Spin Conversion on the Nanoscale

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PROGRESS ARTICLES PUBLISHED ONLINE: 10 JULY 2017 | DOI: 10.1038/NPHYS4192

Spin conversion on the nanoscale

YoshiChika Otani1,2*, Masashi Shiraishi3, Akira Oiwa4, Eiji Saitoh5,6,7,8 and Shuichi Murakami9,10

Spins can act as mediators to interconvert electricity, light, sound, vibration and heat. Here, we give an overview of the recent advances in dierent sub-disciplines of spintronics that can be associated with the developing ﬁeld of spin conversion, and discuss future prospects.

pin conversion is a generic term for the phenomena associated scattering owing to spin–orbit coupling in non-ferromagnetic heavy with the interconversion between different physical entities— metals consisting of 4d and 5d transition metals, such as palladium, Selectricity, light, sound, vibration and heat—that are mediated tantalum, tungsten and platinum. In the reverse process, magnetic by spins (Fig.1). Most spin-conversion phenomena take place at the or spin Hall materials can also detect the spin accumulation, which nanoscale, in the regions near the interface of two diverse varieties is the central method of magneto-electric conversion from spin to of materials, such as magnets, non-magnets, semiconductors and charge information. insulators. In the above physical entities, electricity, light and Spin pumping, however, is a dynamic spin injection that heat represent electrons, photons and phonons, respectively, all relies on the dissipation of spin angular momentum—pure spin of which can transfer angular momentum through spins. Both currents from a resonantly precessing ferromagnetic moment sound and vibration—kinds of phonons in a broad sense—are are injected into an adjacent exchange-coupled non-magnetic mechanical motion, carrying mechanical angular momentum, material. Importantly, spin pumping can inject spin currents across which couples to magnetization, similar to the Barnett1 or any exchange-coupled interface consisting of magnetic insulators, Einstein–de Haas2 effects. semiconductors and metals. We should also note that SHEs can These spin-mediated interconversion phenomena include realize versatile spin-to-charge interconversion without using any of spin-transfer-torque-induced spin dynamics3, direct and inverse the magnetic materials mentioned above. spin Hall effects (SHEs and ISHEs)4, the spin Seebeck effect5, Angular momentum conservation is the fundamental principle the spin Peltier effect6, spin injection into insulators7, electric- that enables pure spin-current-induced magnetization switching— field-controlled magnetic anisotropy in ultrathin ferromagnetic that is, the direct conversion of conduction electron spin angular films8, and more. More recently, a new type of spin-to-charge momenta to the bulk magnetization. This conversion has been interconversion, known as the so-called Edelstein effect, was demonstrated by a variety of methods, such as non-local spin experimentally shown to take place at Rashba interfaces9 or the injection13, spin torque ferromagnetic resonance14, and a spin Hall surface states of topological insulators10. Further development cross15. However, the large current densities required are still a of spin-conversion functionalities may rely on a microscopic challenging issue to be overcome for practical applications. A understanding of interconversions among quasiparticles such as promising alternative approach to efficient reversal may be voltage- electrons, spins, magnons, phonons and photons. induced switching16, which can also be categorized as magnetic In this Progress Article, we outline the research achievements spin conversion. in different sub-disciplines of spintronics that can be associated Injection of a spin current gives rise to a wide variety of with spin-conversion science in terms of magneto-electric, optical, spin-conversion phenomena, such as giant enhancement of spin thermal and mechanical spin conversions, together with theoretical accumulation in metals by using MgO (ref. 17), spin transport efforts on the functional design of spin-conversion processes. in semiconductors (Si; ref. 18, GaAs; ref. 19, Ge; ref. 20 and graphene21), including electrostatic gating effects22, and nonlinear Magneto-electric conversions enhancement23,24. Moreover, recent experimental work has Magneto-electric spin conversion in conductive solids is mediated demonstrated that a superconductive niobium nitride exhibits by spin accumulations and pure spin currents, which can be a nonlinear enhancement of approximately 2,000-fold in its generated by spin injection11, spin pumping12 and SHEs4. Spin- quasiparticle-mediated spin Hall effect25. polarized currents can be electrically injected from ferromagnets A recent notable achievement in spin conversion is the discovery to non-magnetic metals such as copper and silver. And this static of a new conversion mechanism, the inverse Edelstein effect in spin injection results in both spin accumulation and pure spin Bi/Ag interfaces9, where a strong Rashba splitting (approximately currents with no net charge flow near their ohmic or tunnel 200 meV) exists, which has greatly expanded the field of spin junctions. In contrast, SHEs can directly generate transverse pure conversion. This spin conversion takes place at the interface of spin currents without using ferromagnets through spin-dependent Bi/Ag, whereas the length scale of the ISHE is the spin diffusion

1Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan. 2RIKEN-CEMS, 2-1 Hirosawa, Wako 351-0198, Japan. 3Department of Electronic Science and Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. 4Institute of Scientiﬁc and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. 5Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. 6Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan. 7WPI Research Center, Advanced Institute for Material Research, Tohoku University, Sendai 980-8577, Japan. 8ERATO, Japan Science and Technology Agency, Sendai 980-8577, Japan. 9Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan. 10TIES, Tokyo Institute of Technology, Tokyo 152-8551, Japan. *e-mail: [email protected]

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have suggested the microscopic non-equilibrium transient ferromagnetic-like state of antiferromagnetically coupled Gd and Fe sublattices33. Recently, photoinduced spin currents in ultrafast photoinduced magnetization dynamics have been recognized as an important process. Such a concept would provide us with an understanding of the Heat Vibration Sound Light Electricity ultrafast dynamics and new directions for optical spin conversion. Phonon Mechanical motion Photon The milestone work, ultrafast photoinduced demagnetization on the subpicosecond timescale34, has been examined in terms of the 35,36 Spin Seebeck effect transport of spin-polarized photoexcited electrons . In a magnetic bilayer system, the photo-generated spin current induced in one of the magnetic layers exerts a spin-transfer torque on the other layer37. Optical excitation of spin waves has provided rich insight Spin Hall effect into Gilbert damping, which is applicable for studying magnonic crystals38. By extending the wavelength for the excitation to the microwave photon range, fascinating phenomena, such as Bose– 39 Localized spin Conduction Einstein condensation of magnons and single magnon excitation 40 electron spin coupled to a superconducting qubit by a microwave photon , Magnon have opened the way to elucidate the quantum mechanical nature of magnons.

Spin Peltier effect Using semiconductor/non-magnetic metal interfaces and semiconductor structures, the angular momentum of light can be converted into a spin current generated by the spin-selective transition across the bandgap of the semiconductors and detected electrically by ISHE24. Moreover, semiconductor quantum Inverse spin Hall effect structures enable us to study the transfer of angular momentum as well as phase information coherently41,42. To realize the coherent state conversion between single photons to single electron spins, a single photo-generated electron spin readout has been realized in a single-shot manner43. Although we have discussed only the conversion from light to spin in the above, we finally note that the emission of polarized light by the conversion of angular momentum Electricity Light Sound Vibration Heat from spin has to be developed to establish spin light-emitting diodes and spin lasers44.

Figure 1 | Conceptual illustration of nano-spin-conversion science. Driving spins with heat Thermal spin conversion, frequently referred to as spin calori- length of the material. More recently, even a combination of tronics45, is a subfield of spin-conversion science that aims to explore insulating Bi2O3 and metallic Cu layers was found to show a similar new material functionalities based on a spin–heat interconversion. spin-to-charge conversion26. The field has created new phenomena unique to the spin degree of Another significant development in spin generation and freedom. For example, the spin Seebeck effect (SSE) generates pure detection methods has been achieved by using the surface states spin currents owing to the magnetization dynamics induced by a of topological insulators, such as BiSe (ref. 10), BiSbTeSe (ref. 27) temperature gradient applied across the interface of a magnet and and α-Sn (ref. 28), which are regarded as novel topological a metal—longitudinal configuration. spin-conversion schemes. The SSE has been observed in various magnetic materials, and one of the accepted models for the SSE is thermal spin pumping46,47. Controlling spin transfer with light Here, the driving force is the effective temperature difference Optical excitation is an indispensable tool for spin conversion between the magnetization in the magnet and the electrons in the because light can transfer angular momentum to a magnetic metal, which develops under the applied temperature gradient. This system. There have been many challenges to control the magnetic effective temperature difference breaks the balance between spin state using an ultrafast laser pulse and its angular momentum. pumping, driven by thermally excited magnetization motion, and Magnetization precession induced by the helicity of a circularly its reverse process, induced by conduction electron fluctuation, and polarized femtosecond laser pulse, known as the inverse Faraday drives the net spin current across the interface. The reverse process effect, has been demonstrated using rare-earth orthoferrite DyFeO3 of the SSE, the spin Peltier effect, has also been experimentally (ref. 29). The observed effect is non-thermal since the magnetic observed in a Pt/YIG film48. dielectric is transparent. Comprehensive experiments using In metallic magnets, spin degrees of freedom couple with heat by ultrashort optical pulses have been performed to realize a full conduction electron spin currents. A temperature gradient applied magnetization reversal in metals. The ferrimagnetic amorphous to a metallic magnet generates the spin accumulation of conduction alloy, GdFeCo, exhibits a helicity-dependent all-optical switching electrons near the end of the magnet, and this spin-dependent triggered by a subpicosecond light pulse30. The ultrafast switching electron distribution causes a net spin current. The above-described effects have been discussed in terms of strongly non-equilibrium effect is so-called the spin-dependent Seebeck effect (SDSE), which dynamics consisting of the heating process with a rapid elevation was first confirmed by observing a spin current diffusing across of the system temperature and helicity-dependent process30,31. a nanoscale interface between Cu and NiFe (ref. 49). The spin- In the same material system, another all-optical magnetization dependent Peltier effect, which is the reverse effect of the spin- reversal, independent of the helicity of the pulse, has been dependent Seebeck effect, has been experimentally demonstrated in 32 50 observed . Experiments on the X-ray magnetic circular dichroism a (Py: Ni80Fe20)/copper/Py spin valve .

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Spin current generation with mechanical motion domain wall of the ferromagnet is charged and can be controlled A new research field is evolving that combines spin currents with by the external electric field. The chirality and relative stability of mechanical motion, known as spin mechanics or spin mechatronics. the Néel wall and Bloch wall are shown to depend on the position of Classical phenomena that couple magnetism with mechanical the Fermi energy. motion have already been observed 100 years ago in the forms of Another challenge is designing theoretically new spin-conversion the Einstein–de Haas effect2 and the Barnett effect1. In the former, functions in nanostructures, interfaces and thin films, such as a magnetic body starts to rotate along with reversal of the magne- spin pumping, spin Seebeck effect, and interface spin conversion. tization. This can be understood as angular momentum transfer Motivated by spatially resolved femtosecond laser pump and probe from the rotating magnetic body to the electron spins in magnets. experiments59, Shen et al. theoretically derived an equation for The latter effect is its reverse effect; a magnetic body becomes mag- the motion of lattice and magnetization fields with magnetoelastic netized while it rotates. This magnetization means that an effective coupling60. The results show good agreement with experiments, magnetic field is created arising from the rotation on the magnetic implying that the magnetization distributions after an optical body; the field has only recently been measured quantitatively51. excitation indicate the presence of a strong magnon–phonon In contrast to these early-stage experiments carried out on coupling, reflecting the nature of light–spin conversion. macroscopic magnetization, the current microelectromechanical In this way, many theoretical and experimental investigations are systems technology allows us to investigate Einstein–de Haas effects being performed in terms of the above-mentioned viewpoints. For in small samples with high sensitivity52. The recently demonstrated example, the spin conversion, using angular momentum coupled torque-mixing magnetic resonance spectroscopy can also be a with the k-space Berry curvature, could become a key principle powerful spectroscopic tool to investigate a minuscule change in for future spintronics devices that function without spin–orbit spin angular momentum53. More recently, spin current generation coupling. The development of a new chiral material together with from mechanical rotation was observed by Takahashi et al., experimental demonstration is thus an urgent need. Furthermore, who demonstrated spin hydrodynamic generation by the liquid theoretical progress in spin-conversion phenomena often goes dynamics of Hg and GaInSn flowing in a fine channel, with the hand in hand with new measurement methods and new types of spin current detected as a voltage signal through ISHE54. Although probe. For example, the spin Hall effect of electrons and thermal spin mechanics is still in its infancy, the potentiality of controlling Hall effect of magnons have been theoretically proposed in the motions directly by spin currents without using electric motors is a context of k-space Berry curvature, and by using these effects, broad distinction from the existing electronics. we can observe new aspects of spin-conversion phenomena that cannot be determined by other methods. Thus, they are promising Theoretical approaches and prospects for unveiling the properties of magnetic excitations, as has been The notion of spin-conversion phenomena has offered various new demonstrated in a frustrated magnet61. theoretical possibilities in spintronics. The goal of this subfield is For a microscopic understanding in terms of interconversions to construct new theories for spin-conversion phenomena and, at among quasiparticles, such as electrons, spins, magnons, phonons the same time, to propose systems or phenomena as stages for the and photons in nano-spin-conversion science, a close collaboration new theories that can be experimentally examined. There are the between experimental and theoretical studies is indispensable. Such following three important viewpoints: new theoretical frameworks, collaborations have already been fruitful, but this field is still in new materials and systems, and nanostructures, interfaces and its infancy and further breakthrough theories and experiments are thin films. surely on the horizon. Approaches on new theoretical frameworks include the gauge field, Berry curvature, topology and spin electromagnetic field. 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