Anisotropic Altermagnetic and Magnetic Behaviors in YIG/RuO₂ Heterostructure

The field of magnetism has experienced a surge of interest in altermagnetism over the past few years [1]. This growing attention is driven by the unique physical properties of altermagnetic materials, which combine advantages of both conventional antiferromagnets and ferromagnets. Ruthenium dioxide (RuO₂) has emerged as a prototype altermagnetic candidate, but its experimental confirmation faces significant challenges. These challenges include ongoing debates about the existence of its magnetic order and difficulties in manipulating its orientation.

In this study [2], we addressed these challenges by utilizing thermal spin injection from a capping ferrimagnetic insulator yttrium iron garnet (YIG) thin film. This approach allowed us to detect the inverse altermagnetic spin-splitting effect (IASSE) in high-quality epitaxial altermagnetic RuO₂ thin films grown on TiO₂. Our observations revealed two significant findings: 1. The spin-to-charge conversion in the YIG/RuO₂/TiO₂ heterostructure is highly anisotropic. 2. The magnetic hysteresis loop of YIG/RuO₂/TiO₂ is also highly anisotropic. We attributed the anisotropy in the size of thermal voltage to an altermagnetic origin by ruling out the complication of an anisotropic SHE. The anisotropy in the shape of magnetic hysteresis loop verifies the [001] orientation of the RuO₂ Néel vectors, confirming its magnetic order.

We also found that 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. Our study provides significant insights into the magnetic ordering and the spin-splitting effect in altermagnetic materials, paving the way for advancements in spintronic technologies.

[1] L. Šmejkal, J. Sinova, and T. Jungwirth, Phys. Rev. X 12, 040501 (2022).

[2] C.-T. Liao, Y.-C. Wang, Y.-C. Tien, S.-Y. Huang, and D. Qu, Phys. Rev. Lett. 133, 056701 (2024)