From Microscopic Imaging to Ultrasensitive Spin-Orbit Torque Metrology in van der Waals Spintronics and Beyond

The ability to image and manipulate magnetic order at the microscopic scale is essential for advancing spintronic applications in low-dimensional magnetic and topological systems. In this talk, I will present our recent progress in combining nanoscale magnetic imaging and ultrasensitive optical metrology to probe spin-orbit torques (SOTs) across a wide range of quantum materials, from van der Waals (vdW) heterostructures to ferrimagnetic insulators and emerging altermagnets.

First, I will discuss our scanning X-ray magnetic circular dichroism (XMCD) microscopy directly imaging speckled magnetic domains in epitaxial Fe₃GeTe₂/Bi₂Te₃ heterostructures [1]. These complex nanoscale domain textures, which deviate from a simple macrospin picture, are understood through micromagnetic simulations and density functional theory calculations that reveal the critical influence of interfacial interactions in vdW topological magnetic systems.

In parallel, we have developed a fiber-based Sagnac magneto-optic interferometer capable of sub-5 μrad/√Hz Kerr sensitivity under cryogenic temperatures and high magnetic fields [2]. This transport-free approach enables quantitative SOT detection in a wide range of systems, especially those inaccessible by conventional electrical techniques due to insulating behavior, low net magnetization, or superconducting current shunting. Using this platform, we probe current-induced spin torques in a range of systems beyond 2D, including epitaxial ferrimagnetic insulators such as bismuth-substituted yttrium iron garnet (Bi:YIG) [3], low-damping spinel ferrites like Li₀.₅(Al,Fe)₂.₅O₄ [4], and superconducting spintronic heterostructures. Together, these results highlight the synergy between microscopic imaging, precision optical metrology, and spintronic device engineering [5] as an integrated route to uncover spin-torque physics.