Toward the applications to high-performance MRAMs 1, 2, spin-orbit torque (SOT)-induced magnetization switching has been widely investigated in various material systems. While 4d transition metals, such as Ru, exhibit low SOT due to the small spin-orbit coupling 3, recent works have reported the SOT generation in antiferromagnetic RuO2 film thanks to its unique band structure 4, although the detailed understanding is still lacking. Herein, we fabricate the (100)-oriented RuO2 film and evaluate SOT in a RuO2/Co-Fe-B bilayer by harmonic Hall measurement.
RuO2 (4 nm)/Co20Fe60B20 (1.2 nm) bilayer was fabricated on a α-Al2O3 (0001) substrate by DC/RF sputtering, where (100)-oriented RuO2 film with a likely existence of a three-domain structure is expected to be grown 5. The resistivity of the RuO2 film was estimated as 247.4 mΩcm from the sheet resistance. A Ru (4 nm)/Co20Fe60B20 (1.2 nm) bilayer was also fabricated for comparison. The harmonic Hall measurement was performed for Hall bar devices in a rotating magnetic field Fig. 1. A clear second harmonic signal is observed for RuO2/Co-Fe-B while it is negligible for Ru/Co-Fe-B, suggesting the SOT generation owing to the unique band structure of RuO2. For RuO2/Co-Fe-B bilayer, the effective fields with the Slonczewski-like torque (HSL) and field-like torque (HFL) symmetries were quantified by fitting Fig. 2(a, b). Linear relation with the applied current density J Fig. 2(c) proves that these fields are induced by the current. These results would be important for further research on the current-induced magnetization switching using antiferromagnets.
The authors acknowledge JSPS KAKENHI Grants 21K14522, 21K18189, the Core Research Cluster program, the MRAM program in CIES, Tohoku Univ. Fig. 1: The second harmonic Hall resistance vs. field direction curves for Hall bar devices of RuO2/Co-Fe-B and Ru/Co-Fe-B. Fig. 2: Fitting of HSL (a) and HFL (b). J dependence of μ0HSL and μ0HFL (c).
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4 A. Bose et al., Nature Electronics 5, 267 (2022).
5 H. Bai et al., Phys. Rev. Lett. 128, 197202 (2022).