Spin Pumping and Spin Currents in Magnetic Insulators
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We theoretically investigate the fluctuation of a pure spin current induced by the spin Seebeck effect and spin pumping in a normal-metal--(NM-)ferromagnet(FM) bilayer system. Starting with a simple ferromagnet-insulator--(FI-)NM interface model with both spin-conserving and non-spin-conserving processes, we derive general expressions of the spin current and the spin-current noise at the interface within second-order perturbation of the FI-NM coupling strength, and estimate them for a yttrium-iron-garnet--platinum interface. We show that the spin-current noise can be used to determine the effective spin carried by a magnon modified by the non-spin-conserving process at the interface. In addition, we show that it provides information on the effective spin of a magnon, heating at the interface under spin pumping, and spin Hall angle of the NM.
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The flow of electron spin, the so-called "spin current", is a key concept in the recent progress in spintronics. When the spin current interacts with the magnetic moment in a ferromagnetic metal, the angular momentum and energy conservations give rise to the spin transfer torque and spinmotive force, respectively. When it is injected into a non-magnetic metal attached to a ferromagnet, the electric current is induced through the spin-charge conversion mechanism (inverse spin Hall effect). The generation and manipulation of the spin current and a variety of novel phenomena given by the spin current, including the spin Seebeck effect and spinmotive force, are discussed.
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A ``spin-guide'' source for generation of electric currents with a high degree of spin polarization, which allows long-distance transmission of the spin polarization, is proposed. In the spin-guide scheme, a nonmagnetic conducting channel is interfaced or surrounded by a grounded magnetic shell that transmits electrons with a particular spin direction preferentially, resulting in net polarization of the current flowing through the channel parallel to the interface. It is argued that this method is more effective than spin-filter-like schemes where the current flows perpendicular to the interface between a ferromagnetic metal to a non-magnetic conducting material. Under certain conditions a spin-guide may generate an almost perfectly spin-polarized current, even when the magnetic material used is not fully polarized. The spin guide is predicted to allow the transport of spin polarization over long distances that may exceed significantly the spin-flip length in the channel. In addition, it readily permits detection and control of the spin polarization of the current. The spin guide may be employed for spin-flow manipulations in spintronic devices.
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This chapter introduces the basic concept of spin current. It begins with an introduction to the general concept of spin and spin current, which is followed by a discussion of particular spin currents, such as incoherent, exchange, topological, and thermal spin currents. The chapter reviews the definition of charge currents for comparison. It talks about a diffusion spin current due to spatial inhomogeneous spin density and a drift spin current in the absence of coherent dynamics of spin. There are some methods for experimentally detecting pure spin currents, spin currents without accompanying charge currents. One direct method is the utilization of the inverse spin-Hall effect, a method which was demonstrated first by spin pumping and nonlocal technique. The chapter explains the exchange interaction in magnets by introducing a concept of an exchange spin current and then formulates a spin-wave spin current.
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Theory of spin-charge conversion effects in spintronics are presented in terms of correlation functions of physical observables, spin and electric current. Direct and inverse spin Hall effects and spin pumping effect are studied considering metallic systems with random spin-orbit interaction and spatially nonuniform Rashba interaction. The theory is free from ambiguity associated with spin current, and provides a clear physical picture of the spin-charge conversion effects. In the present approach, the spin current transmission efficiency turns out essentially to be the nonuniform component of magnetic susceptibility.
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The field of spintronics is based on the manipulation of the spin degree of freedom. It uses the carrier spin angular momentum as a basic functional unit in addition to the charge. The first requirement of a semiconductor-based spintronic technology is the efficient generation of spin-polarized carriers into the device heterostructure made of Si or Ge (the materials of mainstream microelectronics) at room temperature. In this presentation, we focus on the generation of a sizeable spin population into Ge by spin pumping. Spin pumping corresponds to the generation of a pure spin current in the Ge film by exciting the ferromagnetic resonance of an adjacent ferromagnetic electrode with microwaves. The pure spin current is then detected using spin-orbit based effects. Our aim is to understand the basic mechanisms of spin pumping into Ge as well as the spin-to-charge conversion by inverse spin Hall effect (ISHE, bulk effect) [1-4] and Rashba-Edelstein effect (interface effect) [5]. The influence of interface states is clearly demonstrated. Moreover, using the spin-split Rashba sub-surface states of the Ge(111) surface, we succeeded in demonstrating a giant conversion of a spin current generated by spin pumping into a charge current by the Rashba-Edelstein effect [6,7]. Our experimental findings are supported by ab-initio calculations. 1. Rojas-Sánchez, J.-C. et al. Spin pumping and inverse spin Hall effect in germanium. Phys. Rev. B 88, (2013). 2. Kato, Y. K., Myers, R. C., Gossard, A. C. and Awschalom, D. D. Observation of the spin Hall effect in semiconductors. Science 306, 1910–1913 (2004). 3. Valenzuela, S. O. and Tinkham, M. Direct electronic measurement of the spin Hall effect. Nature 442, 176–179 (2006). 4. Saitoh, E., Ueda, M., Miyajima, H. and Tatara, G. Conversion of spin current into charge current at room temperature: Inverse spin-Hall effect. Appl Phys Lett 88, 2509 (2006). 5. Bychkov, Y. A. and Rashba, E. I. Oscillatory effects and the magnetic susceptibility of carriers in inversion layers. Journal of Physics C: Solid State Physics 17, 6039–6045 (1984). 6. Edelstein, V. M. Spin polarization of conduction electrons induced by electric current in two-dimensional asymmetric electron systems. Solid State Communications 73, 233–235 (1990). 7. Rojas-Sánchez, J.-C. et al. Spin-to-charge conversion using Rashba coupling at the interface between non-magnetic materials. Nat Comms 4, (2013).
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We overview the recent developments in spin current generation mechanisms and study the spin pumping effect and diffusive spin current in detail based on a microscopic theory. The spin-charge conversion using the inverse spin Hall effect is also discussed. Spin chemical potential describing the diffusive spin current is calculated by linear response theory and spin injection effect is discussed based on the result.
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A Spin Current Generated by Spin Pumping in a Ferromagnetic/Nonmagnetic/Spin-Sink Trilayer Film Is Calculated Based on the Spin Pumping Theory and the Standard Spin Diffusion Equation. By Attaching the Spin-Sink Layer, the Injected Spin Current Is Drastically Enhanced when the Interlayer Thickness Is Shorter than the Spin Diffusion Length of the Interlayer. We Also Provided the Formula of the Charge Current which Is Induced from the Pumped Spin Current via the Inverse Spin-Hall Effect.
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There exists a direct connection between spin torque and spin current in two-dimensional electron systems with linear in momentum Rashba spin–orbit (SO) coupling. In terms of the spin-current continuity equation, we show that the spin torque of this type generates a divergent spin current due to spin injection, which we call the spin-current-driven spin pumping. We quantitatively investigate the spin pumping from SO coupled systems in contact with spin-polarized reservoirs using the nonequilibrium Green's function formalism, demonstrating that the spin torque effect efficiently produces a pure spin current which is orders of magnitude larger than the spin Hall current.
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