Y. Kajiwara

Bio: Y. Kajiwara is an academic researcher from Tohoku University. The author has contributed to research in topics: Spin Hall effect & Spin pumping. The author has an hindex of 21, co-authored 43 publications receiving 5246 citations. Previous affiliations of Y. Kajiwara include Keio University.

Transmission of electrical signals by spin-wave interconversion in a magnetic insulator

Abstract: An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. However, the electrons within an insulator possess spin as well as charge, so it is possible for them to transmit a signal in the form of a spin wave. Kajiwara et al. have now developed a hybrid metal–insulator–metal structure in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer. This wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage. The observation of voltage transmission in an insulator raises the prospect of insulator-based spintronics and other novel forms of signal delivery. An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. But an insulator's electrons possess spin in addition to charge, and so can transmit a signal in the form of a spin wave. Here a hybrid metal–insulator–metal structure is reported, in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer; this wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage. The energy bandgap of an insulator is large enough to prevent electron excitation and electrical conduction1. But in addition to charge, an electron also has spin2, and the collective motion of spin can propagate—and so transfer a signal—in some insulators3. This motion is called a spin wave and is usually excited using magnetic fields. Here we show that a spin wave in an insulator can be generated and detected using spin-Hall effects, which enable the direct conversion of an electric signal into a spin wave, and its subsequent transmission through (and recovery from) an insulator over macroscopic distances. First, we show evidence for the transfer of spin angular momentum between an insulator magnet Y3Fe5O12 and a platinum film. This transfer allows direct conversion of an electric current in the platinum film to a spin wave in the Y3Fe5O12 via spin-Hall effects4,5,6,7,8,9,10,11. Second, making use of the transfer in a Pt/Y3Fe5O12/Pt system, we demonstrate that an electric current in one metal film induces voltage in the other, far distant, metal film. Specifically, the applied electric current is converted into spin angular momentum owing to the spin-Hall effect7,8,10,11 in the first platinum film; the angular momentum is then carried by a spin wave in the insulating Y3Fe5O12 layer; at the distant platinum film, the spin angular momentum of the spin wave is converted back to an electric voltage. This effect can be switched on and off using a magnetic field. Weak spin damping3 in Y3Fe5O12 is responsible for its transparency for the transmission of spin angular momentum. This hybrid electrical transmission method potentially offers a means of innovative signal delivery in electrical circuits and devices.

Spin Seebeck insulator.

Abstract: Thermoelectric generation is an essential function in future energy-saving technologies. However, it has so far been an exclusive feature of electric conductors, a situation which limits its application; conduction electrons are often problematic in the thermal design of devices. Here we report electric voltage generation from heat flowing in an insulator. We reveal that, despite the absence of conduction electrons, the magnetic insulator LaY(2)Fe(5)O(12) can convert a heat flow into a spin voltage. Attached Pt films can then transform this spin voltage into an electric voltage as a result of the inverse spin Hall effect. The experimental results require us to introduce a thermally activated interface spin exchange between LaY(2)Fe(5)O(12) and Pt. Our findings extend the range of potential materials for thermoelectric applications and provide a crucial piece of information for understanding the physics of the spin Seebeck effect.

Spin Hall Magnetoresistance Induced by a Nonequilibrium Proximity Effect

Abstract: We report anisotropic magnetoresistance in Pt|Y3Fe5O12 bilayers. In spite of Y3Fe5O12 being a very good electrical insulator, the resistance of the Pt layer reflects its magnetization direction. The effect persists even when a Cu layer is inserted between Pt and Y3Fe5O12, excluding the contribution of induced equilibrium magnetization at the interface. Instead, we show that the effect originates from concerted actions of the direct and inverse spin Hall effects and therefore call it “spin Hall magnetoresistance.”

Inverse spin-Hall effect induced by spin pumping in metallic system

Abstract: The inverse spin-Hall effect (ISHE) induced by the spin pumping has been investigated systematically in simple ferromagnetic/paramagnetic bilayer systems. The spin pumping driven by ferromagnetic resonance injects a spin current into the paramagnetic layer, which gives rise to an electromotive force transverse to the spin current using the ISHE in the paramagnetic layer. In a Ni81Fe19/Pt film, we found an electromotive force perpendicular to the applied magnetic field at the ferromagnetic resonance condition. The spectral shape of the electromotive force is well reproduced using a simple Lorentz function, indicating that the electromotive force is due to the ISHE induced by the spin pumping; extrinsic magnetogalvanic effects are eliminated in this measurement. The electromotive force varies systematically by changing the microwave power, magnetic-field angle, and film size, being consistent with the prediction based on the Landau–Lifshitz–Gilbert equation combined with the models of the ISHE and spin pump...

Spin-electricity conversion induced by spin injection into topological insulators.

Abstract: We report successful spin injection into the surface states of topological insulators by using a spin pumping technique. By measuring the voltage that shows up across the samples as a result of spin pumping, we demonstrate that a spin-electricity conversion effect takes place in the surface states of bulk-insulating topological insulators Bi(1.5)Sb(0.5)Te(1.7)Se(1.3) and Sn-doped Bi(2)Te(2)Se. In this process, the injected spins are converted into a charge current along the Hall direction due to the spin-momentum locking on the surface state.

Spin-torque switching with the giant spin hall effect of tantalum

Abstract: Spin currents can apply useful torques in spintronic devices. The spin Hall effect has been proposed as a source of spin current, but its modest strength has limited its usefulness. We report a giant spin Hall effect (SHE) in β-tantalum that generates spin currents intense enough to induce efficient spin-torque switching of ferromagnets at room temperature. We quantify this SHE by three independent methods and demonstrate spin-torque switching of both out-of-plane and in-plane magnetized layers. We furthermore implement a three-terminal device that uses current passing through a tantalum-ferromagnet bilayer to switch a nanomagnet, with a magnetic tunnel junction for read-out. This simple, reliable, and efficient design may eliminate the main obstacles to the development of magnetic memory and nonvolatile spin logic technologies.

Spin Hall effects

Abstract: In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents. The associated spin Hall and inverse spin Hall effects distinguish between charge and spin current where electron charge is a conserved quantity but its spin direction is not. This review provides a theoretical and experimental treatment of this subfield of spintronics, beginning with distinct microscopic mechanisms seen in ferromagnets and concluding with a discussion of optical-, transport-, and magnetization-dynamics-based experiments closely linked to the microscopic and phenomenological theories presented.

Current-induced switching of perpendicularly magnetized magnetic layers using spin torque from the spin Hall effect.

Abstract: We show that in a perpendicularly magnetized Pt/Co bilayer the spin-Hall effect (SHE) in Pt can produce a spin torque strong enough to efficiently rotate and switch the Co magnetization. We calculate the phase diagram of switching driven by this torque, finding quantitative agreement with experiments. When optimized, the SHE torque can enable memory and logic devices with similar critical currents and improved reliability compared to conventional spin-torque switching. We suggest that the SHE torque also affects current-driven magnetic domain wall motion in Pt/ferromagnet bilayers.

Antiferromagnetic spintronics

Abstract: Antiferromagnetic materials could represent the future of spintronic applications thanks to the numerous interesting features they combine: they are robust against perturbation due to magnetic fields, produce no stray fields, display ultrafast dynamics and are capable of generating large magneto-transport effects Intense research efforts over the past decade have been invested in unraveling spin transport properties in antiferromagnetic materials Whether spin transport can be used to drive the antiferromagnetic order and how subsequent variations can be detected are some of the thrilling challenges currently being addressed Antiferromagnetic spintronics started out with studies on spin transfer, and has undergone a definite revival in the last few years with the publication of pioneering articles on the use of spin-orbit interactions in antiferromagnets This paradigm shift offers possibilities for radically new concepts for spin manipulation in electronics Central to these endeavors are the need for predictive models, relevant disruptive materials and new experimental designs This paper reviews the most prominent spintronic effects described based on theoretical and experimental analysis of antiferromagnetic materials It also details some of the remaining bottlenecks and suggests possible avenues for future research

Spin Caloritronics

Abstract: This is a brief overview of the state of the art of spin caloritronics, the science and technology of controlling heat currents by the electron spin degree of freedom (and vice versa)