Determination of spin Hall angle and spin diffusion length in β -phase-dominated tantalum
TL;DR: In this paper, the dc spin-to-charge conversions of tantalum bilayer structures are investigated utilizing spin pumping and inverse spin Hall effects (ISHE), and it is shown that Ta films, below 30 nm in thickness, are phase dominated.
Abstract: The dc spin-to-charge conversions of tantalum (Ta) in $\mathrm{Ta}/\mathrm{C}{\mathrm{o}}_{40}\mathrm{F}{\mathrm{e}}_{40}{\mathrm{B}}_{20}$ bilayer structures are investigated utilizing spin pumping and inverse spin Hall effects (ISHE). From Ta thickness $({t}_{\mathrm{Ta}})$-dependent resistivity and x-ray diffraction measurements, we found that Ta films, below 30 nm in thickness, are $\ensuremath{\beta}$-phase dominated. The damping enhancement shows a fast increase with ${t}_{\mathrm{Ta}}$ when ${t}_{\mathrm{Ta}}l1\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and reaches a saturation value at $\ensuremath{\sim}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. The ISHE induced charge voltages have opposite signs for Ta and Pt. From ${t}_{\mathrm{Ta}}$-dependent spin pumping produced ISHE voltage and precession angle measurements, the normalized spin-charge conversion signal is found to increase with ${t}_{\mathrm{Ta}}$ and saturate at $\ensuremath{\sim}15\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. Our findings can be understood with a recently developed theory [Phys. Rev. Lett. 114, 126602 (2015)], which includes spin backflow and a spin loss at the interface. With a fitted spin loss factor of $0.02\ifmmode\pm\else\textpm\fi{}0.02$, we extract the spin Hall angle and spin diffusion length of high resistivity Ta to be ${\ensuremath{\theta}}_{\mathrm{SH}}=\ensuremath{-}0.0062\ifmmode\pm\else\textpm\fi{}0.001$ and ${\ensuremath{\lambda}}_{\mathrm{sd}}=5.1\ifmmode\pm\else\textpm\fi{}0.6\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, respectively.
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TL;DR: In this paper, a scheme for efficiently and accurately calculating intrinsic spin Hall conductivity (SHC) based on the maximally localized Wannier function (MLWF) is presented.
Abstract: Ab initio calculation of intrinsic spin Hall conductivity (SHC) generally requires a strict convergence criterion and a dense $k$-point mesh to sample the Brillouin zone, making its convergence challenging and time consuming. Here we present a scheme for efficiently and accurately calculating SHC based on the maximally localized Wannier function (MLWF). The quantities needed by the Kubo formula of SHC are derived in the space of the MLWF and it is shown that only the Hamiltonian, the overlap, and the spin operator matrices are required from the initial ab initio calculation. The computation of these matrices and the interpolation of the Kubo formula on a dense $k$-point mesh can be easily achieved. We validate our results by prototypical calculations on fcc Pt and GaAs, which demonstrate that the Wannier interpolation approach is of high accuracy and efficiency. Calculations of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ta}$ and $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ta}$ show that SHC of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ta}$ is 2.7 times that of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ta}$, while both have the opposite sign relative to fcc Pt and are an order of magnitude smaller than Pt. The calculated spin Hall angle of $\ensuremath{-}0.156$ is quite consistent with previous experiments on $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ta}$, further suggesting intrinsic contribution may dominate in $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ta}$. Our approach could facilitate large-scale SHC calculations and may benefit the discovery of materials with high intrinsic SHC.
59 citations
TL;DR: An all-optical method identifies the spin transparency for pure spin current transport through heavy metal/ferromagnet interface and signifies its role in generation ofpure spin current by spin pumping effect.
Abstract: Generation and utilization of pure spin current have revolutionized energy-efficient spintronic devices. Spin pumping effect generates pure spin current, and for its increased efficiency, spin-mixing conductance and interfacial spin transparency are imperative. The plethora of reports available on generation of spin current with giant magnitude overlook the interfacial spin transparency. Here, we investigate spin pumping in β-Ta/CoFeB thin films by an all-optical time-resolved magneto-optical Kerr effect technique. From variation of Gilbert damping with Ta and CoFeB thicknesses, we extract the spin diffusion length of β-Ta and spin-mixing conductances. Consequently, interfacial spin transparency is derived as 0.50 ± 0.03 from the spin Hall magnetoresistance model for the β-Ta/CoFeB interface. Furthermore, invariance of Gilbert damping with Cu spacer layer thickness inserted between β-Ta and CoFeB layers confirms the absence of other interface effects including spin memory loss. This demonstrates a reliable and noninvasive way to determine interfacial spin transparency and signifies its role in generation of pure spin current by spin pumping effect.
53 citations
Journal Article•
TL;DR: This work presents experiments evidencing a large spin-charge conversion by the Bi/Ag Rashba interface and demonstrates that the Rashba effect at interfaces can be used for efficient charge-spin conversion in spintronics.
Abstract: The Rashba effect is an interaction between the spin and the momentum of electrons induced by the spin-orbit coupling (SOC) in surface or interface states. Its potential for conversion between charge and spin currents has been theoretically predicted but never clearly demonstrated for surfaces or interfaces of metals. Here we present experiments evidencing a large spin-charge conversion by the Bi/Ag Rashba interface. We use spin pumping to inject a spin current from a NiFe layer into a Bi/Ag bilayer and we detect the resulting charge current. As the charge signal is much smaller (negligible) with only Bi (only Ag), the spin-to-charge conversion can be unambiguously ascribed to the Rashba coupling at the Bi/Ag interface. This result demonstrates that the Rashba effect at interfaces can be used for efficient charge-spin conversion in spintronics.
37 citations
TL;DR: In this paper, a low-temperature, long-wavelength theory for the interfacial spin Seebeck effect (SSE) in easy-axis antiferromagnets was developed.
Abstract: Author(s): Reitz, Derek; Li, Junxue; Yuan, Wei; Shi, Jing; Tserkovnyak, Yaroslav | Abstract: We develop a low-temperature, long-wavelength theory for the interfacial spin Seebeck effect (SSE) in easy-axis antiferromagnets. The field-induced spin-flop (SF) transition of N #x27;eel order is associated with a qualitative change in SSE behavior: Below SF, there are two spin carriers with opposite magnetic moments, with the carriers polarized along the field forming a majority magnon band. Above SF, the low-energy, ferromagnetic-like mode has magnetic moment opposite the field. This results in a sign change of the SSE across SF, which agrees with recent measurements on Cr$_2$O$_3$/Pt and Cr$_2$O$_3$/Ta devices [Li $\textit{et al.,}$ $\textit{Nature}$ $\textbf{578,}$ 70 (2020)]. In our theory, SSE is due to a N #x27;eel spin current below SF and a magnetic spin current above SF. Using the ratio of the associated N #x27;eel to magnetic spin-mixing conductances as a single constant fitting parameter, we reproduce the field dependence of the experimental data and partially the temperature dependence of the relative SSE jump across SF.
30 citations
TL;DR: In this article , the authors review the recent developments and applications, the current understanding of the physical processes, and the future challenges and perspectives of broadband spintronic terahertz emitters.
Abstract: Spintronic terahertz emitters are broadband and efficient sources of terahertz radiation, which emerged at the intersection of ultrafast spintronics and terahertz photonics. They are based on efficient spin-current generation, spin-to-charge-current conversion, and current-to-field conversion at terahertz rates. In this Editorial, we review the recent developments and applications, the current understanding of the physical processes, and the future challenges and perspectives of broadband spintronic terahertz emitters.
20 citations
References
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TL;DR: This review describes a new paradigm of electronics based on the spin degree of freedom of the electron, which has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices.
Abstract: This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.
9,917 citations
TL;DR: In this paper, a giant spin Hall effect (SHE) in β-tantalum was shown to generate spin currents intense enough to induce spin-torque switching of ferromagnets at room temperature.
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.
3,330 citations
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TL;DR: In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents as discussed by the authors and 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.
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.
2,178 citations
TL;DR: In this article, a pure spin current was injected into a Pt thin film using spin pumping, and it was observed to generate electromotive force transverse to the spin current, consistent with the spin-Hall effect.
Abstract: The inverse process of the spin-Hall effect (ISHE), conversion of a spin current into an electric current, was observed at room temperature. A pure spin current was injected into a Pt thin film using spin pumping, and it was observed to generate electromotive force transverse to the spin current. By changing the spin-current polarization direction, the magnitude of this electromotive force varies critically, consistent with the prediction of ISHE.
1,835 citations
TL;DR: The precession of the magnetization of a ferromagnet is shown to transfer spins into adjacent normal metal layers, slowing down the precession corresponding to an enhanced Gilbert damping constant in the Landau-Lifshitz equation.
Abstract: The precession of the magnetization of a ferromagnet is shown to transfer spins into adjacent normal metal layers. This ``pumping'' of spins slows down the precession corresponding to an enhanced Gilbert damping constant in the Landau-Lifshitz equation. The damping is expressed in terms of the scattering matrix of the ferromagnetic layer, which is accessible to model and first-principles calculations. Our estimates for permalloy thin films explain the trends observed in recent experiments.
1,558 citations