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We investigated the coherently excited propagating magnons in in-plane magnetized synthetic antiferromagnets using heterodyne-magneto-optical Kerr effect technique. Under the in-plane magnetic field to 45° away from the magnon propagation direction, we observed the mode splitting of magnon dispersion relation due to the magnon-magnon coupling between acoustic and optical magnon modes. The strong coupling with a cooperativity of 8.4 ± 1.3 is achieved for finite wavenumber. These results provide a deep understanding of the properties of coherently excited magnons in synthetic antiferromagnets, which will be helpful for designing advanced magnonic devices.

While in the Pt/Co/MgO films the interfacial DMI correlates with the anisotropy of the orbital magnetic moment and the dipole moment, our XMCD and ab initio study of the Ru/Co/W/Ru system reveals the correlation of DMI with the spin moment of W.

Ferrimagnets composed of multiple and antiferromagnetically coupled magnetic elements have attracted much attention recently as a material platform for spintronics. They offer the combined advantages of both ferromagnets and antiferromagnets, namely the easy control and detection of their net magnetization by an external field, antiferromagnetic-like dynamics faster than ferromagnetic dynamics and the potential for high-density devices. This Review summarizes recent progress in ferrimagnetic spintronics, with particular attention to the most-promising functionalities of ferrimagnets, which include their spin transport, spin texture dynamics and all-optical switching.

A Rayleigh-type surface acoustic wave (SAW) is used in various fields as classical and quantum information carriers because of its surface localization, high electrical controllability, and low propagation loss. Coupling and hybridization between the SAW and other physical systems such as magnetization, electron charge, and electron spin are the recent focuses in phononics and spintronics. A precise measurement of the surface wave amplitude is often necessary to discuss the coupling strengths. However, there are only a few such measurement techniques and they generally require a rather complex analysis. Here we develop and demonstrate a straightforward measurement technique that can quantitatively characterize the SAW. The technique optically detects the surface waving due to the coherently driven SAW by the optical path modulation. Furthermore, when the measurement system operates in the shot-noise-limited regime, the surface slope and displacement at the optical spot can be deduced from the optical path modulation signal. Our demonstrated technique will be an important tool for SAW-related research., 13 pages, 8 figures

Dzyaloshinskii–Moriya interaction (DMI) is considered as one of the most important energies for specific chiral textures such as magnetic skyrmions. The keys of generating DMI are the absence of structural inversion symmetry and exchange energy with spin–orbit coupling. Therefore, a vast majority of research activities about DMI are mainly limited to heavy metal/ferromagnet bilayer systems, only focusing on their interfaces. Here, we report an asymmetric band formation in a superlattices (SL) which arises from inversion symmetry breaking in stacking order of atomic layers, implying the role of bulk-like contribution. Such bulk DMI is more than 300% larger than simple sum of interfacial contribution. Moreover, the asymmetric band is largely affected by strong spin–orbit coupling, showing crucial role of a heavy metal even in the non-interfacial origin of DMI. Our work provides more degrees of freedom to design chiral magnets for spintronics applications.

The diode effect is fundamental to electronic devices and is widely used in rectifiers and AC-DC converters. At low temperatures, however, conventional semiconductor diodes possess a high resistivity, which yields energy loss and heating during operation. The superconducting diode effect (SDE), which relies on broken inversion symmetry in a superconductor may mitigate this obstacle: in one direction a zero-resistance supercurrent can flow through the diode, but for the opposite direction of current flow, the device enters the normal state with ohmic resistance. The application of a magnetic field can induce SDE in Nb/V/Ta superlattices with a polar structure, in superconducting devices with asymmetric patterning of pinning centres, or in superconductor/ferromagnet hybrid devices with induced vortices. The need for an external magnetic field limits their practical application. Recently, a field-free SDE was observed in a NbSe2/Nb3Br8/NbSe2 junction, and it originates from asymmetric Josephson tunneling that is induced by the Nb3Br8 barrier and the associated NbSe2/Nb3Br8 interfaces. Here, we present another implementation of zero-field SDE using noncentrosymmetric [Nb/V/Co/V/Ta]20 multilayers. The magnetic layers provide the necessary symmetry breaking and we can tune the SDE by adjusting the structural parameters, such as the constituent elements, film thickness, stacking order, and number of repetitions. We control the polarity of the SDE through the magnetization direction of the ferromagnetic layers. Artificially stacked structures, as the one used in this work, are of particular interest as they are compatible with microfabrication techniques and can be integrated with devices such as Josephson junctions. Energy-loss-free SDEs as presented in this work may therefore enable novel non-volatile memories and logic circuits with ultralow power consumption., 31 pages, 11 figures

Spin–orbit torque (SOT) switching in chiral antiferromagnets, such as D019-Mn3Sn, has opened prospects in antiferromagnetic spintronics. In this paper, we investigated the critical switching voltage as a function of the voltage pulse width in order to explore the thermal stability factor Δ in the D019-Mn3Sn. Although the critical switching voltage is found to decrease with increasing the voltage pulse width as expected by the thermally activated switching process with Δ of 131.1 at 300 K, our detailed examinations reveal that the Joule heating due to the voltage pulse could induce the magnetic phase transition in the D019-Mn3Sn, which significantly alters the energy landscape relevant to the SOT switching. This assisting mechanism for the SOT switching inhibits an accurate determination of Δ but could deliver a smaller Δ than the actual. Our study provides cautionary note in estimating the thermal stability of the chiral antiferromagnets.

Understanding the nature of recently discovered spin-orbital induced phenomena and a definition of a general approach for "ferromagnet/heavy-metal" layered systems to enhance and manipulate spin-orbit coupling, spin-orbit torque, and the Dzyaloshinskii-Moriya interaction (DMI) assisted by atomic-scale interface engineering are essential for developing spintronics and spin-orbitronics. Here, we exploit X-ray magnetic circular dichroism (XMCD) spectroscopy at the

Nonlinear optical and electrical effects associated with a lack of spatial inversion symmetry allow direction-selective propagation and transport of quantum particles, such as photons1 and electrons2-9. The most common example of such nonreciprocal phenomena is a semiconductor diode with a p-n junction, with a low resistance in one direction and a high resistance in the other. Although the diode effect forms the basis of numerous electronic components, such as rectifiers, alternating-direct-current converters and photodetectors, it introduces an inevitable energy loss due to the finite resistance. Therefore, a worthwhile goal is to realize a superconducting diode that has zero resistance in only one direction. Here we demonstrate a magnetically controllable superconducting diode in an artificial superlattice [Nb/V/Ta]n without a centre of inversion. The nonreciprocal resistance versus current curve at the superconducting-to-normal transition was clearly observed by a direct-current measurement, and the difference of the critical current is considered to be related to the magnetochiral anisotropy caused by breaking of the spatial-inversion and time-reversal symmetries10-13. Owing to the nonreciprocal critical current, the [Nb/V/Ta]n superlattice exhibits zero resistance in only one direction. This superconducting diode effect enables phase-coherent and direction-selective charge transport, paving the way for the construction of non-dissipative electronic circuits.

X-ray magnetic circular dichroism (XMCD) spectroscopy is a key technique for studying magnetic materials, providing element specificity and significantly high detection sensitivity to magnetic mome...

We have developed a method to obtain wideband magnetic polarization selective spectra of magnetic thin films by using circularly polarized microwaves. The combination of an over-coupled crossed microstrip resonator and a hybrid coupler enables broadband and accurate control of circularly polarized microwaves. The performance of the present method was demonstrated with an yttrium iron garnet thin film, and we detected the magnetic polarization dependence of a Kittel mode and a perpendicular standing spin wave mode in the range of 3-20 GHz. In addition, three types of crossed microstrip resonators were systematically studied with experiments and simulations. As a result, we achieved the polarization efficiency of 80% and found how to optimize the crossed microstrip resonator depending on the desired sensitivity, bandwidth, or sample size.

We demonstrated the generation of perpendicularly spin-polarized electronic currents using a tunnel spin-filtering effect through insulative Fe-rich cobalt ferrite Co xFe3− xO4+δ (I-CFO) barriers with perpendicular magnetic anisotropy (PMA). The I-CFO films grown on conductive Fe-rich cobalt ferrite Co yFe3− yO4 (C-CFO) films, which were deposited on additional I-CFO buffer layers on MgO(001) substrates, exhibited PMA induced by an epitaxial strain. Magnetic tunnel junctions (MTJs), which comprise C-CFO electrode layers, I-CFO barrier layers, and perpendicularly magnetized Co/{Tb/Co}15/Co spin detection layers, showed a tunnel magnetoresistance (TMR) effect. This indicated that spin-polarized tunnel currents were injected into the spin detection layers. A spin injection efficiency of −28% was observed for the MTJs with an I-CFO barrier of 3.0 nm in thickness at 100 K. The voltage dependence of the TMR effect indicates that the spin-injection efficiency is affected by voltage-dependent changes in the effective spin-dependent barrier width. The combination of spinel-type C-CFO and I-CFO films with well-controlled compositions and lattice strains is, therefore, applicable as a spin-injection source for spintronics devices when perpendicularly spin-polarized electronic currents are required.

We demonstrate that magnetic crystal anisotropy ( K u ) and spectroscopic g-factor can be modulated simultaneously by gate voltage in a Pt/Co/MgO ultrathin film with our ferromagnetic resonance (FMR) measurement based on the rectification effect. The g-factor reflects on orbital magnetic moment, indicating that the electric field induced-modulations of magnetic anisotropy in cobalt originate from the change of the anisotropy of orbital moments ( ∆ m L ). The quantitative estimation reveals that ∆ m L to K u correlation when the electric field is applied significantly differs from the one inherent at the Pt/Co/MgO interfaces. This means that the correlation itself is also modulated by the electric field. Our results show that the FMR technique is very useful for probing the electric field effect on K u .

Antiferromagnetic materials are outstanding candidates for next generation spintronic applications, because their ultrafast spin dynamics makes it possible to realize several orders of magnitude higher-speed devices than conventional ferromagnetic materials1. Though spin-transfer torque (STT) is a key for electrical control of spins as successfully demonstrated in ferromagnetic spintronics, experimental understanding of STT in antiferromagnets has been still lacking despite a number of pertinent theoretical studies2-5. Here, we report experimental results on the effects of STT on domain-wall (DW) motion in antiferromagnetically-coupled ferrimagnets. We find that non-adiabatic STT acts like a staggered magnetic field and thus can drive DWs effectively. Moreover, the non-adiabaticity parameter {\beta} of STT is found to be significantly larger than the Gilbert damping parameter {\alpha}, challenging our conventional understanding of the non-adiabatic STT based on ferromagnets as well as leading to fast current-induced antiferromagnetic DW motion. Our study will lead to further vigorous exploration of STT for antiferromagnetic spin textures for fundamental physics on spin-charge interaction as wells for efficient electrical control of antiferromagnetic devices., Comment: 19 pages, 4 figures

Symmetry breaking is a fundamental concept that prevails in many branches of physics1–5. In magnetic materials, broken inversion symmetry induces the Dzyaloshinskii–Moriya interaction (DMI), which results in fascinating physical behaviours6–14 with the potential for application in future spintronic devices15–17. Here, we report the observation of a bulk DMI in GdFeCo amorphous ferrimagnets. The DMI is found to increase linearly with an increasing thickness of the ferrimagnetic layer, which is a clear signature of the bulk nature of DMI. We also found that the DMI is independent of the interface between the heavy metal and ferrimagnetic layer. This bulk DMI is attributed to an asymmetric distribution of the elemental content in the GdFeCo layer, with spatial inversion symmetry broken throughout the layer. We expect that our experimental identification of a bulk DMI will open up additional possibilities to exploit this interaction in a wide range of materials. A composition gradient is found to provide the necessary structural inversion asymmetry for a bulk Dzyaloshinskii–Moriya interaction to manifest itself.

We successfully fabricated perpendicularly magnetized cobalt-ferrite Co x Fe 3 - x O 4 + δ (0 0 1) films on nonmagnetic metal TiN (0 0 1) films. We found that the in-plane lattice constant of the Co x Fe 3 - x O 4 + δ (0 0 1) films was larger than the out-of-plane lattice constant. This lattice strain induced perpendicular magnetic anisotropy, and square hysteresis curves were observed in out-of-plane magnetic fields for the Co x Fe 3 - x O 4 + δ (x = 0.4 ∼ 0.6) films. With the decrease of the Co composition from x = 1 to x = 0, the Co x Fe 3 - x O 4 + δ films approached not to conductive Fe 3 O 4 but to insulating γ - Fe 2 O 3 accompanying cation vacancies in the cubic spinel structure. The good tunneling conductivity character was observed for the Co x Fe 3 - x O 4 + δ tunnel junctions. The results open up a possibility that insulating Co x Fe 3 - x O 4 + δ films can be applied to elements of perpendicularly magnetized spintronics devices.

A novel label-free biosensor array is proposed, which uses the ultrafast heat transport from a metallic nano-plate into surrounding solution across proteins captured on the nano-plate. Using electron-beam lithography, 2500 gold nano-plates of 30 nm thick with 5 × 5 μm2 area were fabricated on a fused silica substrate, on which receptor proteins were immobilized. Thermal phonons were induced on the back surface of a single nano-plate by focusing ultrafast light pulses from the substrate side, which propagated toward surrounding solution through surface proteins. Because the ultrafast heat-transport behavior, which is completed within ∼1 ns, is affected by existence of target proteins on the nano-plate, each nano-plate works as a single label-free biosensor. A theoretical calculation was made for predicting the cooling-down behavior of the nano-plate after an impulsive heating, and a pump–probe optics was developed for monitoring the ultrafast temperature change after excitation with a femtosecond fiber laser. We then confirm that each nano-plate acts as a single label-free biosensor by performing antigen–antibody binding reactions, indicating that a large scale number of multichannel measurement is made possible with this phenomenon.

Chiral antiferromagnets are regarded as a new class of antiferromagnets owing to its intriguing spin transport properties such as a sizable anomalous Hall effect (AHE). In this work, we fabricated chiral antiferromagnetic L12-Mn3Ir (111)-oriented films and investigated the anomalous Hall conductivity with respect to the chemical order parameter S . It is found that the anomalous Hall conductivity in (111)-oriented films is much more pronounced compared to (001)-oriented films, which is consistent with the theoretical prediction. We also found that the anomalous Hall conductivity strongly depends on the longitudinal conductivity, which could suggest an extrinsic contribution to the AHE in L12-Mn3Ir.

Surface acoustic wave (SAW) is utilized in diverse fields ranging from physics, engineering, to biology, for transducing, sensing and processing various signals. Optical measurement of SAW provides valuable information since the amplitude and the phase of the displacement field can be measured locally with the resolution limited by the spot size of the optical beam. So far, optical measurement techniques rely on modulation of optical path, phase, or diffraction associated with SAW. Here, we demonstrate that SAW can be measured with an optical polarimeter. We show that the slope of the periodically tilting surface due to the coherently driven SAW is translated into the angle of polarization rotation, which can be straightforwardly calibrated when polarimeters work in the shot-noise-limited regime. The polarimetric measurement of SAW is thus beneficial for quantitative studies of SAW-based technologies., 19 pages, 12 figures

Recently, magnetic multilayer systems have received again a great attention owing to their suitability for the generation of magnetic chiral spin structure. In this study, we experimentally investigated the interfacial Dzyaloshisnkii-Moriaya interaction (DMI) and the spin-Hall angle (SHA) of magnetic Co/Gd/Pt multilayers with respect to the repetition number $N$ of the multilayers. The DMI and SHA are important variables that govern the stability and mobility of a homochiral spin structure, respectively. The experimental results show that the ratio between the DMI and the dipole energy from the domain wall gradually decreases as $N$ increases, which is expected to be hard to achieve homochiral spin structure for larger $N$ and the values of SHA remain constant irrespective of $N$. The observed SHA invariance indicates that the Pt layers in repetition have a negligible effect on the SHA.

Magnetic skyrmion, i.e. a topologically stable swirling spin texture, appears as a particle-like object in the two-dimensional (2D) systems, and has recently attracted attention as a candidate of novel information carrier. In the real three-dimensional (3D) systems, a skyrmion is expected to form a string structure along an extra dimension, while its experimental identification has rarely been achieved. Here, we report the direct visualization of 3D shape of individual skyrmion strings, for the recently discovered room-temperature skyrmion-hosting noncentrosymmetric compound Mn1.4Pt0.9Pd0.1Sn. For this purpose, we have newly developed the magnetic X-ray tomography measurement system that can apply magnetic field, which plays a key role on the present achievement. Through the tomographic reconstruction of the 3D magnetization distribution based on the transmission images taken from various angles, a genuine skyrmion string running through the entire thickness of the sample, as well as various defect structures such as the interrupted and Y-shaped strings, are successfully identified. The observed point defect may represent the emergent magnetic monopole, as recently proposed theoretically. The present tomographic approach with tunable magnetic field paves the way for the direct visualization of the structural dynamics of individual skyrmion strings in the 3D space, which will contribute to the better understanding of the creation, annihilation and transfer process of these topological objects toward the potential device applications., 17 pages, 4 figures

Evidence of an exotic intermediate state---an anapole state---between the liquid and solid phases of electrons in a transition-metal oxide provides the first in-depth look at this long-sought, mysterious phase.

We investigate the magnetoresistance of ferrimagnetic GdFeCo across the magnetization compensation temperature ${T}_{\mathrm{M}}$. The magnetic field dependence of longitudinal resistivity (${\ensuremath{\rho}}_{\mathrm{xx}}$) shows opposite trends below and above ${T}_{\mathrm{M}}$, and the variation of ${\ensuremath{\rho}}_{\mathrm{xx}}$ with $B$ becomes more significant as the temperature decreases. The observed unconventional magnetoresistance is attributed to the sperimagnetism of GdFeCo. Further investigations on the transverse resistivity (${\ensuremath{\rho}}_{\mathrm{xy}}$) of GdFeCo unveils that, contrary to the recent reports that the transition metal dominates transport of rare-earth transition-metal ferrimagnets, the Gd contribution to magnetoresistance is comparable to the FeCo contribution, showing that the transport of GdFeCo is antiferromagnetic. Our results therefore show that ferrimagnets are a convenient platform for studying antiferromagnetic spin transport and also are potential materials that can enable antiferromagnetic spin devices.

The propagation of coherent spin waves in permalloy microstrips was investigated using a heterodyne-magneto-optical Kerr effect technique. The two-dimensional imaging reveals the mode interference with a high-order spin-wave mode along the microstrip width. The spin-wave dispersion relations across the microstrip width were deduced from fast Fourier transformations on the complex spin-wave amplitude of the one-dimensional scan, and multiple spin-wave modes were observed at the edges of the microstrips. These experimental results provide direct evidence for the quantization of spin-wave dispersion due to the finite width of the microstrips and enables us to evaluate the attenuation length for each quantized spin-wave mode.

Most of the existing research on the dynamics of magnetic solitons such as a domain wall (DW) has focused on the effect of DC forces, where the induced velocity is determined by the force strength. Here we show that AC forces such as an oscillating magnetic field or current are also able to move a DW straight via synchronization between the DW angle and the phase of the AC force. The resulting DW velocity is solely proportional to the driving frequency of the AC force, but the strength of the AC field just affects the frequency range for criteria for the phase-locking behavior. The AC-force-driven DW motion is shown to exhibit a phase locking-unlocking transition, a critical phenomenon akin to the Walker breakdown of a DC-bias-driven DW motion. Our work shows that a DW can be driven straight by synchronizing its angle to AC forces and thereby demonstrates a proof-of-concept of magnetic soliton rectifiers (i.e., DC motion induced by AC forces), shedding a light on the hitherto overlooked utility of internal degree of freedom for driving magnetic textures.

The antiferromagnetic order can mediate a transmission of the spin angular momentum flow, or the spin current, in the form of propagating magnons. In this work, we perform laser stimulated THz emission measurements on Pt/CoO/FeCoB multilayers to investigate the spin current transmission through CoO, an antiferromagnetic insulator, on a picosecond timescale. The results reveal a spin current transmission through CoO with the diffusion length of 3.0 nm. In addition, rotation of the polarization of the emitted THz radiation was observed, suggesting an interaction between the propagating THz magnons and the Neel vector in CoO. Our results not only demonstrate the picosecond magnon spin current transmission but also the picosecond interaction of the THz magnons with the Neel vector in the antiferromagnet.

Electric field modulations of magnetism in ferromagnetic metals are in general ascribed to the electric field effects on the orbitals. However, a direct observation of the electric-field-induced changes in the orbital states of a ferromagnetic metal is lacking. In this Rapid Communication, we demonstrate that an electric field applied to an ultrathin Co film can change the electron occupations according to the orbitals, combining x-ray absorption and x-ray magnetic circularly dichroism spectroscopies with the sum rules. Under a positive electric field, the ${d}_{xz(yz)}$-occupied states decrease mainly via the modification of orbital hybridizations, while the ${d}_{{z}^{2}}$-occupied state increases. The changes in the ${d}_{xz}$ and ${d}_{yz}$ states dominantly alter the Curie temperature.