Induction of point and structural mutations in engineered yeast Saccharomyces cerevisiae improve carotenoid production.

ß-Carotene is an attractive compound and that its biotechnological production can be achieved by using engineered Saccharomyces cerevisiae. In a previous study, we developed a technique for the efficient establishment of diverse mutants through the introduction of point and structural mutations into the yeast genome. In this study, we aimed to improve ß-carotene production by applying this mutagenesis technique to S. cerevisiae strain that had been genetically engineered for ß-carotene production. Point and structural mutations were introduced into ß-carotene-producing engineered yeast. The resulting mutants showed higher ß-carotene production capacity than the parental strain. The top-performing mutant, HP100_74, produced 37.6 mg/L of ß-carotene, a value 1.9 times higher than that of the parental strain (20.1 mg/L). Gene expression analysis confirmed an increased expression of multiple genes in the glycolysis, mevalonate, and ß-carotene synthesis pathways. In contrast, expression of ERG9, which functions in the ergosterol pathway competing with ß-carotene production, was decreased in the mutant strain. The introduction of point and structural mutations represents a simple yet effective method for achieving mutagenesis in yeasts. This technique is expected to be widely applied in the future to produce chemicals via metabolic engineering of S. cerevisiae.

Observation of Long-Range Current-Induced Torque in Ni/Pt Bilayers.

The generation of current-induced torques through the spin Hall effect in Pt has been key to the development of spintronics. In prototypical ferromagnetic-metal/Pt devices, the characteristic length of the torque generation is known to be about 1 nm due to the short spin diffusion length of Pt. Here, we report the observation of a long-range current-induced torque in Ni/Pt bilayers. We demonstrate that when Ni is used as the ferromagnetic layer, the torque efficiency increases with the Pt thickness, even when it exceeds 10 nm. The torque efficiency is also enhanced by increasing the Ni thickness, providing evidence that the observed torque cannot be attributed to the spin Hall effect in the Pt layer. These findings, coupled with our semirealistic tight-binding calculations of the current-induced torque, suggest the possibility that the observed long-range torque is dominated by the orbital Hall effect in the Pt layer.

Degradation of glycosylinositol phosphoceramide during plant tissue homogenization.

A convenient method for the determination of plant sphingolipids (glycosylinositol phosphoceramide, GIPC; glucosylceramide, GluCer; phytoceramide 1-phosphate, PC1P and phytoceramide, PCer) was developed. This method includes the extraction of lipids using 1-butanol, alkali hydrolysis with methylamine and separation by TLC. The amounts of sphingolipids in the sample were determined based on the relative intensities of standard sphingolipids visualized by primulin/UV on TLC. Using this method, we found that almost all GIPCs were degraded in response to tissue homogenization in cruciferous plants (cabbage, broccoli and Arabidopsis thaliana). The decrease in GIPCs was compensated for by increases in PC1P and PCer, indicating that GIPC was degraded by hydrolysis at the D and C positions of GIPC, respectively. In carrot roots and leaves, most of GIPC degradation was compensated for by an increase in PCer. In rice roots, the decrease in GIPCs was not fully explained by the increases in PC1P and PCer, indicating that enzymes other than phospholipase C and D activities operated. As the visualization of lipids on TLC is useful for detecting the appearance or disappearance of lipids, this method will be available for the characterization of metabolism of sphingolipids in plants.

Isotropic Spin Hall Effect in an Epitaxial Ferromagnet.

We report the observation of the isotropic spin Hall effect in a ferromagnet. We show that the spin Hall effect in an epitaxially grown Fe_{3}Si generates a sizable spin current with a spin direction that is noncollinear with the magnetization. Furthermore, we find that the spin Hall current is independent of the relative orientation between its spin direction and the magnetization; the spin Hall effect is isotropic. This observation demonstrates that the intrinsically generated transverse spin component is protected from dephasing, providing fundamental insights into the generation and transport of spin currents in ferromagnets.

Impact of inherent energy barrier on spin-orbit torques in magnetic-metal/semimetal heterojunctions.

Spintronic devices are based on heterojunctions of two materials with different magnetic and electronic properties. Although an energy barrier is naturally formed even at the interface of metallic heterojunctions, its impact on spin transport has been overlooked. Here, using diffusive spin Hall currents, we provide evidence that the inherent energy barrier governs the spin transport even in metallic systems. We find a sizable field-like torque, much larger than the damping-like counterpart, in Ni81Fe19/Bi0.1Sb0.9 bilayers. This is a distinct signature of barrier-mediated spin-orbit torques, which is consistent with our theory that predicts a strong modification of the spin mixing conductance induced by the energy barrier. Our results suggest that the spin mixing conductance and the corresponding spin-orbit torques are strongly altered by minimizing the work function difference in the heterostructure. These findings provide a new mechanism to control spin transport and spin torque phenomena by interfacial engineering of metallic heterostructures.

Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten.

The emerging field of orbitronics exploits the electron orbital momentum L. Compared to spin-polarized electrons, L may allow the transfer of magnetic information with considerably higher density over longer distances in more materials. However, direct experimental observation of L currents, their extended propagation lengths and their conversion into charge currents has remained challenging. Here, we optically trigger ultrafast angular-momentum transport in Ni|W|SiO2 thin-film stacks. The resulting terahertz charge-current bursts exhibit a marked delay and width that grow linearly with the W thickness. We consistently ascribe these observations to a ballistic L current from Ni through W with a giant decay length (~80 nm) and low velocity (~0.1 nm fs-1). At the W/SiO2 interface, the L flow is efficiently converted into a charge current by the inverse orbital Rashba-Edelstein effect, consistent with ab initio calculations. Our findings establish orbitronic materials with long-distance ballistic L transport as possible candidates for future ultrafast devices and an approach to discriminate Hall-like and Rashba-Edelstein-like conversion processes.

Giant Orbital-to-Spin Conversion for Efficient Current-Induced Magnetization Switching of Ferrimagnetic Insulator.

It has long been believed that the attachment of two heavy metals such as Ta and Pt with opposite spin Hall angles results in a weakened net torque generation efficiency in magnetization switching devices. Here, we report a giant orbital-to-spin conversion in Ta/Pt/Tm3Fe5O12 (TmIG) heterostructures. We show that the torque generation efficiency is enhanced by an order of magnitude in the Ta/Pt/TmIG trilayer compared to that in the Pt/TmIG bilayer. This enhancement is further evidenced by the fact that the critical current density for the magnetization switching of the Ta/Pt/TmIG is an order of magnitude smaller than that of the Pt/TmIG. It is found that the orbital current generated from Ta through the orbital Hall effect (OHE) is converted to the spin current in the interior of Pt. Our discovery offers an extraordinary approach to enhance the torque generation for magnetization switching of insulators and provides an important piece of information for orbitronics.

Optimizing Electromagnetic Interference Shielding of Ultrathin Nanoheterostructure Textiles through Interfacial Engineering.

Strong electromagnetic wave reflection loss concomitant with second emission pollution limits the wide applications of electromagnetic interference (EMI) shielding textiles. Decoration of textiles by using various dielectric materials has been found efficient for the development of highly efficient EMI shielding textiles, but it is still a challenge to obtain EMI shielding composites with thin thickness. A route of interfacial engineering may offer a twist to overcome these obstacles. Here, we fabricated a Ni nanoparticle/SiC nanowhisker/carbon cloth nanoheterostructure, where SiC nanowhiskers were deposited by a simple manufacturing method, namely, laser chemical vapor deposition (LCVD), directly grown on carbon cloth. Through directly constructing a Ni/SiC interface, we find that the formation of Schottky contact can influence the interfacial polarization associated with the generation of dipole electric fields, leading to an enhancement of dielectric loss. A striking feature of this interfacial engineering strategy is able to enhance the absorption of the incident electromagnetic wave while suppressing the reflection. As a result, our Ni/SiC/carbon cloth exhibits an excellent EMI shielding effectiveness of 68.6 dB with a thickness of only 0.39 mm, as well as high flexibility and long-term duration stability benefited from the outstanding mechanical properties of SiC nanowiskers, showing potential for EMI shielding applications.

Generation and manipulation of current-induced spin-orbit torques.

An emerging field of spintronics, spin-orbitronics, aims to discover novel phenomena and functionalities originating from spin-orbit coupling in solid-state devices. The development of spin-orbitronics promises a fundamental understanding of spin physics in condensed matter, as well as smaller, faster, and far-more energy-efficient spin-based devices. Of particular importance in this field is current-induced spin-orbit torques, which trigger magnetic dynamics by the transfer of angular momentum from an atomic lattice to local magnetization through the spin-orbit coupling. The spin-orbit torque has attracted extensive attention for its fascinating relativistic and quantum mechanical nature, as well as prospective nanoelectronic applications. In this article, we review our studies on the generation and manipulation of current-induced spin-orbit torques.

Spin-orbit torque manipulated by fine-tuning of oxygen-induced orbital hybridization.

Current-induced spin-orbit torques provide an effective way to manipulate magnetization in spintronic devices, promising for fast switching applications in nonvolatile memory and logic units. Recent studies have revealed that the spin-orbit torque is strongly altered by the oxidation of heterostructures with broken inversion symmetry. Although this finding opens a new field of metal-oxide spin-orbitronics, the role of the oxidation in the spin-orbit physics is still unclear. Here, we demonstrate a marked enhancement of the spin-orbit torque induced by a fine-tuning of oxygen-induced modification of orbital hybridization. This is evidenced by a concomitant enhancement of the interface spin-orbit torque, interface spin loss, and interface perpendicular magnetic anisotropy within a narrow range of the oxidation level of metallic heterostructures. This result reveals the crucial role of the atomic-scale effects in the generation of the spin-orbit torques, opening the door to atomic-level engineering of the spin-orbit physics.

Spin Pumping Driven by Magnon Polarons.

We report the observation of a resonant enhancement of spin pumping induced by magnon-phonon coupling at room temperature. We show that the spin pumping driven by microwave parametric excitation is enhanced, compared to its purely magnonic value, when the microwave excites dipole-exchange magnons in the proximity of the intersection of the uncoupled magnon and phonon dispersions. This observation is consistent with a model of the spin pumping driven by hybridized magnon-phonon modes, magnon polarons, where the spin-pumping efficiency depends on the relative scattering strengths of the magnons and phonons in a magnetic insulator.

Intrinsic Spin-Orbit Torque Arising from the Berry Curvature in a Metallic-Magnet/Cu-Oxide Interface.

We report the observation of the intrinsic dampinglike spin-orbit torque (SOT) arising from the Berry curvature in metallic-magnet/CuO_{x} heterostructures. We show that a robust dampinglike SOT, an order of magnitude larger than a fieldlike SOT, is generated in the heterostructure despite the absence of the bulk spin-orbit effect in the CuO_{x} layer. Furthermore, by tuning the interfacial oxidation level, we demonstrate that the fieldlike SOT changes drastically and even switches its sign, which originates from oxygen-modulated spin-dependent disorder. These results provide important information for a fundamental understanding of the physics of the SOTs.

Molecular engineering of Rashba spin-charge converter.

In heterostructures with broken inversion symmetry, the electrons' motion is coupled to their spin through interface-driven spin-orbit coupling: the Rashba effect. The Rashba effect enables the interconversion between spin and charge currents, offering a variety of novel spintronic phenomena and functionalities. However, despite the significant progress in Rashba physics, controlling the spin-charge conversion in metallic heterostructures remains a major challenge. We show that molecular self-assembly provides a way to engineer the Rashba spin-charge converters. We demonstrate that magnetoresistance and voltage generation originating from the spin-charge conversion in metallic heterostructures can be manipulated by decorating the surface with self-assembled organic monolayers through the cooperative molecular field effect. We also demonstrate reversible phototuning of the spin-charge conversion through light-driven molecular transformations using a molecule that can photoisomerize between the trans and cis states. These findings, with the almost-infinite chemical tunability of organic monolayers, pave the way toward molecular engineering of spin-orbit devices.

Current-induced magnetization switching using an electrically insulating spin-torque generator.

Current-induced magnetization switching through spin-orbit torques is the fundamental building block of spin-orbitronics, which promises high-performance, low-power memory and logic devices. The spin-orbit torques generally arise from spin-orbit coupling of heavy metals. However, even in a heterostructure where a metallic magnet is sandwiched by two different insulators, a nonzero spin-orbit torque is expected because of the broken inversion symmetry; an electrical insulator can be a source of the spin-orbit torques. We demonstrate current-induced magnetization switching using an insulator. We show that oxygen incorporation into the most widely used spintronic material, Pt, turns the heavy metal into an electrically insulating generator of the spin-orbit torques, which enables the electrical switching of perpendicular magnetization in a ferrimagnet sandwiched by insulating oxides. We also show that the spin-orbit torques generated from the Pt oxide can be controlled electrically through voltage-driven oxygen migration. These findings open a route toward energy-efficient, voltage-programmable spin-orbit devices based on insulating metal oxides.

Spin-torque generator engineered by natural oxidation of Cu.

The spin Hall effect is a spin-orbit coupling phenomenon, which enables electric generation and detection of spin currents. This relativistic effect provides a way for realizing efficient spintronic devices based on electric manipulation of magnetization through spin torque. However, it has been believed that heavy metals are indispensable for the spin-torque generation. Here we show that the spin Hall effect in Cu, a light metal with weak spin-orbit coupling, is significantly enhanced through natural oxidation. We demonstrate that the spin-torque generation efficiency of a Cu/Ni81Fe19 bilayer is enhanced by over two orders of magnitude by tuning the surface oxidation, reaching the efficiency of Pt/ferromagnetic metal bilayers. This finding illustrates a crucial role of oxidation in the spin Hall effect, opening a route for engineering the spin-torque generator by oxygen control and manipulating magnetization without using heavy metals.

Rashba-Edelstein Magnetoresistance in Metallic Heterostructures.

We report the observation of magnetoresistance originating from Rashba spin-orbit coupling (SOC) in a metallic heterostructure: the Rashba-Edelstein (RE) magnetoresistance. We show that the simultaneous action of the direct and inverse RE effects in a Bi/Ag/CoFeB trilayer couples current-induced spin accumulation to the electric resistance. The electric resistance changes with the magnetic-field angle, reminiscent of the spin Hall magnetoresistance, despite the fact that bulk SOC is not responsible for the magnetoresistance. We further found that, even when the magnetization is saturated, the resistance increases with increasing the magnetic-field strength, which is attributed to the Hanle magnetoresistance in this system.

Spin-current emission governed by nonlinear spin dynamics.

Coupling between conduction electrons and localized magnetization is responsible for a variety of phenomena in spintronic devices. This coupling enables to generate spin currents from dynamical magnetization. Due to the nonlinearity of magnetization dynamics, the spin-current emission through the dynamical spin-exchange coupling offers a route for nonlinear generation of spin currents. Here, we demonstrate spin-current emission governed by nonlinear magnetization dynamics in a metal/magnetic insulator bilayer. The spin-current emission from the magnetic insulator is probed by the inverse spin Hall effect, which demonstrates nontrivial temperature and excitation power dependences of the voltage generation. The experimental results reveal that nonlinear magnetization dynamics and enhanced spin-current emission due to magnon scatterings are triggered by decreasing temperature. This result illustrates the crucial role of the nonlinear magnon interactions in the spin-current emission driven by dynamical magnetization, or nonequilibrium magnons, from magnetic insulators.

Nonlinear spin-current enhancement enabled by spin-damping tuning.

When a magnon, the quanta of a spin excitation, is created in a magnet, this quasiparticle can split into two magnons, which triggers an angular momentum flow from the lattice to the spin subsystem. Although this process is known to enhance spin-current emission at metal/magnetic insulator interfaces, the role of interacting magnons in spintronic devices is still not well-understood. Here, we show that the enhanced spin-current emission is enabled by spin-damping tuning triggered by the redistribution of magnons. This is evidenced by time-resolved measurements of magnon lifetimes using the inverse spin Hall effect. Furthermore, we demonstrate nonlinear enhancement of the spin conversion triggered by scattering processes that conserve the number of magnons, illustrating the crucial role of spin-damping tuning in the nonlinear spin-current emission. These findings provide a crucial piece of information for the development of nonlinear spin-based devices, promising important advances in insulator spintronics.

Solution-processed organic spin-charge converter.

Conjugated polymers and small organic molecules are enabling new, flexible, large-area, low-cost optoelectronic devices, such as organic light-emitting diodes, transistors and solar cells. Owing to their exceptionally long spin lifetimes, these carbon-based materials could also have an important impact on spintronics, where carrier spins play a key role in transmitting, processing and storing information. However, to exploit this potential, a method for direct conversion of spin information into an electric signal is indispensable. Here we show that a pure spin current can be produced in a solution-processed conducting polymer by pumping spins through a ferromagnetic resonance in an adjacent magnetic insulator, and that this generates an electric voltage across the polymer film. We demonstrate that the experimental characteristics of the generated voltage are consistent with it being generated through an inverse spin Hall effect in the conducting polymer. In contrast with inorganic materials, the conducting polymer exhibits coexistence of high spin-current to charge-current conversion efficiency and long spin lifetimes. Our discovery opens a route for a new generation of molecular-structure-engineered spintronic devices, which could lead to important advances in plastic spintronics.