The emerging physics of the altermagnetic (AM) phase [1] has garnered significant interest due to the strong nonrelativistic spin splitting in band structure without net magnetization, offering potential for THz and ultra-dense memory technologies beyond traditional ferromagnet-based technologies. Evidence for such spin splitting exists in MnTe, CrSb and RuO2 thanks to the spin-resolved ARPES experiments [2,3] and several transport phenomena which were attributed to altermagnetism in MnTe, Mn5Si3, and RuO₂ [4-6]. However, recent studies [7] have challenged the magnetic order in RuO₂, casting doubt on its magnetic character in its bulk and thick-layer form.

In this study, we employ THz emission spectroscopy to investigate the theoretically predicted AM spin splitting in RuO₂ in the ultrafast regime. This technique is based on a scheme in which an optically excited ferromagnet (FM)/conversion metal (CM) bilayer emits THz pulses via the conversion of spin current pumped from the FM layer into the CM layer [8]. Using RuO₂ as the CM layer, we study the THz spin-to-charge current conversion (SCCC) [1,6]. THz emission was measured for different angles between the injected spin and the crystal orientation of RuO₂ at several temperatures. Symmetry of signals, affected also by anisotropy of THz outcoupling, was compared with the expected AM symmetry. Our findings suggest that the observed SCCC in RuO₂ originates in the inverse spin Hall effect (ISHE), which is strongly anisotropic with the modulation ~ 23%. Moreover, any contribution from AM spin-splitter effect appears small compared to the ISHE. On top of this study, we introduce a potential to study SCCC in other altermagnetic materials such as MnTe.

References

[1] L. Smejkal et al., Phys. Rev. X 12 040501 (2022)

[2] J. Krempaský et al., Nature 626 517 (2024)

[3] S. Reimers et al., Nat. Comm 15, 2116 (2024)

[4] R.D.G. Betancourt et al., Phys. Rev. Lett. 130, 036702 (2023)

[5] H. Reichlova et al., Nat. Comm. 15, 4961 (2024)

[6] A. Bose et al., Nat. Electron. 5, 263 (2022).

[7] P. Keßler et al., Npj Spintron. 2, 50 (2024)

[8] T. Seifert et al., Nat Photonics 10 483 (2016)