Session J28: Focus Session: Spin-Hall Effect I

The Spin Hall Effect in Rare Earth Thin Films

The spin Hall effect results in a spin current which flows transverse to an applied electric field in heavy metals, and which can be used to apply an efficient spin transfer torque to the ferromagnetic layer in heavy metal/ferromagnet bilayer structures. We report experimental investigations of the strength of the spin Hall effect in lanthanide rare earth materials. To ensure trustworthy results, we compare the results of several complementary measurement techniques: off-resonant second harmonic detection of current-induced magnetic tilting, spin-torque ferromagnetic resonance, and spin pumping. We report on both the anti-damping and effective-field components of the spin torque generated by the spin Hall effect.   

Nonequilibrium spin polarization induced charge Hall effect

The nonequilibrium spin polarization lies at the heart of information processing in spin-based devices. The generation and manipulation of the spin polarization have been realized by various approaches, however, the spin polarization is usually considered to have negligible effect on the electric transport property, especially for systems of high electron concentration like metals ($\varepsilon_F\sim$ eV). Here we show that the nonequilibrium spin polarization can cause a novel Hall voltage in a conventional metallic alloy at room temperature, which is due to a new mechanism and closely related to the spin Nernst effect.   

Spin Hall effect by surface roughness

spin Hall effect and its inverse effect, caused by the spin orbit interaction, provide the interconversion between spin current and charge current. Since the effects make it possible to and manipulate spin current electrically, how to realize the large effects is an important in both physics and applications. To do so, materials with heavy elements, which have strong spin orbit interaction, have been examined so far. Here, we propose a new mechanism to enhance the spin Hall effect without heavy elements, i.e. surface roughness in metallic thin films. We examine Cu and Al thin films with surface roughness and find that they give the spin Hall effect comparable to that in bulk Au. We demonstrate that the spin Hall effect induced by surface roughness has the side jump contribution but not skew scattering.   

Optical detection of spin Hall effect in metals

Here we present room temperature measurements of the spin Hall effect in non-magnetic metals such as Pt and $\beta$-W using a standard bench top magneto-optic Kerr effect (MOKE) system.\footnote{O. M. J. van't Erve, A. T. Hanbicki, K. M. McCreary, C. H. Li, and B. T. Jonker, Optical detection of spin Hall effect in metals, \textit{APL} \textbf{104} 172402 (2014)} With this system, one can readily determine the angular dependence of the induced polarization on the bias current direction, the orientation of the spin Hall induced polarization, and the sign of the spin Hall angle. When a bias current is applied, the spin Hall effect causes electrons of opposite spin to be scattered in orthogonal directions, resulting in a spin accumulation at the surface of the film. The MOKE signal tracks the applied square wave bias current with an amplitude and phase directly related to the spin Hall angle. Using this technique, we show that the spin-Hall angle of $\beta$-W is opposite in sign and significantly larger than that of Pt, and follow the structural phase transition from $\beta$-W to $\alpha$-W as the film is annealed through the dependence of the spin Hall angle on crystal structure. We also use this technique to detect spin diffusion from $\beta$-W into Al thin films.   

Extremely large, gate tunable spin Hall angle in 3D Topological Insulator pn junction

The band structure of the surface states of a three dimensional Topological Insulator (3D TI) is similar to that of graphene featuring massless Dirac Fermions. We show that due to this similarity, the chiral tunneling of electron in a graphene pn junction also appears in 3D TI. Electrons with very small incident angle (modes) are allowed to transmit through a TI pn junction (TIPNJ) due to the chiral tunneling. The rest of the electrons are reflected. As a result, the charge current in a TIPNJ is suppressed. Due to the spin momentum locking, all the small angle modes are spin-down states. Therefore, the transmitted end of the TIPNJ becomes highly spin polarized. On the other hand, the spin of the reflected electron is flipped due to spin momentum locking. This enhances the spin current at the injection end. Thus, the interplay between the chiral tunneling and spin momentum locking reduces the charge current but enhances the spin current at the same time, leading to an extremely large ($\sim$20) spin Hall angle. Since the chiral tunneling can be controlled by an external electric field, the spin Hall angle is gate tunable. The spin current generated by a TIPNJ can be used for energy-efficient switching of nanoscaled ferromagnets, which is an essential part of spintronic devices.   

Growth of $\beta $-Tungsten Films Towards a Giant Spin Hall Effect Logic Device

Spin-orbit coupling in metastable $\beta $-W generates spin transfer torques strong enough to flip magnetic moment of an adjacent magnetic layer. In a MTJ stack these torques can be used to switch between high and low resistive states. This technique can be used in designing efficient magnetic memory and non-volatile spin logic devices. Deposition conditions selective to $\beta $- W need to be understood for the large scale fabrication of such devices. The transition from $\beta $ to $\alpha $ phase of Tungsten is strongly governed by thickness of W layer, base pressure and oxygen availability for example, above 5 nm $\beta $ film relaxes and forms an $\alpha $ phase. Resistivity measurements as well as x-ray photoelectron spectroscopy and x-ray diffraction and reflectivity analysis are performed to determine the phase and thickness of tungsten films. We show that $\beta $ phase is influenced by ultrathin thermal oxide of Si layer and the amount of oxygen flow during the growth. These results demonstrate a reliable technique to fabricate $\beta $ W film up to 20 nm on bare Si and silicon dioxide, while providing insight to growing it anywhere in the device stack.   

Spin polarized tunneling study on spin Hall effect metals and topological insulators

Spin orbit interactions give rise to interesting physics phenomena in solid state materials such as the spin Hall effect (SHE) and topological insulator surface states. Those effects have been extensively studied using electrical detection techniques so far. However, to date most experiments focus only on characterizing electrons near Fermi surface, while the spin-orbit interaction is expected to be dependent on electrons' energies. Here we develop a tunneling spectroscopy technique to measure spin Hall materials and topological insulators under finite bias voltages. By electrically injecting spin polarized current into spin Hall metals or topological insulators through nonmagnetic material/oxide/ferromagnet (FM) junctions and measuring the induced transverse voltage, we are able to quantify the magnitude of the SHE in typical 5d transition metals and the spin momentum locking in topological insulators. The obtained spin Hall angles in Ta, Pt, W and Ir at zero bias are consistent with the results from spin torque experiments, verifying the SHE origin of those earlier observations. At finite biases, the transverse signals provide important information in determining the mechanisms of the observed effects, such as intrinsic vs extrinsic, surface vs bulk. Because of the impedance matching capability of tunnel junctions, the spin polarized tunneling spectroscopy technique is expected to be a powerful tool to measure a wide group of matters including the various newly discovered or proposed topological materials.   

Spin current swapping and spin hall effect in disordered metals

The conversion of charge currents into spin currents via the spin Hall effect has attracted intense experimental and theoretical efforts lately, providing an efficient means to generate electric signals [1] and manipulate the magnetization of single layers. More recently, it was proposed that spin-dependent scattering induced by spin-orbit coupled impurities also produces a so-called spin swapping, i.e. an exchange between the spin angular momentum and linear momentum of itinerant electrons [2,3]. In this work, we investigate the nature of spin swapping and its interplay with extrinsic spin Hall effect and spin relaxation in finite size normal metals. We use two complementary methods based on non-equilibrium Green's function technique. The first method consists in rigorously deriving the drift-diffusion equation of the spin accumulation in the presence of spin-orbit coupled impurities from quantum kinetics using Wigner expansion. The second method is the real-space tight binding modeling of a finite system in the presence of spin-orbit coupled disorder. [1] A. Hoffman, Spin Hall Effects in Metals, IEEE Trans. Magn. 49, 5172 (2013). [2] M. B. Lifshits and M. I. Dyakonov, Phys. Rev. Lett. 103 , 186601 (2009) [3] S. Sadjina et al, Phys. Rev. B 85 , 115306 (2012)   

Role of transparency of platinum-ferromagnet interface in determining intrinsic magnitude of spin Hall effect

The spin Hall effect (SHE) converts charge current to pure spin currents in orthogonal directions in materials that have significant spin-orbit coupling. The efficiency of the conversion is described by the spin Hall Angle (SHA). The SHA can most readily be inferred by using the generated spin currents to excite or rotate the magnetization of ferromagnetic films or nano-elements via spin-transfer torques. Some of the largest spin torque derived spin Hall angles (ST-SHA) have been reported in platinum. By using spin torque ferromagnetic resonance (ST-FMR) measurements, we show that the transparency of the Pt-ferromagnet interface to the spin current plays a central role in determining the magnitude of the ST-SHA. We measure a much larger ST-SHA in Pt/cobalt (0.11) compared to Pt/permalloy (0.05) bilayers when the interfaces are assumed to be completely transparent. Taking into account the transparency of these interfaces, as derived from spin--mixing conductivities, we find that the intrinsic SHA in platinum has a much higher value of 0.19 as compared to the ST-SHA. The importance of the interface transparency is further exemplified by the insertion of atomically thin magnetic layers at the Pt/permalloy interface that we show strongly modulates the magnitude of the ST-SHA.   

Spin Hall effects from mesoscopic Pt films with high resistivity

The spin Hall effect (SHE) and inverse spin Hall effect (ISHE) are explored in mesoscopic lateral structures. Each structure consists of a Pt stripe, a Cu channel and a Py spin injector/detector. Low-resistance AlOx layers are placed at all interfaces. Two groups of structures are made with different sizes of the Pt/AlOx/Cu interfaces. The average resistance values of interfaces are 80 ohm in one group and 4 ohm in the other. Despite the resistance difference by a factor of 20, the average SHE signals only differ by a factor of 1.8 with the low-resistance structures showing higher signals. For a low-resistance interface, the ISHE signal is enhanced due to a more efficient absorption of the pure spin current but at the same time the signal reduction due to current shunting is also more severe. We are able to estimate the effect of shunting and the rate of spin absorption and obtain the product of spin Hall angle and the Pt spin diffusion length. It is noteworthy that the resistivity of the Pt stripe is substantially larger than that of an extended film. The large Pt resistivity contributes positively to the size of the signals but also implies short Pt spin diffusion length (\textless 2nm). A sizable Pt spin Hall angle of \textgreater 0.09 is estimated.   

The Enhancement of spin Hall torque efficiency and Reduction of Gilbert damping in spin Hall metal/normal metal/ferromagnetic trilayers

The spin Hall effect (SHE) in ferromagnet/heavy metal bilayer structures has been demonstrated to be a powerful means for producing pure spin currents and for exerting spin-orbit damping-like and field-like torques on the ferromagnetic layer. Large spin Hall (SH) angles have been reported for Pt, beta-Ta and beta-W films and have been utilized to achieve magnetic switching of in-plane and out-of-plane magnetized nanomagnets, spin torque auto-oscillators, and the control of high velocity domain wall motion. For many of the proposed applications of the SHE it is also important to achieve an effective Gilbert damping parameter that is as low as possible. In general the spin orbit torques and the effective damping are predicted to depend directly on the spin-mixing conductance of the SH metal/ferromagnet interface. This opens up the possibility of tuning these properties with the insertion of a very thin layer of another metal between the SH metal and the ferromagnet. Here we will report on experiments with such trilayer structures in which we have observed both a large enhancement of the spin Hall torque efficiency and a significant reduction in the effective Gilbert damping. Our results indicate that there is considerable opportunity to optimize the effectiveness and energy efficiency of the damping-like torque through engineering of such trilayer structures.   

Phase diagram and optimal switching induced by spin Hall effect

In a ferromagnet/heavy-metal bilayer device with strong spin Hall effect, an in-plane current excites magnetic dynamics through spin-torque transfer. We analyze bilayers with a perpendicularly magnetized ferromagnet and calculate three-dimensional phase diagrams describing switching due to application of external magnetic field at a fixed current. For fields applied in the plane formed by the film normal vector and the current direction, we find the location of the additional equilibria created by the spin torque and give analytic expressions for two different destabilization boundaries. We further discuss the nature of switching at each boundary and qualitatively describe the magnetic state evolution. By analyzing the phase portraits of the system we give the condition at which switching from ``up'' to ``down'' state proceeds through this intermediate state. Using numeric simulations we analyze the switching time and compare it to that of a spin valve with a perpendicular polarizer.   

Enhanced anti-damping torque in double-Spin-Hall trilayers

In magnetic thin-film heterostructures, current-induced anti-damping torque can switch magnetization [1], drive domain walls [2], and induce precessional dynamics [3]. The spin Hall effect in ferromagnet/normal-metal bilayers is an especially promising mechanism for generating a robust anti-damping torque. We report on enhanced tuning of resonant magnetization dynamics in in-plane magnetized Ta/CoFeB/Pt trilayers, where both the Ta and Pt layers serve as spin-Hall sources. The change in resonant linewidth induced by in-plane DC current is measured using spin-torque ferromagnetic resonance (FMR) [4] and cavity-based FMR [5]. With optimized Ta and Pt layer thicknesses, we observe in 4-nm thick CoFeB a damping modification of $> 3\times10^{-3}$ per $10^{11}$ A/m$^2$ of DC current, effectively more than doubling the anti-damping torque compared to conventional spin-Hall bilayers. This finding presents a new possibility for increasing the efficiency of spin-Hall driven devices.\\[4pt] [1] L. Liu et al. Science. 336, 555 (2012).\\[0pt0 [2] S. Emori et al. Nat. Mater. 12, 611 (2013).\\[0pt] [3] V. E. Demidov et al. Nat. Mater. 11, 1028 (2012).\\[0pt] [4] L. Liu et al. Phys. Rev. Lett. 106, 036601 (2011).