See-Hun Yang

Chiral spin torque at magnetic domain walls

Spin-polarized currents provide a powerful means of manipulating the magnetization of nanodevices, and give rise to spin transfer torques that can drive magnetic domain walls along nanowires. In ultrathin magnetic wires, domain walls are found to move in the opposite direction to that expected from bulk spin transfer torques, and also at much higher speeds. Here we show that this is due to two intertwined phenomena, both derived from spin-orbit interactions. By measuring the influence of magnetic fields on current-driven domain-wall motion in perpendicularly magnetized Co/Ni/Co trilayers, we find an internal effective magnetic field acting on each domain wall, the direction of which alternates between successive domain walls. This chiral effective field arises from a Dzyaloshinskii-Moriya interaction at the Co/Pt interfaces and, in concert with spin Hall currents, drives the domain walls in lock-step along the nanowire. Elucidating the mechanism for the manipulation of domain walls in ultrathin magnetic films will enable the development of new families of spintronic devices.

Domain-wall velocities of up to 750 m s −1 driven by exchange-coupling torque in synthetic antiferromagnets

The operation of racetrack memories is based on the motion of domain walls in atomically thin, perpendicularly magnetized nanowires, which are interfaced with adjacent metal layers with high spin-orbit coupling. Such domain walls have a chiral Néel structure and can be moved efficiently by electrical currents. High-capacity racetrack memory requires closely packed domain walls, but their density is limited by dipolar coupling from their fringing magnetic fields. These fields can be eliminated using a synthetic antiferromagnetic structure composed of two magnetic sub-layers, exchange-coupled via an ultrathin antiferromagnetic-coupling spacer layer. Here, we show that nanosecond-long current pulses can move domain walls in synthetic antiferromagnetic racetracks that have almost zero net magnetization. The domain walls can be moved even more efficiently and at much higher speeds (up to ∼750 m s(-1)) compared with similar racetracks in which the sub-layers are coupled ferromagnetically. This is due to a stabilization of the Néel domain wall structure, and an exchange coupling torque that is directly proportional to the strength of the antiferromagnetic exchange coupling between the two sub-layers. Moreover, the dependence of the wall velocity on the magnetic field applied along the nanowire is distinct from that of the single-layer racetrack due to the exchange coupling torque. The high domain wall velocities in racetracks that have no net magnetization allow for densely packed yet highly efficient domain-wall-based spintronics.

Memory on the racetrack.

Racetrack memory stores digital data in the magnetic domain walls of nanowires. This technology promises to yield information storage devices with high reliability, performance and capacity.

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

The spin Hall effect induces spin currents in nonmagnetic layers, which can control the magnetization of neighbouring ferromagnets. The transparency of the interface is shown to strongly influence the efficiency of such manipulation.

Chiral spin torque arising from proximity-induced magnetization

Domain walls can be driven by current at very high speeds in nanowires formed from ultra-thin, perpendicularly magnetized cobalt layers and cobalt/nickel multilayers deposited on platinum underlayers due to a chiral spin torque. An important feature of this torque is a magnetic chiral exchange field that each domain wall senses and that can be measured by the applied magnetic field amplitude along the nanowire where the domain walls stop moving irrespective of the magnitude of the current. Here we show that this torque is manifested when the magnetic layer is interfaced with metals that display a large proximity-induced magnetization, including iridium, palladium and platinum but not gold. A correlation between the strength of the chiral spin torque and the proximity-induced magnetic moment is demonstrated by interface engineering using atomically thin dusting layers. High domain velocities are found where there are large proximity-induced magnetizations in the interfaced metal layers.

High speed 45nm gate length CMOSFETs integrated into a 90nm bulk technology incorporating strain engineering

A leading edge 90 nm logic bulk foundry technology with 45 nm gate length devices, incorporating strain engineering, is described in this paper. Gate length and dielectric scaling, along with optimized strain engineering, enable high performance devices, which are amongst the best reported to date. Short channel effect control down to 35 nm is demonstrated. Both NMOS and PMOS performance are improved through careful optimization of stress effects from both trench isolation and contact etch stop nitride film. Furthermore, analysis of the channel mobility and current enhancement is used to gain understanding of the stress mechanisms, and hence layout design practice should be optimized for performance.

Enhanced interface perpendicular magnetic anisotropy in Ta|CoFeB|MgO using nitrogen doped Ta underlayers

We show that the magnetic characteristics of Ta|CoFeB|MgO magnetic heterostructures are strongly influenced by doping the Ta underlayer with nitrogen. In particular, the saturation magnetization drops upon doping the Ta underlayer, suggesting that the doped underlayer acts as a boron diffusion barrier. In addition, the thickness of the magnetic dead layer decreases with increasing nitrogen doping. Surprisingly, the interface magnetic anisotropy increases to ∼1.8 erg/cm2 when an optimum amount of nitrogen is introduced into the Ta underlayer. These results show that nitrogen doped Ta serves as a good underlayer for spintronic applications including magnetic tunnel junctions and domain wall devices.

Enhanced spin–orbit torques by oxygen incorporation in tungsten films

The origin of spin–orbit torques, which are generated by the conversion of charge-to-spin currents in non-magnetic materials, is of considerable debate. One of the most interesting materials is tungsten, for which large spin–orbit torques have been found in thin films that are stabilized in the A15 (β-phase) structure. Here we report large spin Hall angles of up to approximately –0.5 by incorporating oxygen into tungsten. While the incorporation of oxygen into the tungsten films leads to significant changes in their microstructure and electrical resistivity, the large spin Hall angles measured are found to be remarkably insensitive to the oxygen-doping level (12–44%). The invariance of the spin Hall angle for higher oxygen concentrations with the bulk properties of the films suggests that the spin–orbit torques in this system may originate dominantly from the interface rather than from the interior of the films.

Domain wall trajectory determined by its fractional topological edge defects

When a domain wall of a given chirality is injected into a magnetic nanowire, its trajectory through a branched network of Y-shaped nanowire junctions—such as a honeycomb lattice, for instance—can be pre-determined. This property has implications for data storage and processing.

Current Induced Tilting of Domain Walls in High Velocity Motion along Perpendicularly Magnetized Micron-Sized Co/Ni/Co Racetracks

Kerr microscopy is used to investigate domain wall motion in response to nanosecond-long current pulses in perpendicularly magnetized micron-sized Co/Ni/Co racetracks. Domain wall velocities greater than 300 m/s are observed. The velocity is independent of the pulse length for a wide range of current densities. However, the domain wall dynamics depends on the pulse length just above the threshold current for motion, where slow creep motion occurs, and at very high current densities, where domain nucleation takes place. We also observe a tilting of the domain wall that cannot be accounted for by the Oersted field from the driving current.