Andrei Zholud

Spin-current nano-oscillator based on nonlocal spin injection

Nonlocal spin injection has been recognized as an efficient mechanism for creation of pure spin currents not tied to the electrical charge transfer. Here we demonstrate experimentally that it can induce coherent magnetization dynamics, which can be utilized for the implementation of novel microwave nano-sources for spintronic and magnonic applications. We show that such sources exhibit a small oscillation linewidth and are tunable over a wide frequency range by the static magnetic field. Spatially resolved measurements of the dynamical magnetization indicate a relatively large oscillation area, resulting in a high stability of the oscillation with respect to thermal fluctuations. We propose a simple quasilinear dynamical model that reproduces well the oscillation characteristics.

Synchronization of spin Hall nano-oscillators to external microwave signals

Recently, a novel type of spin-torque nano-oscillators driven by pure spin current generated via the spin Hall effect was demonstrated. Here we report the study of the effects of external microwave signals on these oscillators. Our results show that they can be efficiently synchronized by applying a microwave signal at approximately twice the frequency of the auto-oscillation, which opens additional possibilities for the development of novel spintronic devices. We find that the synchronization exhibits a threshold determined by magnetic fluctuations pumped above their thermal level by the spin current, and is significantly influenced by the nonlinear self-localized nature of the auto-oscillatory mode.

Microwave generation by spin Hall nanooscillators with nanopatterned spin injector

We experimentally study spin Hall nano-oscillators based on Pt/ferromagnet bilayers with nanopatterned Pt spin injection layer. We demonstrate that both the spectral characteristics and the electrical current requirements can be simultaneously improved by reducing the spin injection area. Moreover, devices with nanopatterned Pt spin injector exhibit microwave generation over a wide temperature range that extends to room temperature. Studies of devices with additional Pt spacers under the device electrodes show that the oscillation characteristics are affected not only by the spin injection geometry but also by the effects of Pt/ferromagnet interface on the dynamical properties of the ferromagnet.

Spin Transfer due to Quantum Magnetization Fluctuations

We utilize a nanoscale magnetic spin-valve structure to demonstrate that current-induced magnetization fluctuations at cryogenic temperatures result predominantly from the quantum fluctuations enhanced by the spin transfer effect. The demonstrated spin transfer due to quantum magnetization fluctuations is distinguished from the previously established current-induced effects by a nonsmooth piecewise-linear dependence of the fluctuation intensity on current. It can be driven not only by the directional flows of spin-polarized electrons, but also by their thermal motion and by scattering of unpolarized electrons. This effect is expected to remain non-negligible even at room temperature, and entails a ubiquitous inelastic contribution to spin-polarizing properties of magnetic interfaces.

Measurement of the Hall effect at nanoscale with three probes

The Hall effect and its varieties such as quantum, anomalous, and spin Hall effects provide indispensable tools for the characterization of electronic and magnetic properties of materials, metrology, and spintronics. The conventional four-probe Hall configuration is generally not amenable to measurements at nanoscale due to current shunting by the Hall electrodes. We demonstrate that Hall measurements on the nanoscale can be facilitated by the three-probe Hall configuration that avoids the shunting problem. We illustrate the efficiency of the proposed approach with anomalous Hall effect-based measurements of individual activation events during domain wall motion in magnetic films with perpendicular anisotropy.

Field-induced waveguides for spin-torque magnonics

The development of magnonic devices over the last decade has significantly benefitted from the advances in the physics of nanomagnetism, such as the recent demonstration of the possibility to utilize the current-induced spin-transfer torque (STT) for the excitation of spin waves, which can expand the functionality of magnonic devices, increase their efficiency, and enable downscaling.

Nanoconstriction-based spin-Hall oscillators

Recent observations of coherent microwave magnetization oscillations driven by pure spin current have provided novel opportunities for the development of active spintronic devices. It was demonstrated that pure spin current produced by the spin-Hall effect (SHE) can be utilized to implement magnetic nano-oscillators that do not require electrical current flows through the active magnetic layer, allowing one to avoid detrimental effects associated with large densities of the electric current in the magnetic layer typical for the traditional spin-transfer torque (STT) spintronic devices. The previously demonstrated spin-Hall nano-oscillators (SHNO) were found to exhibit a relatively large power and small auto-oscillation linewidth at cryogenic temperatures. However, both of these characteristics significantly degrade at increased temperatures. This drawback can be avoided if a single dynamical mode with a significant spatial extent can be selectively excited in an SHNO. One can expect that the auto-oscillation area should depend on the spin injection geometry. However, in practice local injection of spin current leads to the spontaneous formation of the so-called “bullet” auto-oscillation mode, whose spatial dimensions are determined by the nonlinear self-localization effects rather than the spin-current injection area. Here, we demonstrate SHNOs characterized by efficient room-temperature generation of coherent microwave signals, achieved by controlling the auto-oscillation characteristics using magnetic dipolar effects instead of the self-localization.