Anisotropic Altermagnetic and Magnetic Behaviors in Ruthenium Dioxide

Danru Qu^1,3*, Yu-Chun Wang², Ching-Te Liao², Yu-Cheng Tien², Ssu-Yen Huang^2,3

¹Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
²Department of Physics, National Taiwan University, Taipei, Taiwan
³Center of Atomic Initiatives for New Materials, National Taiwan University, Taipei, Taiwan

* Presenter:Danru Qu, email:danru@ntu.edu.tw

Altermagnetism has attracted intensive attention recently since altermagnets have zero magnetization like antiferromagnets but with time-reversal breaking behaviors like ferromagnets. Ruthenium dioxide (RuO₂), a prototype altermagnet proposed by theoreists, however, has faced severe experimental challenges in confirming its magnetic order and controlling the orientation of its Néel vectors. To overcome these challenges and firmly establish the altermagnetism in RuO2, we investigate the magnetic and altermagnetic behaviors of YIG/RuO₂ heterostructures. By leveraging thermal spin injection from yttrium iron garnet (YIG), we successfully observed the inverse altermagnetic spin-splitting effect (IASSE) in epitaxial RuO₂ thin films grown on TiO₂ substrates. Our results revealed two important findings: (1) the spin-to-charge conversion and (2) the magnetic hysteresis loop in the YIG/RuO₂/TiO₂ heterostructure were highly anisotropic. These anisotropies, attributed to the alignment of RuO₂'s Néel vectors in the [001] orientation, confirm its magnetic order. Moreover, the IASSE displayed an opposite sign compared to the inverse spin Hall effect (ISHE) and exhibited remarkable efficiency—consistently 70% of ISHE across a range of RuO₂ thicknesses from 5 to 32 nm. Our work not only advances the understanding of RuO₂'s magnetic properties but also introduces new ways to manipulate altermagnetic behaviors via substrate-induced modifications to RuO₂ crystallinity. These insights in altermagnets open new pathways for developing spintronic devices without ferromagnets.

[1] Libor Šmejkal, Jairo Sinova, and Tomas Jungwirth, Emerging Research Landscape of Altermagnetism. Phys. Rev. X 12, 040501 (2022).

[2] C.-T. Liao, Y.-C. Wang, Y.-C. Tien, S.-Y. Huang, and D. Qu, Separation of Inverse Altermagnetic Spin-Splitting Effect from Inverse Spin Hall Effect in RuO2. Phys. Rev. Lett. 133, 056701 (2024), Editors’ suggestion

Keywords: Altermagnetism, Spin-splitting effect, Spin Hall effect, Magnetism, Spintronics