spin, charge and lattice degrees of freedom. In this study, we exemplify the strain-engineered magnetism of La$_{2/3}$Sr$_{1/3}$MnO$_3$ in freestanding and rippled membrane forms without and with process-induced strain, respectively, prepared by epitaxial lift-off technique. We find that the deposition of Pt/Ti stressor suppresses the crack formation in the lift-off process and induces a ripple structure in the La$_{2/3}$Sr$_{1/3}$MnO$_3$ membrane. Laser micrograph and Raman spectroscopy show a ripple period of about 30 um and a height of a few um, where alternating convex and concave structures are subjected to tensile strain of 0.6% and compressive strain of 0.5%, respectively. While the freestanding La$_{2/3}$Sr$_{1/3}$MnO$_3$ membrane exhibits room-temperature ferromagnetism, the macroscopic magnetic transition temperature (TC) of the rippled membrane is reduced by as large as 27%. Temperature-variable Kerr microscopy observation in the rippled membrane reveals that the spatial variation of TC to be approximately 4% of the macroscopic TC, which coincides with the local strains at convex and concave structures. The large reduction of macroscopic TC in the rippled membrane may be ascribed to the lattice disorders due to strain gradient. Our demonstration of tuning ferromagnetism by the ripple structure validates the high potential of the process-induced strain in epitaxial lift-off technique and paves the way for strain-mediated emerging physical properties in various transition metal oxides.

cond-mat.mtrl-sci↗

arXiv2024

Wideband wide-field imaging of spin-wave propagation using diamond quantum sensors

Imaging spin-wave propagation in magnetic materials in a wide frequency range is crucial for understanding and applying spin-wave dynamics. Recently, nitrogen-vacancy (NV) centers in diamond have attracted attention as sensors capable of quantitatively measuring the amplitude and phase of coherent spin waves. However, the conventional sensing protocol has been limited to detecting spin waves whose frequencies match the resonance frequency of the NV spins. We demonstrate that by utilizing the AC Zeeman effect, it is possible to image spin waves propagating in a yttrium iron garnet (YIG) thin film over a wide frequency range up to a maximum detuning of 567 MHz without changing the external magnetic field. Our results expand the applicability of NV centers for spin-wave sensing and pave the way for quantitative investigations of the dynamics in various magnetic materials, such as metallic ferromagnets and van der Waals magnets.

cond-mat.mes-hall↗

arXiv2024

Non-coplanar spin structure in a metallic thin film of triangular lattice antiferromagnet CrSe

An antiferromagnetic metal with two-dimensional triangular network offers a unique playground of intriguing magneto-transport properties and functionalities stemming from interplay between conducting electrons and intricate magnetic phases. A NiAs-type CrSe is one of the candidates owing to alternate stackings of Cr and Se triangular atomic networks in its crystal structure. While fabrication of CrSe thin films is indispensable to develop functional devices, studies on its thin-film properties have been limited to date due to the lack of metallic samples. Here, we report on realization of metallic conductivities of CrSe thin films, which allows to investigate their intrinsic magneto-transport properties. The metallic sample exhibits co-occurrence of a weak ferromagnetism with perpendicular magnetic anisotropy and the antiferromagnetic behavior, indicating the presence of non-coplanar spin structures. In addition, control of polarity and tilting angle of the non-coplanar spin structure is accomplished by a sign of cooling magnetic fields. The observed non-coplanar spin structure, which can be a source of emergent magnetic field acting on the conducting electrons, highlights a high potential of the triangular lattice antiferromagnet and provide unique platform for functional thin-film devices composed of NiAs-type derivative Cr chalcogenides and pnictides.

cond-mat.mtrl-sci↗

arXiv2023

Effect of interface quality on spin Hall magnetoresistance in Pt/MgFe$_{2}$O$_{4}$ bilayers

We report on spin Hall magnetoresistance (SMR) in bilayers composed of Pt and magnetic insulator MgFe$_{2}$O$_{4}$ (MFO) with spinel structure. The Pt thickness dependence of the SMR reveals that annealing of the MFO surface before depositing the Pt layer is crucial for a large SMR with better interface quality. We also found that oxygen pressure during the MFO growth hardly affects the SMR while it influences on magnetic property of the MFO film. Our findings provide important clues to further understanding the spin transport at interfaces containing magnetic insulators, facilitating development of low power consumption devices.

cond-mat.mtrl-sci↗

arXiv2023

Impact of epitaxial strain relaxation on ferromagnetism in a freestanding La2/3Sr1/3MnO3 membrane

Manganite perovskites host emerging physical properties of strongly-correlated electrons with charge, spin, and lattice degrees of freedom. Using epitaxial lift-off technique, we report enhancement of saturation magnetization and ferromagnetic transition temperature of the freestanding La2/3Sr1/3MnO3 membrane compared with the as-grown film on SrTiO3 substrate involving lateral tensile strain. Structural analysis reveals shrinkage of unit-cell volume by tensile strain relaxation in the freestanding membrane, which causes enhancement of the ferromagnetic interaction. The impact of the microscopic lattice deformation on the ferromagnetism of La2/3Sr1/3MnO3 indicates a high potential of this material for flexible electronics application with intriguing functionalities in strongly-correlated electron systems.

cond-mat.str-el↗

arXiv2023

Spin-orbit torque generation in bilayers composed of CoFeB and epitaxial SrIrO$_{3}$ grown on an orthorhombic DyScO$_{3}$ substrate

We report on the highly efficient spin-orbit torque (SOT) generation in epitaxial SrIrO$_{3}$(SIO), which is grown on an orthorhombic DyScO$_{3}$(110) substrate. By conducting harmonic Hall measurement in Co$_{20}$Fe$_{60}$B$_{20}$ (CoFeB)/SIO bilayers, we characterize two kinds of the SOTs, i.e., dampinglike (DL) and fieldlike ones to find that the former is much larger than the latter. By comparison with the Pt control sample with the same CoFeB thickness, the observed DL SOT efficiency $\xi$$_{DL}$ of SIO ($\sim$0.32) is three times higher than that of Pt ($\sim$0.093). The $\xi$$_{DL}$ is nearly constant as a function of the CoFeB thickness, suggesting that the SIO plays a crucial role in the large SOT generation. These results on the CoFeB/SIO bilayers highlight that the epitaxial SIO is promising for low-current and reliable spin-orbit torque-controlled devices.

cond-mat.mtrl-sci↗

arXiv2023

Spin Hall magnetoresistance in Pt/Y$_{3}$Fe$_{5}$O$_{12}$ bilayers grown on Si and Gd$_{3}$Ga$_{5}$O$_{12}$ substrates

We study spin Hall magnetoresistance (SMR) in Pt/ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ (YIG) bilayers by focusing on crystallinity, magnetization, and interface roughness by controlling post-annealing temperatures. The SMR in the Pt/YIG grown on Si substrate is comparable to that grown on widely used Gd$_{3}$Ga$_{5}$O$_{12}$ substrate, indicating that the large SMR can be achieved irrespective to the crystallinity. We deduced the spin mixing conductance from the Pt thickness dependence of the SMR to find the high interface quality of the optimized Pt/YIG grown on Si in terms of spin current. We also clarified that the SMR correlates well with the magnetization, the interface roughness, and carrier density. These findings highlight that optimizing YIG properties is a key to control of magnetization by spin current, leading to the development of low power consumption spintronic device based on the magnetic insulator.

cond-mat.mtrl-sci↗

arXiv2023

Spin current generation from an epitaxial tungsten dioxide WO$_{2}$

We report on efficient spin current generation at room temperature in rutile type WO$_{2}$ grown on Al$_{2}$O$_{3}$(0001) substrate. The optimal WO$_{2}$ film has (010)-oriented monoclinically distorted rutile structure with metallic conductivity due to 5$\it{d}$$^2$ electrons, as characterized by x-ray diffraction, electronic transport, and x-ray photoelectron spectroscopy. By conducting harmonic Hall measurement in Ni$_{81}$Fe$_{19}$/WO$_{2}$ bilayer, we estimate two symmetries of the spin-orbit torque (SOT), i.e., dampinglike (DL) and fieldlike ones to find that the former is larger than the latter. By comparison with the Ni$_{81}$Fe$_{19}$/W control sample, the observed DL SOT efficiency $\xi$$_{DL}$ of WO$_{2}$ (+0.174) is about two thirds of that of W (-0.281) in magnitude, with a striking difference in their signs. The magnitude of the $\xi$$_{DL}$ of WO$_{2}$ exhibits comparable value to those of widely reported Pt and Ta, and Ir oxide IrO$_{2}$. The positive sign of the $\xi$$_{DL}$ of WO$_{2}$ can be explained by the preceding theoretical study based on the 4$\it{d}$ oxides. These results highlight that the epitaxial WO$_{2}$ offers a great opportunity of rutile oxides with spintronic functionalities, leading to future spin-orbit torque-controlled devices.

cond-mat.mtrl-sci↗

arXiv2021

Stacking-order effect on spin-orbit torque, spin-Hall magnetoresistance, and magnetic anisotropy in Ni$_{81}$Fe$_{19}$-IrO$_2$ bilayers

The 5d transition-metal oxides have been an intriguing platform to demonstrate efficient charge to spin current conversion due to a unique electronic structure dominated by strong spin-orbit coupling. Here, we report on stacking-order effect of spin-orbit torque (SOT), spin-Hall magnetoresistance, and magnetic anisotropy in bilayer Ni$_{81}$Fe$_{19}$-5d iridium oxide, IrO$_2$. While all the IrO$_2$ and Pt control samples exhibit large dampinglike-SOT generation stemming from the efficient charge to spin current conversion, the magnitude of the SOT is larger in the IrO$_2$ (Pt)-bottom sample than in the IrO$_2$ (Pt)-top one. The fieldlike-SOT has even more significant stack order effect, resulting in an opposite sign in the IrO$_2$ samples in contrast to the same sign in the Pt samples. Furthermore, we observe that the magnetic anisotropy energy density and the anomalous Hall effect are increased in the IrO$_2$ (Pt)-bottom sample, suggesting enhanced interfacial perpendicular magnetic anisotropy. Our findings highlight the significant influence of the stack order on spin transport and magnetotransport properties of Ir oxide/ferromagnet systems, providing useful information on design of SOT devices including 5d transition-metal oxides.

cond-mat.mtrl-sci↗

arXiv2021

Spin-orbit torque generation in NiFe/IrO2 bilayers

The 5d transition-metal oxides have a unique electronic structure dominated by strong spin-orbit coupling and hence they can be an intriguing platform to explore spin-current physics. Here, we report on room-temperature generation of spin-orbit torque (SOT) from a conductive 5d iridium oxide, IrO2. By measuring second-harmonic Hall resistance of Ni81Fe19/IrO2 bilayers, we find both dampinglike and fieldlike SOTs. The former is larger than the latter, enabling easier control of magnetization. We also observe that the dampinglike SOT efficiency has a significant dependence on IrO2 thickness, which is well described by the drift-diffusion model based on the bulk spin Hall effect. We deduce the effective spin Hall angle of +0.093 +- 0.003 and the spin-diffusion length of 1.7 +- 0.2 nm. By comparison with control samples Pt and Ir, we show that the effective spin Hall angle of IrO2 is comparable to that of Pt and seven times higher than that of Ir. The fieldlike SOT efficiency has a negative sign without appreciable dependence on the thickness, in contrast to the dampinglike SOT. This suggests that the fieldlike SOT likely stems from the interface. These experimental findings suggest that the uniqueness of the electronic structure of 5d transition-metal oxides is crucial for highly efficient charge to spin-current conversion.

cond-mat.mtrl-sci↗

arXiv2017

Microscopic Origin of Interfacial Dzyaloshinskii-Moriya Interaction

Chiral spin textures at the interface between ferromagnetic and heavy nonmagnetic metals, such as Neel-type domain walls and skyrmions, have been studied intensively because of their great potential for future nanomagnetic devices. The Dyzaloshinskii-Moriya interaction (DMI) is an essential phenomenon for the formation of such chiral spin textures. In spite of recent theoretical progress aiming at understanding the microscopic origin of the DMI, an experimental investigation unravelling the physics at stake is still required. Here, we experimentally demonstrate the close correlation of the DMI with the anisotropy of the orbital magnetic moment and with the magnetic dipole moment of the ferromagnetic metal. The density functional theory and the tight-binding model calculations reveal that asymmetric electron occupation in orbitals gives rise to this correlation.

cond-mat.mtrl-sci↗

arXiv2015

Soliton-like magnetic domain wall motion induced by the interfacial Dzyaloshinskii-Moriya interaction

Topological defects such as magnetic solitons, vortices, Bloch lines, and skyrmions have started to play an important role in modern magnetism because of their extraordinary stability, which can be exploited in the production of memory devices. Recently, a novel type of antisymmetric exchange interaction, namely the Dzyaloshinskii-Moriya interaction (DMI), has been uncovered and found to influence the formation of topological defects. Exploring how the DMI affects the dynamics of topological defects is therefore an important task. Here we investigate the dynamic domain wall (DW) under a strong DMI and find that the DMI induces an annihilation of topological vertical Bloch lines (VBLs) by lifting the four-fold degeneracy of the VBL. As a result, velocity reduction originating from the Walker breakdown is completely suppressed, leading to a soliton-like constant velocity of the DW. Furthermore, the strength of the DMI, which is the key factor for soliton-like DW motion, can be quantified without any side effects possibly arising from current-induced torques or extrinsic pinnings in magnetic films. Our results therefore shed light on the physics of dynamic topological defects, which paves the way for future work in topology-based memory applications.

cond-mat.mtrl-sci↗