Bio: Danru Qu is an assistant research fellow at the Center for Condensed Matter Sciences (CCMS), National Taiwan University. Before joining CCMS, she worked as a postdoctoral fellow at the Institute of Physics (IOP), Academia Sinica and the Institute for Solid State Physics (ISSP), University of Tokyo. She was awarded the Japan Society for the Promotion of Science (JSPS) postdoctoral fellowship during her stay in Japan. She got her Ph. D degree in Physics from the Johns Hopkins University, and bachelor degree from the University of Science and Technology of China. Her research interest is the interactions among spin, charge and heat in novel magnetic materials, topological materials, magnetic insulators and their heterostructures. She recently received the 2030 Research Project for Emerging Young Scholars from the National Science and Technology Council.
Abstract: The field of magnetism has seen a surge of interest in a novel classification known as altermagnetism over the past few years [1]. This interest is driven by the unique physical properties of altermagnetic materials, which takes advantages of both conventional antiferromagnets and ferromagnets through the altermagnetic spin-splitting effect (ASSE). The non-relativistic ASSE enables the generation of not only a longitudinal spin polarized current, but also a transverse pure spin current upon the injection of a charge current along certain crystal orientations. However, experimentally unequivocal observation of the ASSE is challenging. This difficulty arises from the inevitable mixing of ASSE with the spin Hall effect (SHE) caused by the material's relativistic spin-orbit coupling and the ASSE's dependence on the hard-to-probe and hard-to-control Néel vectors.
In this talk, I will discuss our approach in addressing these challenges in the study of ASSE in RuO2 [2]. Our experimental observations revealed a highly anisotropic spin-to-charge conversion in the epitaxial RuO2 thin film. We attribute the anisotropy to an altermagnetic origin by ruling out the complication of an anisotropic SHE. We found the IASSE exhibited an opposite sign compared to the inverse spin Hall effect (ISHE). Remarkably, the efficiency of the IASSE was found to be consistently 70% of that of the ISHE in RuO2 for thicknesses ranging from 5 nm to 32 nm. Furthermore, we demonstrated that the ASSE/IASSE effects are observable only when the Néel vectors are well-aligned by modifying the Néel vector domains via RuO2 crystallinity through different substrates. Interestingly, the shape of the thermal voltage hysteresis loop is anisotropic and is consistent with the anisotropic magnetic hysteresis loop of YIG, which verifies the [001] orientation of the RuO2 Néel vectors. Our study provides significant insights into the spin-splitting effect in altermagnetic materials, paving the way for future advancements in spintronic technologies.