arXiv subjects

Shutaro Karube

Publications and source records attributed to Shutaro Karube.

13 recordsLinked to original sources

arXiv2026

Coexistence of Magnon-Induced Optical Vortex and Gaussian Beam Scattering Assisted by Rotational Umklapp Process

The exploitation of crystal-lattice symmetries to engineer angular momentum transfer between structured light and magnons marks a novel frontier for optomagnonic research. In Brillouin light scattering, when focused light propagates parallel to an external magnetic field and interacts with ferromagnetic uniform magnons, only optical-vortex scattering is expected to be permitted. Due to the combined effects of magneto-optical coupling and optical spin-orbit interaction, the transfer of magnon spin angular momentum to photon orbital angular momentum allows for this distinctive scattering phenomenon. Here, we experimentally demonstrate that, for a specific ferromagnetic crystal orientation, Gaussian-beam scattering coexists with the optical-vortex scattering, contrary to conventional expectations based on angular momentum conservation between magnons and photons. We show that the crystal lattice, via the rotational Umklapp process, provides the missing angular momentum required for the Gaussian-beam scattering. Furthermore, we predict that as the degree of light focusing increases, the relative efficiencies of the Gaussian-beam and optical-vortex scattering processes reverse.

physics.optics↗

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

Optical vortex generation by magnons with spin-orbit-coupled light

Light possesses both spin and orbital angular momentum. In spatially asymmetric optical fields, these properties undergo spontaneous coupling, referred to as optical spin-orbit coupling. The study of the coupling has recently become central in modern optics due to its substantial applications in communications, sensing, and quantum control. A key challenge is to clarify the relationship between the origins of spatially asymmetric optical fields and the resulting spin-orbit coupling. Current research focuses on materials and configurations exhibiting spatial asymmetry, such as focusing lenses, interfaces, inhomogeneous media, and metasurfaces. However, Maxwell's equations indicate that matter can introduce both spatial and temporal asymmetry into optical fields. For instance, magnetic ordering breaks the time-reversal symmetry of interacting optical fields via the magneto-optic effect, introducing nonreciprocity in the resulting optical phenomena. Despite the importance, optical phenomena involving both spatially and temporally asymmetric optical fields remain unexplored. Here, we demonstrate that breaking time and spatial symmetries through magnons and light focusing, respectively, transforms an input Gaussian beam into a specific optical vortex beam in a nonreciprocal manner. This phenomenon is quantitatively explained by integrating the physics of magnon-induced Brillouin light scattering with optical spin-orbit coupling. The observed conservation of total angular momentum, encompassing both magnons and photons, further indicates that magnons can control both spin and orbital angular momentum of light. Finally, we outline future research directions enabled by asymmetric optical fields in both space and time.

physics.optics↗

arXiv2025

Néel vector rotation driven by spin-orbit torque in amorphous ferrimagnetic GdCo

Spin superfluidity, a phenomenon enabling low dissipative spin transport analogous to superfluidity in liquid helium and superconductivity in electronic systems, has remained a theoretical concept. To realize the spin superfluidity in an antiferromagnet, it is necessary to excite a Néel vector rotation within the magnetic easy-plane, which has been elusive so far. In this study, we demonstrated spin-orbit torque-driven Néel vector rotation in amorphous ferrimagnetic GdCo. A pseudo-magnetic easy-plane is formed in a spin-flop state under an external magnetic field at the vicinity of the magnetization compensation temperature, and we observed stochastic binary switching in the anomalous Hall resistance, directly attributed to Néel vector rotation. Furthermore, homodyne detection confirmed rotation frequencies in the GHz range as expected from atomic spin simulations, providing evidence of dynamic Néel vector behavior. These findings represent a crucial advance towards the realization of spin superfluidity.

cond-mat.mtrl-sci↗

arXiv2024

Quantitative evaluation method for magnetoelastic coupling between surface acoustic waves and spin waves using electrical and optical measurements

Coupling and hybridization of different elementary excitations leads to new functionalities. In phononics and spintronics, magnetoelastic coupling between Rayleigh-type surface acoustic wave (SAW) and spin wave (SW) has recently attracted much attention. Quantitatively evaluating and comparing the coupled system are essential to develop the study of the magnetoelastic SAW-SW coupling. So far, previous studies of SAW-SW coupling have employed a quantity called coupling strength. However, it is still challenging to compare the coupling strength values among studies fairly because the quantity depends on the device geometry and the applied magnetic field angle, which are not unified among the previous studies. Here, we focus on a practical constant composed of a magnetoelastic constant and a strain amplitude that depends only on the material properties. We demonstrate a versatile evaluation technique to evaluate the practical constant by combining electrical measurements and optical imaging. An essential part of the technique is an analysis that can be used under off-resonance conditions where SAW and SW resonance frequencies do not match. Existing analysis can only handle the case under on-resonance conditions. Our analysis makes it possible to observe the magnetoelastic couplings between SAW with resonance frequencies that can be imaged optically and SW with resonance frequencies in the gigahertz range. Our demonstrated technique, which uses electrical and optical measurements under off-resonance conditions, can significantly advance research on SAW-SW coupled systems.

cond-mat.mes-hall↗

arXiv2024

Handedness manipulation of propagating antiferromagnetic magnons

Antiferromagnetic magnons possess a distinctive feature absent in their ferromagnetic counterparts: the presence of two distinct handedness modes, the right-handed (RH) and left-handed (LH) precession modes. The magnon handedness determines the sign of spin polarization carried by the propagating magnon, which is indispensable for harnessing the diverse functionalities. However, the control of coherently propagating magnon handedness in antiferromagnets has remained elusive so far. Here we demonstrate the manipulation and electrical readout of propagating magnon handedness in perpendicularly magnetized synthetic antiferromagnets (SAF). We find that the antiferromagnetic magnon handedness can be directly identified by measuring the inverse spin Hall effect (ISHE) voltage, which arises from the spin pumping effect caused by the propagating antiferromagnetic magnons in the SAF structure. The RH and LH modes of the magnon can be distinguishable particularly when the SAF structure is sandwiched by heavy metals with the same sign of spin Hall angle. Moreover, we succeed in controlling the handedness of propagating antiferromagnetic magnons by tuning the excitation microwave frequency. This work unveils promising avenues for harnessing magnon unique properties in antiferromagnet-based magnonic applications.

cond-mat.mtrl-sci↗

arXiv2021

Observation of spin-splitter torque in collinear antiferromagnetic RuO$_2$

The spin-splitter effect is theoretically predicted to generate an unconventional spin current with $\mathit{x}$- and $\mathit{z}$- spin polarization via the spin-split band in antiferromagnets. The generated torque, namely spin-splitter torque, is effective for the manipulation of magnetization in an adjacent magnetic layer without an external magnetic field for spintronic devices such as MRAM. Here, we study the generation of torque in collinear antiferromagnetic RuO$_2$ with (100), (101), and (001) crystal planes. Next we find all $\mathit{x}$-, $\mathit{y}$-, and $\mathit{z}$-polarized spin currents depending on the Néel vector direction in RuO$_2$(101). For RuO$_2$(100) and (001), only $\mathit{y}$-polarized spin current was present, which is independent of the Néel vector. Using the $\mathit{z}$-polarized spin currents, we demonstrate field-free switching of the perpendicular magnetized ferromagnet at room temperature. The spin-splitter torque generated from RuO$_2$ is verified to be useful for the switching phenomenon and paves the way for a further understanding of the detailed mechanism of the spin-splitter effect and for developing antiferromagnetic spin-orbitronics.

cond-mat.mtrl-sci↗

arXiv2021

Observation of unconventional spin polarization induced spin orbit torque in L1$_2$-ordered antiferromagnetic Mn$_3$Pt thin films

Non-collinear antiferromagnets exhibits richer magneto-transport properties due to the topologically nontrivial spin structure they possess compared to conventional nonmagnetic materials, which allows us to manipulate the charge-spin conversion more freely by taking advantage of the chirality. In this work, we explore the unconventional spin orbit torque of L1$_2$-ordered Mn$_3$Pt with a triangular spin structure. We observed an unconventional spin orbit torque along the $\mathbf{x}$-direction for the (001)-oriented L1$_2$ Mn$_3$Pt, and found that it has a unique sign reversal behavior relative to the crystalline orientation. This generation of unconventional spin orbit torque for L1$_2$-ordered Mn$_3$Pt can be interpreted as stemming from the magnetic spin Hall effect. This report help clarify the correlation between the topologically nontrivial spin structure and charge-spin conversion in non-collinear antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous spin-orbit field via Rashba-Edelstein effect at W/Pt interface

We have studied spin-orbit (SO) field in Ni$_{80}$Fe$_{20}$(Py)/W/Pt trilayer by means of spin-torque ferromagnetic resonance, and demonstrated that the W/Pt interface generates an extra SO field acting on the Py layer. This unprecedented field originates from the following three processes, 1) spin accumulation at W/Pt interface via the Rashba-Edelstein effect, 2) diffusive spin transport in the W layer, and 3) spin absorption into the Py layer through accumulation at the Py/W interface. Our result means that we can create extra SO field away from the ferromagnet/ metal interface and control its strength by a combination of two different metals.

cond-mat.mes-hall↗

arXiv2018

Clear variation of spin splitting by changing electron distribution at non-magnetic metal/Bi2O3 interfaces

Large spin splitting at Rashba interface, giving rise to strong spin-momentum locking, is essential for efficient spin-to-charge conversion. Recently, a Cu/Bismuth oxide (Bi2O3) interface has been found to exhibit an efficient spin-to-charge conversion similar to a Ag/Bi interface with large Rashba spin splitting. However, the guiding principle of designing the metal/oxide interface for the efficient conversion has not been clarified yet. Here we report strong non-magnetic (NM) material dependence of spin splitting at NM/Bi2O3 interfaces. We employed spin pumping technique to inject spin current into the interface and evaluated the magnitude of interfacial spin-to-charge conversion. We observed large modulation and sign change in conversion coefficient which corresponds to the variation of spin splitting. Our experimental results together with first-principles calculations indicate that such large variation is caused by material dependent electron distribution near the interface. The results suggest that control of interfacial electron distribution by tuning the difference in work function across the interface may be an effective way to tune the magnitude and sign of spin-to-charge conversion and Rashba parameter at interface.

cond-mat.mtrl-sci↗

arXiv2017

Evaluation of bulk-interface contributions to Edelstein magnetoresistance at metal/oxide interfaces

We report a systematic study on Edelstein magnetoresistance (Edelstein MR) in Co25Fe75/Cu/Bi2O3 heterostructures with a strong spin-orbit interaction at the Cu/Bi2O3 interface. We succeed in observing a significant dependence of the Edelstein MR on both Cu layer thickness and temperature, and also develop a general analytical model considering distinct bulk and interface contributions on spin relaxation. Our analysis, based on the above model, quantitatively illustrates a unique property of the spin transport near the Rashba interface, revealing a prominent role of the spin relaxation process by determining the ratios of the spin relaxation inside and outside the interface. We further find the characteristic spin transport is unaffected by temperature. Our results provide an essential tool for exploring the transport in a system with spin-momentum-locked two-dimensional states.

cond-mat.mes-hall↗

arXiv2016

Experimental observation of spin to charge current conversion at non-magnetic metal/Bi2O3 interfaces

We here demonstrate the interfacial spin to charge current conversion by means of spin pumping from a ferromagnetic Permalloy (Py: Ni80Fe20) to a Cu/Bi2O3 interface. A clear signature of the spin to charge current conversion was observed in voltage spectrum of a Py/Cu/Bi2O3 trilayer film whereas no signature in a Py/Cu and Py/Bi2O3 bilayer films. We also found that the conversion coefficient strongly depended on Cu thickness, reflecting the thickness dependent momentum relaxation time in Cu layer.

cond-mat.mes-hall↗

arXiv2015

Spin relaxation characteristics in Ag nanowire covered with various oxides

We have studied spin relaxation characteristics in a Ag nanowire covered with various oxide layers of Bi2O3, Al2O3, HfO2, MgO or AgOx by using non-local spin valve structures. The spin-flip probability, a ratio of momentum relaxation time to spin relaxation time at 10 K, exhibits a gradual increase with an atomic number of the oxide constituent elements, Mg, Al, Ag and Hf. Surprisingly the Bi2O3 capping was found to increase the probability by an order of magnitude compared with other oxide layers. This finding suggests the presence of an additional spin relaxation mechanism such as Rashba effect at the Ag/Bi2O3 interface, which cannot be explained by the simple Elliott-Yafet mechanism via phonon, impurity and surface scatterings. The Ag/Bi2O3 interface may provide functionality as a spin to charge interconversion layer.

cond-mat.mtrl-sci↗