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.

Spin-torque switching with the giant spin hall effect of tantalum

The 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 possibility of manipulating magnetic-device elements without applying cumbersome magnetic fields. 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, but existing measurement techniques have provided only indirect evidence for dynamical states. 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 d.c. spin-polarized current. We show that spin transfer can produce several different types of magnetic excitation. Although there is no mechanical motion, a simple magnetic-multilayer structure acts like a nanoscale motor; it converts energy from a d.c. electrical current into high-frequency magnetic rotations that might be applied in new devices including microwave sources and resonators.

Microwave oscillations of a nanomagnet driven by a spin-polarized current

Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems1. Experiments to date have examined a multitude of molecules conducting in parallel2,3, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions4,5 or scanning probes6,7 as well as three-terminal single-molecule transistors made from carbon nanotubes8, C60 molecules9, and conjugated molecules diluted in a less-conducting molecular layer10. 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.

Coulomb blockade and the Kondo effect in single-atom transistors

https://ralphgroup.lassp.cornell.edu/papers/JMMM-320-1190-2008.pdf

Spin transfer torques

We report a giant spin Hall effect in β-W thin films. Using spin torque induced ferromagnetic resonance with a β-W/CoFeB bilayer microstrip, we determine the spin Hall angle to be |θSHβ-W|=0.30±0.02, large enough for an in-plane current to efficiently reverse the orientation of an in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction adjacent to a thin β-W layer. From switching data obtained with such 3-terminal devices, we independently determine |θSHβ-W|=0.33±0.06. We also report variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable (α and β) phase composition.

Spin transfer torque devices utilizing the giant spin Hall effect of tungsten

We have measured the statistical properties of magnetic reversal in nanomagnets driven by a spin-polarized current. Like reversal induced by a magnetic field, spin-transfer-driven reversal near room temperature exhibits the properties of thermally activated escape over an effective barrier. However, the spin-transfer effect produces qualitatively different behaviors than an applied magnetic field. We discuss an effective current vs field stability diagram. If the current and field are tuned so that their effects oppose one another, the magnet can exhibit telegraph-noise switching.

/pdf/thermally-activated-magnetic-reversal-induced-by-a-spin-39srd7pu51.pdf

Thermally-Activated Magnetic Reversal Induced by a Spin-Polarized Current

Molecular Magnetism: Single-Molecule Magnets and Molecular Quantum Spintro

We consider the form of the current-voltage curves generated when tunneling spectroscopy is used to measure the energies of individual electronic energy levels in nanometer-scale systems. We point out that the voltage positions of the tunneling resonances can undergo temperature-dependent shifts, leading to errors in spectroscopic measurements that are proportional to the temperature. We do this by solving the set of rate equations that can be used to describe electron tunneling via discrete quantum states, for a number of cases important for comparison to experiments, including (1) when just one spin-degenerate level is accessible for transport, (2) when two spin-degenerate levels are accessible, with no variation in electron-electron interactions between eigenstates, and (3) when two spin-degenerate levels are accessible, but with variations in electron-electron interactions. We also comment on the general case with an arbitrary number of accessible levels. In each case we analyze the voltage positions, amplitudes, and widths of the current steps due to the quantum states.

/pdf/solving-rate-equations-for-electron-tunneling-via-discrete-3za9mlfa38.pdf

Solving rate equations for electron tunneling via discrete quantum states

We report the synthesis and characterization of well-defined CoPt clusters with a mean diameter of 3 nm, produced in ultrahigh vacuum conditions following a physical route. Samples made of diluted layers of CoPt clusters embedded in amorphous carbon have been studied by transmission electron microscopy. High-resolution observations have revealed the appearance of $L{1}_{0}$ chemical order upon annealing, even for clusters with a 2 nm diameter, without cluster coalescence. The magnetic properties of both chemically disordered and ordered CoPt clusters embedded in amorphous carbon have then been measured by x-ray magnetic circular dichroism and superconducting quantum interference device magnetometry. Despite a striking change of the Co magnetic moment, the magnetic anisotropy of chemically ordered nanoparticles increases, with respect to the chemically disordered $A1$ phase, in much lower proportions than what is observed for the bulk.

Edgar Bonet

Papers

Thermally-Activated Magnetic Reversal Induced by a Spin-Polarized Current

Molecular Magnetism: Single-Molecule Magnets and Molecular Quantum Spintro

Solving rate equations for electron tunneling via discrete quantum states

Solving rate equations for electron tunneling via discrete quantum states

Evidence of L 1 0 chemical order in CoPt nanoclusters: Direct observation and magnetic signature