Optical spin-orbit torque in heavy metal-ferromagnet heterostructures

Laser-Induced Ultrafast Spin Injection in All-Semiconductor Magnetic CrI3/WSe2 Heterobilayer

Spin injection stands out as a crucial method employed for initializing, manipulating, and measuring the spin states of electrons, which are fundamental to the creation of qubits in quantum computing. However, ensuring efficient spin injection while maintaining compatibility with standard semiconductor processing techniques is a significant challenge. Herein, we demonstrate the capability of inducing an ultrafast spin injection into a WSe2 layer from a magnetic CrI3 layer on a femtosecond time scale, achieved through real-time time-dependent density functional theory calculations upon a laser pulse. Following the peak of the magnetic moment in the CrI3 sublayer, the magnetic moment of the WSe2 layer reaches a maximum of 0.89 μB (per unit cell containing 4 WSe2 and 1 CrI3 units). During the spin dynamics, spin-polarized excited electrons transfer from the WSe2 layer to the CrI3 layer via type-II band alignment. The large spin splitting in conduction bands and the difference in the number of spin-polarized local unoccupied states available in the CrI3 layer lead to a net spin in the WSe2 layer. Furthermore, we confirmed that the number of available states, the spin-flip process, and the laser pulse parameters play important roles during the spin injection process. This work highlights the dynamic and rapid nature of spin manipulation in layered all-semiconductor systems, offering significant implications for the development and enhancement of quantum information processing technologies.

Perspectives: Light Control of Magnetism and Device Development

Accurately controlling magnetic and spin states presents a significant challenge in spintronics, especially as demands for higher data storage density and increased processing speeds grow. Approaches such as light control are gradually supplanting traditional magnetic field methods. Traditionally, the modulation of magnetism was predominantly achieved through polarized light with the help of ultrafast light technologies. With the growing demand for energy efficiency and multifunctionality in spintronic devices, integrating photovoltaic materials into magnetoelectric systems has introduced more physical effects. This development suggests that sunlight will play an increasingly pivotal role in manipulating spin orientation in the future. This review introduces and concludes the influence of various light types on magnetism, exploring mechanisms such as magneto-optical (MO) effects, light-induced magnetic phase transitions, and spin photovoltaic effects. This review briefly summarizes recent advancements in the light control of magnetism, especially sunlight, and their potential applications, providing an optimistic perspective on future research directions in this area.

Observation of Terahertz Spin Hall Conductivity Spectrum in GaAs with Optical Spin Injection

We report the first observation of the spin Hall conductivity spectrum in GaAs at room temperature. Our terahertz polarimetry with a precision of several μrads resolves the Faraday rotation of terahertz pulses arising from the inverse spin Hall effect of optically injected spin-polarized electrons. The obtained spin Hall conductivity spectrum exhibits an excellent quantitative agreement with theory, demonstrating a crossover in the dominant origin from impurity scattering in the dc regime to the intrinsic Berry-curvature mechanism in the terahertz regime. Our spectroscopic technique opens a new pathway to analyze anomalous transports related to spin, valley, or orbital degrees of freedom.

Nonthermal Ultrafast Optical Control of Magnetization Dynamics by Linearly Polarized Light in Metallic Ferromagnet

Coherent optical control of the magnetization in ferromagnetic (FM) mediums using ultrafast nonthermal effect paves a promising avenue to improve the speed and repetition rate of the magnetization manipulation. Whereas previously, only heat-induced or helicity-dependent magnetization dynamics are demonstrated in metallic ferromagnets. Here, the linearly-polarized light control of magnetization is demonstrated in FM Co coupled with ferroelectric (FE) BiFeO₃ by tuning the light polarization direction. It is revealed that in the Co/BiFeO₃ heterostructure excited by femtosecond laser pulses, the magnetization precession amplitude follows a sinusoidal dependence on the laser polarization direction. This nonthermal control of coherent magnetization rotation is attributed to the optical rectification effect in the BiFeO₃ layer, which yields a FE polarization depending on the light polarization, and the subsequent modulation of magnetic energy in Co by the electrostriction-induced strain. This work demonstrates an effective route to nonthermally manipulate the ultrafast magnetization dynamics in metallic ferromagnets.

Evaluating Lead Halide Perovskite Nanocrystals as a Spin Laser Gain Medium

Spin-polarized charge endows conventional lasers with not only new functionalities but also reduced lasing thresholds thanks to the lifting of spin degeneracy. II-VI and III-V semiconductors have been extensively investigated as spin laser gain mediums; however, the degree of polarization is limited by the light hole and heavy hole degeneracy. Herein, we evaluate the potential of CsPbBr₃ nanocrystals─ones that are featured with low band-edge degeneracy and therefore a high degree of polarization as a result of inverted band structure and large spin-orbit coupling─as a gain medium for spin lasers. Our experiment and numerical modeling results reveal that, within the spin relaxation lifetime, the optical gain threshold can be depressed by polarizing the charge using circularly polarized photoexcitation. However, prolonging the spin relaxation lifetime is required to realize a spin laser.

Prospect of Spin-Orbitronic Devices and Their Applications

Science, engineering, and medicine ultimately demand fast information processing with ultra-low power consumption. The recently developed spin-orbit torque (SOT)-induced magnetization switching paradigm has been fueling opportunities for spin-orbitronic devices, i.e., enabling SOT memory and logic devices at sub-nano second and sub-picojoule regimes. Importantly, spin-orbitronic devices are intrinsic of nonvolatility, anti-radiation, unlimited endurance, excellent stability, and CMOS compatibility, toward emerging applications, e.g., processing in-memory, neuromorphic computing, probabilistic computing, and 3D magnetic random access memory. Nevertheless, the cutting-edge SOT-based devices and application remain at a premature stage owing to the lack of scalable methodology on the field-free SOT switching. Moreover, spin-orbitronics poises as an interdisciplinary field to be driven by goals of both fundamental discoveries and application innovations, to open fascinating new paths for basic research and new line of technologies. In this perspective, the specific challenges and opportunities are summarized to exert momentum on both research and eventual applications of spin-orbitronic devices.

Detection of femtosecond spin injection into a thin gold layer by time and spin resolved photoemission

The ultrafast demagnetization effect allows for the generation of femtosecond spin current pulses, which is expected to extend the fields of spin transport and spintronics to the femtosecond time domain. Thus far, directly observing the spin polarization induced by spin injection on the femtosecond time scale has not been possible. Herein, we present time- and spin-resolved photoemission results of spin injection from a laser-excited ferromagnet into a thin gold layer. The injected spin polarization is aligned along the magnetization direction of the underlying ferromagnet. Its decay time depends on the thickness of the gold layer, indicating that transport as well as storage of spins are relevant. This capacitive aspect of spin transport may limit the speed of future spintronic devices.