Session W51: Invited Session: Recent Advances in Spin Transport: Spin Pumping and the Spin Hall Effect

Electrical detection of current-induced spin polarization due to spin-momentum locking in the topological insulator Bi$_{2}$Se$_{3}$

Topological insulators (TIs) exhibit topologically protected metallic surface states populated by massless Dirac fermions with spin-momentum locking -- the carrier spin lies in-plane, locked at right angle to the carrier momentum. An unpolarized charge current should thus create a net spin polarization whose amplitude and orientation are controlled by the charge current. Here we show direct electrical detection of this bias current induced spin polarization as a voltage measured on a ferromagnetic (FM) metal tunnel barrier surface contact [1]. The magnetization of the contact determines the spin detection axis, and the voltage measured at this contact is proportional to the projection of the TI spin polarization onto this axis. When the charge current is orthogonal to the magnetization of the FM detector contact, the TI spin is parallel (or antiparallel) to the magnetization, and a spin-related signal is detected at the FM contact proportional to the magnitude of the charge current. The voltage measured scales inversely with Bi$_{2}$Se$_{3}$ film thickness, and its sign is that expected from spin-momentum locking and opposite that of a Rashba effect [2]. Similar data are obtained for two different FM contact structures, Fe/Al$_{2}$O$_{3}$ and Co/MgO/graphene, underscoring the fact that these behaviors are due to bias current induced spin polarization in the TI surface states rather than the bulk, and are independent of the details of the contact. These results demonstrate simple and direct electrical access to the TI Dirac surface state spin system, provide clear evidence for the spin-momentum locking and bias current-induced spin polarization, and enable utilization of these remarkable properties for future technological applications. \\[4pt] [1] C. H. Li, O. M. J. van't Erve, J. T. Robinson, Y. Liu, L. Li , and B. T. Jonker, Nature Nanotech. 9, 218 (2014). DOI: 10.1038/NNANO.2014.16\\[0pt] [2] S. Hong, V. Diep, S. Datta and Y.P. Chen, Phys. Rev. B. 86, 085131 (2012).   

Spin pumping and spin-transfer torques in antiferromagnet

Spin pumping and spin-transfer torques are key elements of coupled dynamics of magnetization and conduction electron spin, which have been widely studied in various ferromagnetic materials. Recent progress in spintronics suggests that a spin current can significantly affects the behavior of an antiferromagnetic material [1], and the electron motion become adiabatic when the staggered field varies sufficiently slowly [2]. However, pumping from antiferromagnets and its relation to current-induced torques is yet unclear. In a recent study [3], we have solved this puzzle analytically by calculating how electrons scatter off a normal metal-antiferromagnetic interface. The pumped spin and staggered spin currents are derived in terms of the staggered field, the magnetization, and their rates of change. We find that for both compensated and uncompensated interfaces, spin pumping is of a similar magnitude as in ferromagnets; the direction of spin pumping is controlled by the polarization of the driving microwave. Via the Onsager reciprocity relations, the current-induced torques are also derived, the salient feature of which is illustrated by a terahertz nano-oscillator. \\[4pt] [1] R. Cheng and Q. Niu, Phys. Rev. B \textbf{89}, 081105(R) (2014).\\[0pt] [2] R. Cheng and Q. Niu, Phys. Rev. B \textbf{86}, 245118 (2012).\\[0pt] [3] R. Cheng, J. Xiao, Q. Niu, and A. Brataas, Phys. Rev. Lett. \textbf{113}, 057601 (2014).   

Pure Spin Current in a broad range of materials generated by YIG-based spin pumping

Spintronics relies on the generation, manipulation, and detection of spin current mediated by itinerant charges or magnetic excitations. FMR spin pumping is a powerful technique in understanding pure spin current. Building on our high-quality Y$_{3}$Fe$_{5}$O$_{12}$ (YIG) films and the large inverse spin Hall effect (ISHE) signals enabled by these films [1-10], we have characterized spin currents in several classes of materials with different magnetic structures, including: nonmagnetic and ferromagnetic metals, nonmagnetic insulators, and antiferromagnetic (AF) insulators. The spin Hall angles determined for a series of 3d, 4d, and 5d metals show that both atomic number and d-electron count play important roles in spin Hall physics [1, 6]. Strikingly, we achieved robust spin transport from YIG to Pt across AF insulators, which initially enhances the ISHE signals and can transmit spin currents up to 100 nm thickness, demonstrating highly efficient spin transport through an AF insulator carried by magnetic excitations [3].\\[4pt] [1] Du, et al. PRB 90, 140407(R) (2014).\\[0pt] [2] Adur, et al. PRL 113, 176601 (2014).\\[0pt] [3] Wang, et al. PRL 113, 097202 (2014).\\[0pt] [4] Wang, et al. APL 104, 202405 (2014).\\[0pt] [5] Du, et al. PR Appl. 1, 044004 (2014). \\[0pt] [6] Wang, et al. PRB 112, 197201 (2014).\\[0pt] [7] Wolfe, et al. PRB 89, 180406(R) (2014).\\[0pt] [8] Wang, et al. PRB 89, 134404 (2014).\\[0pt] [9] Du, et al. PRL 111, 247202 (2013).\\[0pt] [10] Wang, et al. PRB 88, 100406(R) (2013).   

The essential role of spin-memory loss at 3d/5d metallic interfaces in spin pumping

I will present a review of experiments and theory of spin-pumping in Co/(Cu)/Pt 3d/5d metallic systems in the ferromagnetic resonance (FMR) regime of spin injection [1]. By combining i) FMR analyses of the resonance linewidth of the Co spectra in contact with the Pt (or Cu/Pt) reservoir and ii) detection of the inverse spin-hall effect signal vs. Pt thickness, we were able to evidence two different lengthscales for the spin-current profile generated or absorbed at the interfaces [2]. The first lenghscale, extracted from FMR analyses and of the order of 2 nm, represents a typical interface length characteristic of a spin memory loss at the Co/Pt and Co/Cu/Pt interfaces. This represent a typical region of spin-current dissipation by which almost 60-70 {\%} of the total current generated is lost before conversion in bulk Pt. The second lengthscale, roughly equal to 3.4 nm, like determined by Inverse Spin Hall Effect (ISHE) transverse voltage measurement, is more characteristic of the spin-diffusion length of the bulk Pt that governs a part of the spin-to-charge conversion efficiency by ISHE. After careful analyses, we determined a spin-hall angle of 5.6 {\%} for Pt and an intrinsic spin hall conductivity of 3200 (Ohm.cm)$^{-1}$ for our corresponding Pt resistivity [2]. In the end, I will focus on the physical description of our experiments within a derived Valet-Fert model describing the spin transport/relaxation in a diffusive approach and using relevant boundary conditions for spin-pumping (constant spin accumulation in the ferromagnet). The origin of the spin-memory loss and spin-current discontinuity, also proposed in a very recent work [3], will be explained in terms of atomic intermixing at interfaces or possible Rashba-split states at Co/Pt interfaces.\\[4pt] [1] Y. Tserkovnyak et al., Rev. Mod. Phys. 77, 1375 (2005) and references therein.\\[0pt] [2] J. C. Rojas-Sanchez et al., Phys. Rev. Lett. 112 106602 (2014)\\[0pt] [3] Liu Yi et al., Phys. Rev. Lett. 113, 207202 (2014)