Giant Spin Hall Effect in Cu-Tb Alloy Thin Films

Anti-Poiseuille flow by spin Hall effect

Hydrodynamics is known to emerge in electron flow when the electron-electron interaction dominates over the other momentum-nonconserving scatterings. The hydrodynamic equation that describes the electric current includes viscosity, extending beyond the Ohmic flow. The laminar flow of such a viscous electron fluid in a sample with finite width is referred to as the Poiseuille flow, where the flow velocity is maximum at the center and decreases towards the edges of the sample. In this paper, we show a unique viscous electron fluid arising in electron systems exhibiting the spin Hall effect (spin Hall systems), where the charge and spin currents are coupled. Such a viscous electron fluid emerges even in noninteracting electron systems, and the current density exhibits a minimum at the center of a flow and a maximum at the edges, i.e. an anti-Poiseuille flow realizing. We also find that the spin accumulation by the spin Hall effect is connected to the electric current vorticity in two-dimensional (2D) spin Hall systems. Furthermore, we propose a novel guiding principle to manipulate topological magnetic textures from the hydrodynamic viewpoint. By solving the hydrodynamic equation in a 2D spin Hall system with a cavity and employing micromagnetic simulations for an attached chiral magnetic insulator, we demonstrate that spin accumulation near the cavity's boundary leads to creating a magnetic skyrmion. Our research illuminates new aspects of electron hydrodynamics and spintronics, contributing significant insights to the fields.

Strain-Mediated Spin-Orbit Torque Enhancement in Pt/Co on Flexible Substrate

Current-induced magnetization switching by spin-orbit torque generated in heavy metals offers an enticing realm for energy-efficient memory and logic devices. The spin Hall efficiency is a key parameter in describing the generation of spin current. Recent findings have reported enhancement of spin Hall efficiency by mechanical strain, but its origin remains elusive. Here, we demonstrate a 45% increase in spin Hall efficiency in the platinum/cobalt (Pt/Co) bilayer, of which 78% of the enhancement was preserved even after the strain was removed. Spin transparency and X-ray magnetic circular dichroism revealed that the enhancement was attributed to a bulk effect in the Pt layer. This was further confirmed by the linear relationship between the spin Hall efficiency and resistivity, which indicates an increase in skew-scattering. These findings shed light on the origin of enhancement and are promising in shaping future utilization of mechanical strain for energy-efficient devices.

A scalable molecule-based magnetic thin film for spin-thermoelectric energy conversion

Spin thermoelectrics, an emerging thermoelectric technology, offers energy harvesting from waste heat with potential advantages of scalability and energy conversion efficiency, thanks to orthogonal paths for heat and charge flow. However, magnetic insulators previously used for spin thermoelectrics pose challenges for scale-up due to high temperature processing and difficulty in large-area deposition. Here, we introduce a molecule-based magnetic film for spin thermoelectric applications because it entails versatile synthetic routes in addition to weak spin-lattice interaction and low thermal conductivity. Thin films of Cr^II[Cr^III(CN)₆], Prussian blue analogue, electrochemically deposited on Cr electrodes at room temperature show effective spin thermoelectricity. Moreover, the ferromagnetic resonance studies exhibit an extremely low Gilbert damping constant ~(2.4 ± 0.67) × 10^-4, indicating low loss of heat-generated magnons. The demonstrated STE applications of a new class of magnet will pave the way for versatile recycling of ubiquitous waste heat.