Weng-Lee Lim

Spectral Characteristics of the Microwave Emission by the Spin Hall Nano-Oscillator

We utilized microwave spectroscopy to study the magnetization oscillations locally induced in a Permalloy film by a pure spin current, which is generated due to the spin Hall effect in an adjacent Pt layer. The oscillation frequency is lower than the ferromagnetic resonance of Permalloy, indicating that the oscillation forms a self-localized nonpropagating spin-wave soliton. At cryogenic temperatures, the spectral characteristics are remarkably similar to the traditional spin-torque nano-oscillators driven by spin-polarized currents. However, the linewidth of the oscillation increases exponentially with temperature and an additional peak appears in the spectrum below the ferromagnetic resonance, suggesting that the spectral characteristics are determined by interplay between two localized dynamical states.

Dynamical skyrmion state in a spin current nano-oscillator with perpendicular magnetic anisotropy.

We study the spectral characteristics of spin current nano-oscillators based on the Pt/[Co/Ni] magnetic multilayer with perpendicular magnetic anisotropy. By varying the applied magnetic field and current, both localized and propagating spin wave modes of the oscillation are achieved. At small fields, we observe an abrupt onset of the modulation sidebands. We use micromagnetic simulations to identify this state as a dynamical magnetic skyrmion stabilized in the active device region by spin current injection, whose current-induced dynamics is accompanied by the gyrotropic motion of the core due to the skew deflection. Our results demonstrate a practical route for controllable skyrmion manipulation by spin current in magnetic thin films.

Optimization of Pt-based spin-Hall-effect spintronic devices

We study experimentally the routes to improve the characteristics of the spin-Hall-effect devices based on permalloy/Pt bilayers by optimization of the Pt layer thickness and by the addition of an antiferromagnetic spin-sinking layer. We experimentally determine the spin-diffusion length in Pt and show that Pt thickness can be reduced down to 2 nm without degradation of the device characteristics caused by the spin accumulation effects, which provides possibilities for significant reduction of the required driving currents. We also show that the addition of a spin-sinking layer results in a non-monotonic dependence of device efficiency on the Pt thickness.

Temperature-dependent proximity magnetism in Pt

We study experimentally the magnetic coupling between ferromagnets separated by a thin Pt layer. The coupling remains ferromagnetic regardless of the Pt thickness and exhibits a significant dependence on temperature. We show that these results are consistent with the effects of temperature-dependent magnetism induced in Pt at the interfaces with ferromagnets. Analysis shows that the characteristic magnetization decay length in Pt increases from approximately 0.2 nm at room temperature to 0.8 nm at 25 K.

Control of current-induced spin-orbit effects in a ferromagnetic heterostructure by electric field

We study the effects of electrostatic gating on the current-induced phenomena in ultrathin ferromagnet/heavy metal heterostructures. We utilize heterodyne detection and analysis of symmetry with respect to the direction of the magnetic field to separate electric field contributions to the magnetic anisotropy, current-induced field-like torque, and damping torque. Analysis of the electric field effects allows us to estimate the Rashba and the spin Hall contributions to the current-induced phenomena. Electrostatic gating can provide insight into the spin-orbit phenomena, and enable new functionalities in spintronic devices.

Fast chirality reversal of the magnetic vortex by electric current

The possibility of high-density information encoding in magnetic materials by topologically stable inhomogeneous magnetization configurations such as domain walls, skyrmions, and vortices has motivated intense research into mechanisms enabling their control and detection. While the uniform magnetization states can be efficiently controlled by electric current using magnetic multilayer structures, this approach has proven much more difficult to implement for inhomogeneous states. Here, we report direct observation of fast reversal of magnetic vortex by electric current in a simple planar structure based on a bilayer of spin Hall material Pt with a single microscopic ferromagnetic disk contacted by asymmetric electrodes. The reversal is enabled by a combination of the chiral Oersted field and spin current generated by the nonuniform current distribution in Pt. Our results provide a route for the efficient control of inhomogeneous magnetization configurations by electric current.