Speaker
Description
Controlling the magnetization direction of a ferromagnet is crucial for magnetic memories, including cassette tapes, hard disk drives, and the next-generation magnetoresistive random access memory (MRAM)—a type of non-volatile memory that stores data by manipulating magnetic states. However, the current method of controlling magnetization—by generating a spin current converted to charge through the spin Hall effect—relies heavily on heavy metals, limiting the technology's scalability and cost-effectiveness. The recent discovery of the orbital Hall effect (OHE)—which refers to the charge-to-orbital conversion—offers an alternative means of controlling magnetization using an orbital current [1]. This mechanism is present in commonly used light metals like vanadium and titanium (Ti). Despite its significance, the direct observation of OHE remains challenging due to the need to distinguish the orbital angular momentum from other effects, such as spin angular momentum.
In our study, we explore innovative optical magnetic imaging techniques to address this challenge. These non-contact, sensitive methods provide spatial resolution capabilities that electrical measurements cannot achieve. I will introduce several state-of-the-art techniques we are currently examining, including the laser-based magneto-optical Kerr effect (MOKE), single-spin-based scanning magnetometry (nitrogen-vacancy scanning, or NV scanning), and X-ray magnetic circular dichroism (XMCD) ptychography. These techniques have unique advantages: time-resolved MOKE offers spatial resolution and ultrafast time-resolving measurements; NV scanning enables sensitive magnetic field sensing at a nanometer scale from cryogenic to room temperature; XMCD ptychography can distinguish spin and orbitals with tens of nanometer spatial resolution.
Specifically, our experimental observations have directly confirmed the presence of OHE in the light metal Ti using MOKE [2]. We found an unexpectedly long orbital relaxation length of approximately 70 nm. These results have significant implications for the electrical control of magnetism and emphasize the need for a better understanding of OHE dynamics across a range of materials. In addition, I will discuss the future directions of optical imaging techniques and how they can be used in future studies to elucidate the origin of OHE and explore their potential applications in other materials, such as two-dimensional magnets.
References
[1] Bernevig, B. A., Hughes, T. L. & Zhang, S. C. Orbitronics: the intrinsic orbital current in p-doped silicon. Phys. Rev. Lett. 95, 066601 (2005).
[2] Choi, Y.-G., Jo, D., Ko, K.-H., Kim, K.-H. Park, H. G., Kim, C., Min. B.-C., Choi, G.-M., and Lee, H.-W. Nature (accepted) (2023), Preprint: https://arxiv.org/abs/2109.14847.
| Keywords | Magnetic imaging, orbital Hall effect, spin Hall effect, magneto-optic Kerr effect, MOKE, x-ray, circular dichroism, nitrogen-vacancy scanning, NV scanning |
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