Kwang-Su Ryu

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.

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.

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.

Magneto-optical microscope magnetometer for simultaneous local probing of magnetic properties

The design of a magneto-optical microscope magnetometer (MOMM) for simultaneously probing local magnetic properties is described. The MOMM consists of an optical polarizing microscope capable of magneto-optical contrast that is used as a magnetometer by sweeping a magnetic field from an electromagnet. Due to full-field optical imaging, as opposed to single photodiode detection, the system is capable of simultaneous measurement of magnetic hysteresis loops and magnetization viscosity curves on 8000 individual local regions of 400×400 nm2 area in ferromagnetic materials. The most striking feature of the system is that it provides two-dimensional maps of the local magnetic properties including the coercivity, the switching time, and the activation magnetic moment from two-dimensional arrays of the hysteresis loops and the viscosity curves. We present the local magnetic properties and their correlations in Co/Pd multilayer films prepared by electron-beam evaporation.

Large converse magnetoelectric coupling effect at room temperature in CoPd/PMN-PT (001) heterostructure

We report a large converse magnetoelectric (CME) effect at room temperature in a multiferroic heterostructure formed from thin layers of perpendicularly magnetized CoxPd1-x alloys deposited on a piezoelectric single-crystal of lead magnesium niobate-lead titanate PMN-PT(001). The CME results from a strain-induced reorientation of the CoPd magnetization. By varying the composition and thickness of the CoxPd1-x film, a large converse magnetoelectric coupling constant, α=8×10−7 s/m, at room temperature was found for 10 nm Co0.25Pd0.75. This large CME effect results from combining a highly magnetostrictive CoPd alloy with highly piezoelectric PMN-PT.

Thickness-dependent magnetic domain change in epitaxial MnAs films on GaAs(001)

The authors report the change of the magnetic domain structure, dependent on the film thickness of MnAs films epitaxially grown on GaAs(001), investigated by magnetic force microscopy. Interestingly, as the film thickness decreases, the domain structure within the ferromagnetic α-MnAs stripes changes from a head-on domain structure to a simple 180° one around a thickness of 250nm. This result is understood by the change in the demagnetizing factor of the ferromagnetic stripes with the film thickness.

Origin of uniaxial magnetic anisotropy in epitaxial MnAs film on GaAs(001) substrate

We investigate the origin of in-plane uniaxial magnetic anisotropy of epitaxial ferromagnetic MnAs film on GaAs(001). Interestingly, as temperature increases, the in-plane uniaxial magnetic anisotropy along the MnAs[112¯0] direction changes and then disappears. Direct microscopic domain observations show that the type of domain structure changes from a simple domain to a closure one with increasing temperature. From these results, the temperature-dependent change of the in-plane magnetic anisotropy is ascribed to a decrease in the shape anisotropy induced by the decrease in the width of the ferromagnetic α-stripe.

Real-time direct observation of temperature-dependent domain reversal behavior in epitaxial MnAs film on GaAs(001)

We have investigated temperature-dependent domain reversal behavior in the MnAs film epitaxially grown on GaAs(001) using a magneto-optical microscope capable of real-time direct observation of domain evolution. Interestingly enough, the domain reversal in the temperature range of 20°C∼35°C shows the domain wall-motion process with the sawtooth type and, then, it changes to the nucleation-dominant process above 37.5°C. This change could be understood by the decrease of the dipolar interaction energy and the disconnection of the ferromagnetic α-MnAs stripes, induced by the decrease of the α-MnAs volume ratio with increasing temperature.

Transition from the thermal activation process to the viscous process in magnetization reversal behavior of the Co/Pd multilayer

We have investigated the transition from thermal activation process to viscous process in magnetization reversal behavior of the Co/Pd multilayer from the determination of the wall-motion speed and the nucleation rate via time-resolved domain observation. Interestingly, we find that the field dependencies of two activation volumes in the thermal activation regime are different from each other, which reveals that the wall-motion and nucleation experience completely different interactions. We also find that the wall-mobility in the viscous regime is much smaller than a typical value for the sandwiched Co films, which implies that the Co/Pd interfaces substantially contribute to the dynamic dissipation.