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.

In the spin Hall effect (SHE) 1-3 the application of a charge current through a nonmagnetic material results in the generation of a transverse spin current due to the spin-orbit interaction.This transverse spin current can be described by2e is the spin current density, e J the charge current density ,  the spin polarization unit vector and / SH s e J J   is the material-dependent spin Hall angle. Previous studies have demonstrated that the magnitudes of the spin Hall angles in the 5d elements Pt 4-9 and high resistivity β-Ta 10 can be relatively large, Pt SH   0.07 and -Ta

We report that spin current transport across Pt-ferromagnet (FM) interfaces is strongly dependent on the type and the thickness of the FM layer and on post-deposition processing protocols. By employing both harmonic voltage measurements and spin-torque ferromagnetic resonance measurements, we find that the efficiency of the Pt spin Hall effect in exerting a damping-like spin torque on the FM ranges from < 0.05 to > 0.10 under different interfacial conditions. We also show that the temperature dependence of the spin torque efficiencies for both the dampinglike torque and field-like torque is dependent upon the details of the Pt-FM interface. The "internal" spin Hall angle of the Pt thin films used in this study, after taking the interfacial spin transmission factor into account, is estimated to be ~ 0.20. This suggests that a careful engineering of Pt-FM interfaces can improve the spin-Hall-torque efficiency of Pt-based spintronic devices. The spin Hall effect (SHE) [1, 2] causes an electrical current density e J flowing through a material with strong spin-orbit interactions to generate a transverse spin current density s J . The amplitude of s J is characterized by the spin Hall angle . The most straightforward way to determine a lower bound [3], θ SH LB , on the spin Hall angle in normal metal (NM) systems is to measure the current-dependent torque that is exerted on an adjacent ferromagnet (FM) when spin current flows to the NM-FM interface. Research has shown [4, 5]that there are two different components of torque that can be observed in this case: a "dampinglike" torque , where m is the orientation of the ferromagnetic moment, and a "field-like" torque . The determination via τ DL measurements of a large θ SH LB ≈ 0.07, in Pt-FM thin film bilayers [3, 6, 7], and the subsequent observation of even larger, "giant" spin Hall angles for high resistivity Ta (amorphous or β phase Ta), θ SH LB ≈ 0.12 [8] and β-W, θ SH LB ≈ 0.33 [9, 10], have opened up a very active area for research and technology development.Recent calculations utilizing Boltzmann analysis [11] and the drift-diffusion approximation [12] have noted that if the NM-FM interface is not completely transparent to the flow of the spin current then spin backflow will reduce the torque applied to the FM by the SHE.This reduction can be characterized, as suggested above, by defining a damping-like spin torque efficiency ξ DL , and also a field-like torque efficiency ξ FL , for a particular NM-FM interface, such that ξ DL can be less than or equal to the "internal" spin Hall angle θ SH that quantifies the spin current generated in the absence of an adjacent FM. Within a diffusive model, the effects of spin backflow are expected to modify the spin torque efficiencies in the form [11,12]

We show that a direct current in a tantalum microstrip can induce steady-state magnetic oscillations in an adjacent nanomagnet through spin torque from the spin Hall effect (SHE). The oscillations are detected electrically via a magnetic tunnel junction (MTJ) contacting the nanomagnet. The oscillation frequency can be controlled using the MTJ bias to tune the magnetic anisotropy. In this 3-terminal device, the SHE torque and the MTJ bias therefore provide independent controls of the oscillation amplitude and frequency, enabling new approaches for developing tunable spin torque nano-oscillators.

We report that by measuring a current-induced hysteresis loop shift versus in-plane bias magnetic field, the spin-Hall effect (SHE) contribution of the current-induced effective field per current density χ SHE can be estimated for Pt-and Ta-based magnetic heterostructures with perpendicular magnetic anisotropy. We apply this technique to a Pt-based sample with its ferromagnetic (FM) layer being wedged deposited and discover an extra effective field contribution χ Wedged due to the asymmetric nature of the deposited FM layer. We confirm the correlation between χ Wedged and the asymmetric depinning process in FM layer during magnetization switching by magneto-optical Kerr microscopy. These results indicate the possibility of engineering deterministic spin-orbit torque switching by controlling the symmetry of domain expansion through the materials growth process. DOI: 10.1103/PhysRevB.93.144409 Current-induced spin-orbit torque (SOT) has been shown to be an efficient way of manipulating the magnetization in heavy-metal/ferromagnet (HM/FM) heterostructures. Unlike conventional spin transfer torque [1,2] in which the source of spin angular momentum comes from a ferromagnetic polarizer layer, SOTs arise from either the bulklike spin-Hall effect (SHE) [3,4] of the nonmagnetic (NM) HM layer or Rashbatype spin-orbit interaction [5,6] at the interface. SOTs can be utilized to achieve efficient magnetization switching [7][8][9][10], ultrafast domain-wall (DW) motion [6,11,12], and microwave generation through magnetic oscillations [13,14] in spintronic device applications.SOTs are typically studied in magnetic heterostructures with perpendicular magnetic anisotropy (PMA), and in general both a Slonczewski-like and a field-like torque can be present. The Slonczewski-like torque is most relevant to magnetization switching: It manifests as an effective field H eff with an out-of-plane (easy-axis) component that can reverse the magnetization or drive DWs if a component of the magnetization lies along the current-flow direction. The most common measurement schemes used to quantify the Slonczewski-like SOT efficiency χ ≡ H eff /J e (effective field per unit current density J e ) include ferromagnetic resonance techniques [15][16][17], low-frequency harmonic voltage measurements using small ac currents [18][19][20], and analysis of current-induced DW motion in thin magnetic strips [11,12,21,22]. Current-induced SOT switching of PMA films under an in-plane bias field is another convenient means for determining the sign of χ ; however, a quantitative estimate of its magnitude is usually difficult to obtain in such measurements due to the complicated magnetization reversal process [23,24].In this paper we examine the role of domain nucleation and DW propagation in SOT-assisted magnetization switching in HM/FM bilayer systems with PMA. We show that the currentinduced shift of the out-of-plane hysteresis loop as a function of in-plane bias field can be well explained by a simple currentassisted DW propagation model. This simple mea...