arXiv subjects

Hidekazu Kurebayashi

Publications and source records attributed to Hidekazu Kurebayashi.

29 records · Page 2Linked to original sources

arXiv2022

Reconfigurable Training and Reservoir Computing in an Artificial Spin-Vortex Ice via Spin-Wave Fingerprinting

Strongly-interacting artificial spin systems are moving beyond mimicking naturally-occurring materials to emerge as versatile functional platforms, from reconfigurable magnonics to neuromorphic computing. Typically artificial spin systems comprise nanomagnets with a single magnetisation texture: collinear macrospins or chiral vortices. By tuning nanoarray dimensions we achieve macrospin/vortex bistability and demonstrate a four-state metamaterial spin-system 'Artificial Spin-Vortex Ice' (ASVI). ASVI can host Ising-like macrospins with strong ice-like vertex interactions, and weakly-coupled vortices with low stray dipolar-field. Vortices and macrospins exhibit starkly-differing spin-wave spectra with analogue-style mode-amplitude control and mode-frequency shifts of df = 3.8 GHz. The enhanced bi-textural microstate space gives rise to emergent physical memory phenomena, with ratchet-like vortex training and history-dependent nonlinear fading memory when driven through global field cycles. We employ spin-wave microstate fingerprinting for rapid, scaleable readout of vortex and macrospin populations and leverage this for spin-wave reservoir computation. ASVI performs linear and non-linear mapping transformations of diverse input signals as well as chaotic time-series forecasting. Energy costs of machine learning are spiralling unsustainably, developing low-energy neuromorphic computation hardware such as ASVI is crucial to achieving a zero-carbon computational future.

cond-mat.mes-hall↗

arXiv2021

Tunable gigahertz dynamics of low-temperature skyrmion lattice in a chiral magnet

Recently, it has been shown that the chiral magnetic insulator Cu$_2$OSeO$_3$ hosts skyrmions in two separated pockets in temperature and magnetic field phase space. It has also been shown that the predominant stabilization mechanism for the low-temperature skyrmion (LTS) phase is via the crystalline anisotropy, opposed to temperature fluctuations that stabilize the well-established high-temperature skyrmion (HTS) phase. Here, we report on a detailed study of LTS generation by field cycling, probed by GHz spin dynamics in Cu$_2$OSeO$_3$. LTSs are populated via a field cycling protocol with the static magnetic field applied parallel to the $\langle{100}\rangle$ crystalline direction of plate and cuboid-shaped bulk crystals. By analyzing temperature-dependent broadband spectroscopy data, clear evidence of low-temperature skyrmion excitations with clockwise (CW), counterclockwise (CCW), and breathing mode (BR) character at temperatures below $T$ = 40 K are shown. We find that the mode intensities can be tuned with the number of field-cycles below the saturation field. By tracking the resonance frequencies, we are able to map out the field-cycle-generated LTS phase diagram, from which we conclude that the LTS phase is distinctly separated from the high-temperature counterpart. We also study the mode hybridization between the dark CW and the BR modes as a function of temperature. By using two Cu$_2$OSeO$_3$ crystals with different shapes and therefore different demagnetization factors, together with numerical calculations, we unambiguously show that the magnetocrystalline anisotropy plays a central role for the mode hybridization.

cond-mat.mtrl-sci↗

arXiv2021

Magnetism, symmetry and spin transport in van der Waals layered systems

The discovery of an ever increasing family of atomic layered magnetic materials, together with the already established vast catalogue of strong spin-orbit coupling (SOC) and topological systems, calls for some guiding principles to tailor and optimize novel spin transport and optical properties at their interfaces. Here we focus on the latest developments in both fields that have brought them closer together and make them ripe for future fruitful synergy. After outlining fundamentals on van der Waals (vdW) magnetism and SOC effects, we discuss how their coexistence, manipulation and competition could ultimately establish new ways to engineer robust spin textures and drive the generation and dynamics of spin current and magnetization switching in 2D materials-based vdW heterostructures. Grounding our analysis on existing experimental results and theoretical considerations, we draw a prospective analysis about how intertwined magnetism and spin-orbit torque (SOT) phenomena combine at interfaces with well-defined symmetries, and how this dictates the nature and figures-of-merit of SOT and angular momentum transfer. This will serve as a guiding role in designing future non-volatile memory devices that utilize the unique properties of 2D materials with the spin degree of freedom.

cond-mat.mtrl-sci↗

arXiv2021

Charge density waves in electron-doped molybdenum disulfide

We present the discovery of a charge density wave (CDW) ground state in heavily electron-doped molybdenum disulfide (MoS$_2$). This is the first observation of a CDW in any $d^2$ (column 6) transition metal dichalcogenide (TMD). The band structure of MoS$_2$ is distinct from the $d^0$ and $d^1$ TMDs in which CDWs have been previously observed, facilitating new insight into CDW formation. We demonstrate a metal-insulator transition at 85 K, a 25 meV gap at the Fermi level, and two distinct CDW modulations, $(2\sqrt{3}\times2\sqrt{3})$R$30^\circ$ and $2\times2$, attributable to Fermi surface nesting (FSN) and electron-phonon coupling (EPC), respectively. This simultaneous exhibition of FSN and EPC CDW modulations is unique among observations of CDW ground states, and we discuss this in the context of band folding. Our observations provide a route toward the resolution of controversies surrounding the origin of CDW modulations in TMDs.

cond-mat.str-el↗

arXiv2021

Coupling microwave photons to topological spin-textures in Cu$_2$OSeO$_3$

Topologically protected nanoscale spin textures, known as magnetic skyrmions, possess particle-like properties and feature emergent magnetism effects. In bulk cubic heli-magnets, distinct skyrmion resonant modes are already identified using a technique like ferromagnetic resonance in spintronics. However, direct light-matter coupling between microwave photons and skyrmion resonance modes has not been demonstrated yet. Utilising two distinct cavity systems, we realise to observe a direct interaction between the cavity resonant mode and two resonant skyrmion modes, the counter-clockwise gyration and breathing modes, in bulk Cu$_2$OSeO$_3$. For both resonant modes, we find the largest coupling strength at 57 K indicated by an enhancement of the cavity linewidth at the degeneracy point. We study the effective coupling strength as a function of temperature within the expected skyrmion phase. We attribute the maximum in effective coupling strength to the presence of a large number of skyrmions, and correspondingly to a completely stable skyrmion lattice. Our experimental findings indicate that the coupling between photons and resonant modes of magnetic skyrmions depends on the relative density of these topological particles instead of the pure spin number in the system.

cond-mat.mes-hall↗

arXiv2021

Spin-orbit coupling suppression and singlet-state blocking of spin-triplet Cooper pairs

An inhomogeneous magnetic exchange field at a superconductor/ferromagnet interface converts spin-singlet Cooper pairs to a spin-aligned (i.e. spin-polarized) triplet state. Although the decay envelope of such triplet pairs within ferromagnetic materials is well studied, little is known about their decay in non-magnetic metals and superconductors, and in particular in the presence of spin-orbit coupling (SOC). Here we investigate devices in which triplet supercurrents are injected into the s-wave superconductor Nb. In the normal state of Nb, triplet supercurrents decay over a distance of 5 nm, which is an order of magnitude smaller than the decay of spin singlet pairs due to the SOC interacting with the spin associated with triplet pairs. In the superconducting state of Nb, triplet supercurrents are not able to couple with the singlet wavefunction and thus blocked by the absence of available equilibrium states in the singlet gap. The results offer new insight into the dynamics between s-wave singlet and s-wave triplet states.

cond-mat.supr-con↗

arXiv2020

Tailoring Magnetic Anisotropy in Cr$_2$Ge$_2$Te$_6$ by Electrostatic Gating

Electrical control of magnetism of a ferromagnetic semiconductor offers exciting prospects for future spintronic devices for processing and storing information. Here, we report observation of electrically modulated magnetic phase transition and magnetic anisotropy in thin crystal of Cr$_2$Ge$_2$Te$_6$ (CGT), a layered ferromagnetic semiconductor. We show that heavily electron-doped ($\sim$ $10^{14}$ cm$^{-2}$) CGT in an electric double-layer transistor device is found to exhibit hysteresis in magnetoresistance (MR), a clear signature of ferromagnetism, at temperatures up to above 200 K, which is significantly higher than the known Curie temperature of 61 K for an undoped material. Additionally, angle-dependent MR measurements reveal that the magnetic easy axis of this new ground state lies within the layer plane in stark contrast to the case of undoped CGT, whose easy axis points in the out-of-plane direction. We propose that significant doping promotes double-exchange mechanism mediated by free carriers, prevailing over the superexchange mechanism in the insulating state. Our findings highlight that electrostatic gating of this class of materials allows not only charge flow switching but also magnetic phase switching, evidencing their potential for spintronics applications.

cond-mat.mes-hall↗

arXiv2019

Tunable pure spin supercurrents and the demonstration of a superconducting spin-wave device

Recent ferromagnetic resonance experiments and theory of Pt/Nb/Ni8Fe2 proximity-coupled structures strongly suggest that spin-orbit coupling (SOC) in Pt in conjunction with a magnetic exchange field in Ni8Fe2 are the essential ingredients to generate a pure spin supercurrent channel in Nb. Here, by substituting Pt for a perpendicularly magnetized Pt/Co/Pt spin-sink, we are able to demonstrate the role of SOC, and show that pure spin supercurrent pumping efficiency across Nb is tunable by controlling the magnetization direction of Co. By inserting a Cu spacer with weak SOC between Nb and Pt/(Co/Pt) spin-sink, we also prove that Rashba type SOC is key for forming and transmitting pure spin supercurrents across Nb. Finally, by engineering these properties within a single multilayer structure, we demonstrate a prototype superconducting spin-wave (SW) device in which lateral SW propagation is gateable via the opening or closing of a vertical pure spin supercurrent channel in Nb.

cond-mat.mes-hall↗

arXiv2019

Abrikosov vortex nucleation and its detrimental effect on superconducting spin pumping in Pt/Nb/Ni80Fe20/Nb/Pt proximity structures

We report Abrikosov vortex nucleation in Pt/Nb/Ni80Fe20/Nb/Pt proximity-coupled structures under oblique ferromagnetic resonance (FMR) that turns out to be detrimental to superconducting spin pumping. By measuring an out-of-plane field-angle θH dependence and comparison with Pt-absent control samples, we show that as θH increases, the degree of enhancement (suppression) of spin pumping efficiency in the superconducting state for the Pt-present (Pt-absent) sample diminishes and it reverts to the normal state value at θH = 90°. This can be explained in terms of a substantial out-of-plane component of the resonance field for the Ni80Fe20 layer (with in-plane magnetization anisotropy and high aspect ratio) that approaches the upper critical field of the Nb, turning a large fraction of the singlet superconductor volume into the normal state.

cond-mat.mes-hall↗

arXiv2019

Effect of Meissner screening and trapped magnetic flux on magnetization dynamics in thick Nb/Ni80Fe20/Nb trilayers

We investigate the influence of Meissner screening and trapped magnetic flux on magnetization dynamics for a Ni80Fe20 film sandwiched between two thick Nb layers (100 nm) using broadband (5-20 GHz) ferromagnetic resonance (FMR) spectroscopy. Below the superconducting transition Tc of Nb, significant zero-frequency line broadening (5-6 mT) and DC resonance field shift (50 mT) to a low field are both observed if the Nb thickness is comparable to the London penetration depth of Nb films (>= 100 nm). We attribute the observed peculiar behaviors to the increased incoherent precession near the Ni80Fe20/Nb interface and the effectively focused magnetic flux in the middle Ni80Fe20 caused by strong Meissner screening and (defect-)trapped flux of the thick adjacent Nb layers. This explanation is supported by static magnetic properties of the samples and comparison with FMR data on thick Nb/Ni80Fe20 bilayers. Great care should therefore be taken in the analysis of FMR response in ferromagnetic Josephson structures with thick superconductors, a fundamental property for high-frequency device applications of spin-polarized supercurrents.

cond-mat.mes-hall↗

arXiv2014

Magnonic Charge Pumping via Spin-Orbit Coupling

The interplay between spin, charge, and orbital degrees of freedom has led to the development of spintronic devices like spin-torque oscillators, spin-logic devices, and spin-transfer torque magnetic random-access memories. In this development spin pumping, the process where pure spin-currents are generated from magnetisation precession, has proved to be a powerful method for probing spin physics and magnetisation dynamics. The effect originates from direct conversion of low energy quantised spin-waves in the magnet, known as magnons, into a flow of spins from the precessing magnet to adjacent normal metal leads. The spin-pumping phenomenon represents a convenient way to electrically detect magnetisation dynamics, however, precessing magnets have been limited so far to pump pure spin currents, which require a secondary spin-charge conversion element such as heavy metals with large spin Hall angle or multi-layer layouts to be detectable. Here, we report the experimental observation of charge pumping in which a precessing ferromagnet pumps a charge current, demonstrating direct conversion of magnons into high-frequency currents via the relativistic spin-orbit interaction. The generated electric current, differently from spin currents generated by spin-pumping, can be directly detected without the need of any additional spin to charge conversion mechanism and amplitude and phase information about the relativistic current-driven magnetisation dynamics. The charge-pumping phenomenon is generic and gives a deeper understanding of the recently observed spin-orbit torques, of which it is the reciprocal effect and which currently attract interest for their potential in manipulating magnetic information. Furthermore, charge pumping provides a novel link between magnetism and electricity and may find application in sourcing alternating electric currents.

cond-mat.mes-hall↗