Spintronics and topological Berry phase effect in two-dimensional systems

Abstract

Spintronics, or spin electronics, is an emergent field of nanoscale electronics that uses the spin of electrons in addition to their charge to influence an electrical current. Spin transport phenomena, pivotal in this domain, are heavily affected by the preser- vation of spin information at interfaces. This dissertation delves into spin-memory loss, a common issue in spintronic heterostructures composed of a ferromagnet (FM) and a nonmagnetic metal (NM) with strong spin-orbit coupling (SOC). We explore the fundamental mechanism of spin-memory loss using a tight-binding approach, empha- sizing the key role of bulk SOC in driving spin-flips through the complex intertwining of spin and orbital states. The advent of two-dimensional (2D) van der Waals (vdW) materials has paral- leled the rise of spintronics, attracting substantial interest in materials research since the graphene isolation. Fe3GeTe2 (FGT), a vdW topological ferromagnetic semimetal, has emerged as a subject of significant attention. It is known for its intriguing phe- nomena, such as considerable Berry phase effects. Driven by the large anomalous Hall effect (AHE) observed in FGT, an outcome of its nontrivial band topology, this dis- sertation will investigate the microscopic mechanism of the AHE and the robustness of topological features against external perturbations, such as uniaxial tensile strain. The analysis is performed through first-principles calculations and model analysis in- corporating symmetries. This study further explores spin-orbit torque (SOT) in FGT. It is noteworthy that the strength of the effective field for SOT in FGT considerably surpasses that in heavy metals like Platinum (Pt), with the field strength stemming from the orbital origin being especially larger. These findings bear significant implications, suggesting that orbital-origin torques may have a more pivotal role in spintronic systems than previ- ously anticipated. The structure of this dissertation is as follows: In Ch. 1, we introduce spintronics and the topological Berry phase effect, laying out the fundamental physics. Ch. 2 provides foundational knowledge on spin transport, delineating the physical picture of transport in mesoscopic systems within the ballistic regime. This foundation sets the stage for the findings presented in Ch. 3, which articulates the formal aspects of our computation of interfacial spin transport, with a specific focus on the mechanism of spin-memory loss. Ch. 4 delves into the topological Berry phase effect in FGT, including the anomalous Hall effect. Finally, Ch. 5 addresses the calculation of the SOT in FGT from the perspective of the orbital degree of freedom.