Spin Current Noise of the Spin Seebeck Effect and Spin Pumping
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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.Keywords:
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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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Nonlocal spin transport in nanostructured devices with ferromagnetic injector (F1) and detector (F2) electrodes connected to a normal conductor (N) is studied. We reveal how the spin transport depends on interface resistance, electrode resistance, spin polarization and spin diffusion length, and obtain the conditions for efficient spin injection, spin accumulation and spin current in the device. It is demonstrated that the spin Hall effect is caused by spin–orbit scattering in nonmagnetic conductors and gives rise to the conversion between spin and charge currents in a nonlocal device. A method of evaluating spin–orbit coupling in nonmagnetic metals is proposed.
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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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The spin-orbit coupling provides a versatile tool to generate and to manipulate the spin degree of freedom in low-dimensional semiconductor structures. The spin Hall effect, where an electric current drives a transverse spin current and causes a nonequilibrium spin accumulation near the sample boundary, 1,2 the spin-galvanic effect, where a nonequilibrium spin polarization drives an electric current 3,4 or the reverse process, in which an electrical current generates a non-equilibrium spin-polarization, 5–9 are all consequences of spin-orbit coupling. In order to observe a spin Hall effect a bias driven current is an essential prerequisite. Then spin separation is caused via spin-orbit coupling either by Mott scattering (extrinsic spin Hall effect) or by spin splitting of the band structure (intrinsic spin Hall effect). Recently an elementary effect causing spin separation which is fundamentally different from that of the spin Hall effect has been observed. 10 In contrast to the spin Hall effect it does not require an electric current to flow: it is spin separation achieved by spin-dependent scattering of electrons in media with suitable symmetry. It is show that by free carrier (Drude) absorption of terahertz radiation spin separation is achieved in a wide range of temperatures from liquid helium temperature up to room temperature. Moreover the experimental results demonstrate that simple electron gas heating by any means is already sufficient to yield spin separation due to spin-dependent energy relaxation processes of non-equilibrium carriers. In order to demonstrate the existence of the spin separation due to asymmetric scattering the pure spin current was converted into an electric current. It is achieved by application of a magnetic field which polarizes spins. This is analogues to spin-dependent scattering in transport experiments: spin-dependent scattering in an unpolarized electron gas causes the extrinsic spin Hall effect, whereas in a spin-polarized electron gas a charge current, the anomalous Hall effect, can be observed. As both magnetic fields and gyrotropic mechanisms were used authors introduced the notation "magneto-gyrotropic photogalvanic effects" for this class of phenomena. The effect is observed in GaAs and InAs low dimensional structures at free-carrier absorption of terahertz radiation in a wide range of temperatures from liquid helium temperature up to room temperature. The results are well described by the phenomenological description based on the symmetry. Experimental and theoretical analysis evidences unumbiguously that the observed photocurrents are spin-dependent. Microscopic theory of this effect based on asymmetry of photoexcitation and relaxation processes are developed being in a good agreement with experimental data. Note from Publisher: This article contains the abstract only.
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Spin Hall effects intermix spin and charge currents even in nonmagnetic materials and, therefore, ultimately may allow the use of spin transport without the need for ferromagnets. We show how spin Hall effects can be quantified by integrating Ni{80}Fe{20}|normal metal (N) bilayers into a coplanar waveguide. A dc spin current in N can be generated by spin pumping in a controllable way by ferromagnetic resonance. The transverse dc voltage detected along the Ni{80}Fe{20}|N has contributions from both the anisotropic magnetoresistance and the spin Hall effect, which can be distinguished by their symmetries. We developed a theory that accounts for both. In this way, we determine the spin Hall angle quantitatively for Pt, Au, and Mo. This approach can readily be adapted to any conducting material with even very small spin Hall angles.
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Spin wave resonance in Ni81Fe19/Pt thin wire arrays has been investigated using the inverse spin-Hall effect (ISHE). The spin wave in the Ni81Fe19 layer drives spin pumping, generation of spin currents from magnetization precession, and the pumped spin current is converted into a charge current by ISHE in the Pt layer. We found an electromotive force transverse to the spatial and the spin-polarization directions of the spin current. The experimental results indicate that the amplitude of the electromotive force is proportional to the spin wave resonance absorption intensity, enabling the electric measurement of spin wave resonance in nanostructured magnetic systems.
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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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