Spin-orbit torque: spin accumulation techniques and interfacial investigation of topological insulator

• Main Author: Poh, Han Yin
• Other Authors: Lew Wen Siang
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Physics; Spintronics
• Online Access: https://hdl.handle.net/10356/176696
• Institution: Nanyang Technological University

Spin-orbit torque (SOT) represents a promising direction in the field of spintronics, allowing the manipulation of magnetization via electric current. Originating from strong spin-orbit coupling in materials such as heavy metals or topological insulators, it allows the generation of large SOT. However, a comprehensive understanding of the SOT mechanism such as spin accumulation dynamics in these materials remains elusive. Spin accumulation has been a subject of continued pursuit to understand SOT switching mechanisms for spintronics device applications. Effective methods to precisely characterize spin accumulation in a ferromagnet are yet to be demonstrated. Enhancement in SOT efficiency is highly desired as it allows low-power magnetization switching. As such, recent SOT studies focus on exploring novel materials and structures to improve efficiency. The topological insulator has recently drawn much attention as it demonstrates high charge-spin conversion efficiency due to its spin-momentum locking at the Dirac surface states. However, integrating the topological insulator into a magnetic heterostructure remains a challenge as the topological surface state is highly sensitive to perturbations. In this thesis, a technique to quantify spin accumulations directly in a heavy metal/ferromagnet heterostructure via DC-biased harmonic Hall measurement was presented. This technique relies on the induced anomalous resistance, which is generated by the spin Hall effect, and alters its sign based on the alignment of accumulated spins to local magnetization. Spin transport in topological insulators, particularly Bi0.85Sb0.15, was explored to enhance the SOT efficiency. Here, I investigate the use of different insertion layers between the TI and FM, to promote the crystallinity of the TI. The improvement of crystallinity will affect the topological surface state (TSS), which results in the enhancement of spin-orbit torque. Lastly, the influence of magnetic moment on SOT generated by the TSS from TI in a TI/FM heterostructure was studied. The experimental and analytical results reveal that the out-of-plane magnet moments disrupts the TSS of the TI, which induces a bandgap. The increment in magnetic moment induces a larger bandgap, which decreases SOT generated from the TI.