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.

Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems. Experiments to date have examined a multitude of molecules conducting in parallel, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions or scanning probes as well as three-terminal single-molecule transistors made from carbon nanotubes, C(60) molecules, and conjugated molecules diluted in a less-conducting molecular layer. The ultimate limit would be a device where electrons hop on to, and off from, a single atom between two contacts. Here we describe transistors incorporating a transition-metal complex designed so that electron transport occurs through well-defined charge states of a single atom. We examine two related molecules containing a Co ion bonded to polypyridyl ligands, attached to insulating tethers of different lengths. Changing the length of the insulating tether alters the coupling of the ion to the electrodes, enabling the fabrication of devices that exhibit either single-electron phenomena, such as Coulomb blockade, or the Kondo effect.

We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance (FMR) dynamics. The Oersted field from the current also generates an FMR signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current and the spin Hall angle. We study Pt/Permalloy bilayer films with a microwave-frequency (RF) charge current applied in the film plane (Permalloy = Py = Ni 81 Fe 19 ). An oscillating transverse spin current is generated in the Pt by the SHE and injected into the adjacent Py ( Fig. 1(a)), thereby exerting an oscillating spin torque (ST) on the Py that induces magnetization precession. This leads to an oscillation of the bilayer resistance due to the anisotropic magnetoresistance (AMR) of Py. A DC voltage signal is generated across the sample from the mixing of the RF current and the oscillating resistance, similar to the signal that arises from ST induced FMR in spin valves and magnetic tunnel junctions [12][13][14][15]. The resonance properties enable a quantitative measure of the spin current absorbed by the Py.Our measurement setup is shown in Fig. 1(c). Pt/Py bilayers were grown by DC magnetron sputter deposition. The starting material for the Pt was 99.95% pure. Highly resistive 3 Ta (1 nm) was employed as the capping layer to prevent oxidation of the Py. The bilayers were subsequently patterned into microstrips of 1 to 20 μm wide and 3 to 250 μm long. By using a bias tee, we were able to apply a microwave current and at the same time measure the DC voltage. A sweeping magnetic field H ext was applied in the film plane, with the angle θ between H ext and microstrip kept at 45° unless otherwise indicated. The output power of the microwave signal generator was varied from 0 to 20 dBm and the measured DC voltage was proportional to the applied power, indicating that the induced precession was in the small angle regime. All the measurements we present were performed at room temperature with a power of 10 dBm.We model the motion of the Py magnetic moment m by the Landau-Lifshitz-GilbertHere γ is the gyromagnetic ratio, α is the Gilbert damping coefficient, μ 0 is the permeability in vacuum, M s is the saturation magnetization of Py, t is the thickness of the Py layer, , / 2 S RF J erepresents the oscillating spin current density injected into Py, H RF is the Oersted field generated by the RF current, H eff is the sum of H ext and the demagnetization field 4π M eff , and σ is the direction of the injected spin moment. The third and fourth terms on the right hand side of Eq. (1) are the result of in-plane spin torque and the out-of-plane torque due to the Oersted field, respectively ( Fig. 1(a)). The mixing signal in response to a combination of in...

† These authors contributed equally to this workThe recent discovery that a spin-polarized electrical current can apply a large torque to a ferromagnet, through direct transfer of spin angular momentum, offers the intriguing possibility of manipulating magnetic-device elements without applying cumbersome magnetic fields. 1-16 However, a central question remains unresolved:What type of magnetic motions can be generated by this torque? Theory predicts that spin transfer may be able to drive a nanomagnet into types of oscillatory magnetic modes not attainable with magnetic fields alone, 1-3 but existing measurement techniques have provided only indirect evidence for dynamical states. 4,6-8,12,14-16 The nature of the possible motions has not been determined. Here we demonstrate a technique that allows direct electrical measurements of microwave-frequency dynamics in individual nanomagnets, propelled by a DC spin-polarised current. We show that in fact spin transfer can produce several different types of magnetic excitations. Although there is no mechanical motion, a simple magnetic-multilayer structure acts like a nanoscale motor; it converts energy from a DC electrical current into high-frequency magnetic rotations that might be applied in new devices including microwave sources and resonators. 2 We examine samples made by sputtering a multilayer of 80 nm Cu / 40 nm Co / 10 nm Cu / 3 nm Co / 2 nm Cu / 30 nm Pt onto an oxidized silicon wafer and then milling through part of the multilayer (Fig. 1a) to form a pillar with an elliptical cross section of lithographic dimensions 130 nm ¥ 70 nm. 17 Top contact is made with a Cu electrode.Transmission or reflection of electrons from the thicker "fixed" Co layer produces a spinpolarised current that can apply a torque to the thinner "free" Co layer. Subsequent oscillations of the free-layer magnetization relative to the fixed layer change the device resistance 18 so, under conditions of DC current bias, magnetic dynamics produce a timevarying voltage (with typical frequencies in the microwave range). If the oscillations were exactly symmetric relative to the direction to the fixed-layer moment, voltage signals would occur only at multiples of twice the fundamental oscillation frequency, f. To produce signal strength at f, we apply static magnetic fields (H) in the sample plane a few degrees away from the magnetically-easy axis of the free layer. All data are taken at room temperature, and by convention positive current I denotes electron flow from the free to the fixed layer.In characterization measurements done at frequencies < 1 kHz, the samples exhibit the same spin-transfer-driven changes in resistance reported in previous experiments 7,9 (Fig. 1b). For H smaller than the coercive field of the free layer (H c ~ 600 Oe), an applied current produces hysteretic switching of the magnetic layers between the low-resistance parallel (P) and high-resistance antiparallel (AP) states. Sweeping H can also drive switching between the P and AP states (Fig 1b, inset). For H larger ...

We show that in a perpendicularly magnetized Pt/Co bilayer the spin-Hall effect (SHE) in Pt can produce a spin torque strong enough to efficiently rotate and switch the Co magnetization. We calculate the phase diagram of switching driven by this torque, finding quantitative agreement with experiments. When optimized, the SHE torque can enable memory and logic devices with similar critical currents and improved reliability compared to conventional spin-torque switching. We suggest that the SHE torque also affects current-driven magnetic domain wall motion in Pt/ferromagnet bilayers.