Research on the anisotropic absorption mechanism of spin current due to the band curvature of topological surface states has made progress

In ferromagnetic/non-magnetic (FM/NM) heterojunctions, the nanosecond-scale spin pumping effect and the femtosecond-scale spin superdiffusion process are two main mechanisms for injecting spin currents from the ferromagnetic layer to the non-magnetic layer, providing a theoretical basis for the development of ultrafast and low-energy consumption spin logic devices. Topological insulators (TI) demonstrate higher spin-electron conversion efficiency than traditional heavy metal materials due to their strong spin-orbit coupling and spin-momentum locking surface states, and have great application potential in the fields of new-generation spin logic devices and terahertz wave generation. 

The previous research work of the team led by Cheng Zhaohua from the Institute of Physics of the Chinese Academy of Sciences and the Beijing National Center for Condensed Matter Physics revealed that the non-magnetic band structure can regulate the magnetic damping factor of the ferromagnetic layer. In the Fe/α-GeTe heterojunction, researchers observed that the anisotropic Rashba split band, under the spin pumping effect, induced enhanced anisotropic Gilbert damping. Similarly, the higher-order terms of the k·p theory in topological insulators can also cause the Fermi surface to bend, forming a Fermi contour resembling a snowflake. However, the influence of this deformed topological surface state (TSS) on the pure spin absorption in the FM/TI heterostructure has not yet been clarified. 

Recently, the research team based on the Boltzmann equation of the spin potential, took into account the spin accumulation intensity, and theoretically calculated and predicted the anisotropic absorption mechanism of spin flow caused by the curvature of the topological surface states. The theory predicts that if the Fermi surface is dominated by TSS, the curvature effect will lead to anisotropic Gilbert damping at the nanosecond time scale. As the thickness of Bi2Te3 increases, the contribution of the bulk state becomes stronger, and the accumulated spins diffusing to the bulk state will dilute this effect, causing the anisotropy to disappear; while at the femtosecond time scale, the quasi-equilibrium state has not yet been established, so there will be no anisotropic spin dynamics behavior, and it will manifest as isotropic ultrafast demagnetization. 

Experimentally, the research team utilized ultra-high vacuum molecular beam epitaxy technology to fabricate high-quality, thickness-controllable Bi2Te3 ultrathin films. By measuring the Fermi surface of Bi2Te3 of different thicknesses using angle-resolved photoelectron spectroscopy, the variation law of the energy band bending effect with thickness was fitted. After constructing an Fe/Bi2Te3 heterojunction, combined with the planar waveguide ferromagnetic resonance testing system and the time-resolved magneto-optical Kerr effect testing platform, the spin dynamics were investigated at the nanosecond and femtosecond time scales. The experimental results were consistent with the theoretical predictions: At the nanosecond scale, an anisotropic enhanced Gilbert damping was observed, and its anisotropic degree showed a trend of increasing first and then decreasing with the increase of Bi2Te3 layer thickness; while at the femtosecond scale, an almost isotropic ultrafast demagnetization process was detected. 

This study proposed and verified the anisotropic Gilbert damping mechanism induced by the band curvature of topological surface states, laying the foundation for the advancement of anisotropic spin electronics research. 

The related research results were published under the title 'Warping Effect-Induced Spin Current Absorption at Various Timescales in Fe/Bi2Te3 Heterostructures' in the journal Physical Review Letters. This research was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China.