The quantum physics of electron spins and photons



Spin currents in Pt-Bi alloy

We have studied the spin torque efficiency of Pt_{1-x}Bi_{x} alloy.
The spin Hall angle increases with Bi concentration and exceeds 0.3 when x is close to 0.9.
As the magnetic easy axis of a 0.6 nm thick Co layer deposited on the alloy points along the film normal, the system serves as an ideal system for spin–orbit torque magnetization switching devices.

Charge–spin conversion in Pt_{1-x}Bi_{x} alloys for spin–orbit torque switching
Z. Chi, Y.-C. Lau, M. Kawaguchi, M. Hayashi
APL Mater. 9, 061111 (2021).

Helicity dependent photocurrent in metal/semimetal bilayers

We have studied helicity dependent photocurrent (HDP) in metal/semimetal (Bi) bilayers.
Compared to Bi single layer films, we find the HDP is enhanced in metal/Bi bilayers.
For the bilayers, the sign of HDP under back illumination reverses from that of front illumination.
Using spin transport calculations, we show that the HDP sign reversal under back illumination is caused by spin absorption and spin to charge conversion at the metal/Bi interface.
These results show that the HDP can be used to assess interface states with strong spin orbit coupling.

Interface-enhanced helicity dependent photocurrent in metal/semimetal bilayers
H. Hirose, M. Kawaguchi, Y.-C. Lau, Z. Chi, F. Freimuth, K. Takanashi, M. Hayashi
Phys. Rev. B 103, 174437 (2021).

Polarized THz pulses from a spintronic THz emitter!

Two fundamental terahertz cylindrical vector beams (CVBs), azimuthally- and radially-polarized THz pulses, are generated from a spintronic THz emitter.
A metallic bilayer, consisting of a ferromagnetic layer and heavy metal layer, is irradiated with a femtosecond laser pulse.
By applying appropriate magnetic fields and using a triangular Si prism, THz CVBs are observed from the emitter.
The approach facilitates access of CVBs and paves the way toward polarization control in the THz regime.

Switchable generation of azimuthally-and radially-polarized terahertz beams from a spintronic terahertz emitter
H. Niwa, N. Yoshikawa, M. Kawaguchi, M. Hayashi, R. Shimano
Optics Express 29, 13331 (2021).
[Editors' pick]

Naruo Ohga won the Unviersity of Tokyo, Faculty of Science award!

Great work! Congratulations!!

Machine learning and pattern recognition are used to determine the DMI from a single magnetic domain image!

The Dzyaloshinskii–Moriya interaction (DMI) and the magnetic anisotropy distribution of thin-film heterostructures, parameters critical in developing next-generation storage class magnetic memory technologies, are estimated from a single magnetic domain image using machine learning and pattern recognition.
Micromagnetic simulation is used to generate thousands of random images for training and a convolutional neural network system is employed as the learning tool.
We show that the trained system can independently determine the DMI and magnetic anisotropy distribution, demonstrating the potential of image recognition for materials research.

Determination of the Dzyaloshinskii-Moriya interaction using pattern recognition and machine learning
M. Kawaguchi, K. Tanabe, K. Yamada, T. Sawa, S. Hasegawa, M Hayashi, Y. Nakatani
npj Computational Materials 7, 1 (2021).
[Press release]

First principles studies on the relation between the anomalous Hall and spin Hall effects in ferromagnets

The symmetry of the Berry curvature and the spin Berry curvature in ferromagnets are studied to reveal the correlation between the anomalous Hall and the spin Hall conductivities.
We find the Berry curvature preserves the crystal rotation symmetry whereas the symmetry of the spin Berry curvature is reduced.
Such symmetry reduction is most notable at band crossing points where the Bloch states have opposite spin characters.
The symmetry reduction implies that the anomalous Hall conductivity does not simply scale with the spin Hall conductivity with a scaling factor being the spin polarization at the Fermi level.

First Principles Investigation of Anomalous Hall and Spin Hall Effects in Ferromagnetic CoPt
G. Qu, K. Nakamura, M. Hayashi
J. Phys. Soc. Jpn. 90, 024707 (2021).

Observation of the acoustic spin Hall effect in strong spin-orbit metals

Under excitation of surface acoustic wave (SAW), we find a spin current flows orthogonal to the SAW propagation in nonmagnetic metals.
The spin current scales with the spin orbit interaction (SOI) and the time derivative of the lattice displacement.
These results, which imply the strong coupling of electron spins with rotating lattices via the SOI, show the potential of lattice dynamics to supply spin current in strong spin-orbit metals.

Acoustic spin Hall effect in strong spin-orbit metals
T. Kawada, M. Kawaguchi, T. Funato, H. Kohno, M. Hayashi
Science Advances 7, eabd9697 (2021).
[Press release]


Bismuth thin films emit light in the terahertz frequency range!

Circularly polarized femtosecond laser pulses are irradiated to Bismuth thin films, from which THz emmision is observed via ultrafast electric current.
The origin of terahertz emission is attributed to photoinduced inverse spin Hall effect in Bi.
The results demonstrate the potential of Dirac electrons in bismuth as a simple and controllable terahertz source.

Terahertz Emission from Thin Films Induced by Excitation with Circularly Polarized Light
Y. Hirai, N. Yoshikawa, H. Hirose, M. Kawaguchi, M. Hayashi, R. Shimano
Phys. Rev. Appl. 14, 064015 (2020).
[Editors' suggestion]


Spin Hall effect of ferromagnets depends on their magnetization direction.

Using first principle calculations and modeling, we show that the spin Hall conductivity of bcc Fe and fcc Ni depends on the direction of magnetization.
The spin Hall conductivity of electrons with their spin orientation orthogonal to the magnetization are found to be larger than that when the two are parallel.
These results show that ferromagnets can be used to generate spin current and its magnitude can be controlled by the magnetization direction.

Magnetization direction dependent spin Hall effect in ferromagnets
G. Qu, K. Nakamura, M. Hayashi
Phys. Rev. B 102, 144440 (2020).

Superconducting properties of W thin films on topological insulators

An anomalously large in-plane upper critical field, exceeding at least 2.5 times the Pauli paramagnetic limit, is found in a thin superconducting W film grown on a topological insulator (Bi,Sb)2Te3.
The results suggest the universality of the spin–orbit scattering formalism for describing the transport involving the diffusive carriers as well as the Cooper pairs in systems with strong spin–orbit coupling.

Concomitance of superconducting spin–orbit scattering length and normal state spin diffusion length in W on (Bi,Sb)2Te3
Y.-C. Lau, R. Akiyama, H. T. Hirose, R. Nakanishi, T. Terashima, S. Uji, S. Hasegawa, M. Hayashi
J. Phys. Mater. 3, 034001 (2020).

Hana Hirose and Takuya Kawada won the Unviersity of Tokyo, School of Science award!

Great work! Congratulations!!

Giant spin orbit torque found in systems with Dirac electrons.

Dirac-like electrons in BiSb semimetallic alloys are found to be extremely effective in generating spin current.
The film structure, composition, layer thickness and temperature dependences of the spin Hall conductivity show that it is the bulk of the alloy that predominantly contributes to the generation of spin current; the topological surface states, if present in the films, play little role.
The thermally-excited Dirac-like electrons in the L-valley of the semimetal possess an extremely large spin current mobility, leading to a spin Hall angle that exceeds 1.

The spin Hall effect of Bi-Sb alloys driven by thermally excited Dirac-like electrons
Z. Chi, Y.-C. Lau, X. Xu, T. Ohkubo, K. Hono, M. Hayashi
Science Advances, 6, eaay2324 (2020).

Our paper on highly efficient spin orbit torque in sythetic antiferromagnets published.

The spin orbit torque exerted on sythetic antiferromagnetic multilayers is found to be ~15 times more efficient than that of ferrogmagnetically coupled multilayers.
Model calculations based on the Landau-Lifshitz-Gilbert equation predicts an enhancement factor of 1.2-1.4.
Other sources of spin current may account for the highly efficient SOT in synthetic antiferromagnet multilayers.

Highly efficient spin-orbit torque in Pt/Co/Ir multilayers with antiferromagnetic interlayer exchange coupling
Y. Ishikuro, M. Kawaguchi, T. Taniguchi, M. Hayashi
Phys. Rev. B 101, 014404 (2020).


Takuya Kawada selected as the Finalist of the "Best student presentation award" at MMM!

Place: 64th Annual Conference on Magnetism and Magnetic Materials (MMM) at Las Vegas.
Title: Surface acoustic wave induced DC spin current in spin-orbit heterostructures.

Our paper on the giant perpendicular magnetic anisotropy in Ir/Co/Pt multilayers is published.

Ir/Co/Pt multilayers exhibit oscillatory interlayer exchange coupling, a magnetic exchange coupling that oscillates between ferromagnetic coupling and antiferromagnetic coupling depending on the thickness of Ir.
We find a large perpendicular magnetic aniostropy appears, with the anisotropy field exceeding 10 T, when the Co layers are coupled antiferromagnetically.

Giant perpendicular magnetic anisotropy in Ir/Co/Pt multilayers
Y.-C. Lau, Z. Chi, T. Taniguchi, M. Kawaguchi, G. Shibata, N. Kawamura, M. Suzuki, S. Fukami, A. Fujimori, H. Ohno, M. Hayashi
Phys. Rev. Mater. 3, 104419 (2019).

DMI modulation with current.

We find the Dzyaloshinskii-Moriya interaction (DMI) that emerges at heavy metal/ferromagnetic metal interfaces can be modulated with current.
The DMI increases with increasing current with a rate that agrees with what is predicted by the model based on the spin Doppler effect.
The results imply that the interfacial DMI may partly originate from equilibrium spin current which can be modulated externally.

Current induced modulation of interfacial Dzyaloshinskii-Moriya interaction
N. Kato, M. Kawaguchi, Y.-C. Lau, T. Kikuchi, Y. Nakatani, M. Hayashi
Phys. Rev. Lett. 122, 257205 (2019).

A giant acoustic planar Hall effect observed.

We find a planar-Hall like transverse voltage develops in magnetic thin film when surface acoustic waves (SAW) are excited in feromagnetic thin films.
The sign of the field angle dependent transverse voltage does not depend on the SAW propagation direction but its size is more than an order of magnitude larger than its electronic counterpart (i.e. planar Hall effect).
These results imply that acoustic current can generate magneto-galvanic effects similar to the electric and thermo-electric currents but the effect can be significantly larger.

Unidirectional planar Hall voltages induced by surface acoustic waves in ferromagnetic thin films
T. Kawada, M. Kawaguchi, M. Hayashi
Phys. Rev. B 99, 184435 (2019).

Studies on the DMI and spin orbit torque at the Ir/Co interface published.

The DMI of the Ir and Co interface is found to be similar in magnitude with that of the Pt and Co interface.
The sign of the Ir/Co DMI is the same with that of the Pt/Co interface.
Ir has small a spin Hall angle of 0.01 but acts as a good spin sink, which can be exploited for engineering spin orbit torque in spin orbit heterostructures.

Dzyaloshinskii-Moriya interaction and spin-orbit torque at the Ir/Co interface
Y. Ishikuro, M. Kawaguchi, N. Kato, Y.-C. Lau, M. Hayashi
Phys. Rev. B 99, 134421 (2019).

Our paper on the spin Hall effect in Co-Ga alloy is published.

Spin Hall angle of ~0.05 is found in an alloy formed from elements with relatively small spin spin orbit coupling.
First principle calculations show that hybridization of the Co 3d and Ga 4p orbitals contribute to the large spin Hall effect in this alloy.

Spin Hall effect from 3d-4p hybridized orbitals
Y.-C. Lau, H. Lee, G. Qu, K. Nakamura, M. Hayashi
Phys. Rev. B 99, 064410 (2019).


Hana Hirose won the JSAP young scientist presentation award!

Hana Hirose won the 45th young scientist presentation award for her presentation at the 79th Japan Applied Physics Society (JSAP) autumn meeting.
Presentation title: "Material dependence of interfacial spin-momentum locked bands in metallic heterostructures."

Our paper was selected as a featured article!

Circular photogalvanic effect in Cu/Bi bilayers
H. Hirose, N. Ito, M. Kawaguchi, Y. C. Lau, M. Hayashi
Appl. Phys. Lett. 113, 222404 (2018).

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Spin orbit induced photocurrent found in Bi/Cu bilayer.

Irradiation of circularly polarized light in the visible range generates a helicity dependent photocurrent in Cu/Bi bilayers.
The effect can arise from a spin momentum locked band at the Cu/Bi interface.

Circular photogalvanic effect in Cu/Bi bilayers
H. Hirose, N. Ito, M. Kawaguchi, Y. C. Lau, M. Hayashi
Appl. Phys. Lett. 113, 222404 (2018).

Spin orbit torque detected optically!

The spin orbit torque of W/CoFeB/MgO and Pt/CoFeB/MgO heterostructures are studied using magneto-optical Kerr effect. Signals from the current induced torque and current induced heating are separated using a double modulation scheme.

Optical Detection of Spin-Orbit Torque and Current-Induced Heating
Y. Marui, M. Kawaguchi, M. Hayashi
Appl. Phys. Express 11, 5 093001 (2018).

Hana Hirose won the Best presentation award!

Hana Hirose won the Best presentation award at the Condensed Matter Physics Summer School.

Our paper on the resistance of chiral domain walls is published.

Resistance associated with chiral domain walls is studied. Contributon form the anomalous Hall effect dominates the domain wall resistance, but we also find effects that can be attributed to the chiral magnetoresistance.

Domain-Wall Resistance in Cofeb-Based Heterostructures with Interface Dzyaloshinskii-Moriya Interaction
Y. Ishikuro, M. Kawaguchi, Y. C. Lau, Y. Nakatani, M. Hayashi
Appl. Phys. Express 11, 5 073001 (2018).

Our paper on the spin Hall magnetoresistance in Pt/Co bilayers is published.

An anomalously large spin Hall magnetoresistance (SMR) is found in Co/Pt bilayers. The effective spin Hall angle derived from the SMR approaches 0.4, indicating that the measured SMR includes contribution from other spin dependent effect.

Anomalous Spin Hall Magnetoresistance in Pt/Co Bilayers
M. Kawaguchi, D. Towa, Y. C. Lau, S. Takahashi, M. Hayashi
Appl. Phys. Lett. 112, 5 202405 (2018).

Naoaki Kato won the Unviersity of Tokyo, Faculty of Science award!

Great work! Congratulations!!