MMM 2022

November 07, 2022

Minneapolis, United States

Observation of non local orbital transport and sign reversal of damping

We study the efficiencies of damping-like torque in Nb/Ni and Ta/Ni bilayers by the spin-torque ferromagnetic resonance (ST-FMR) technique1 and compare it with Nb/Ni81Fe19(Py) and Ta/Py as a function of the thickness of ferromagnet. In both these systems, we observe that the sign of the damping-like torque is negative for the Nb/Py and it is positive for the Nb/Ni system suggesting two competing mechanisms responsible for the torque on the magnet: (1) spin-current generated from the bulk of Nb and Ta due to spin Hall effect (SHE) producing the spin-orbit torques that are predicted to have a negative sign1 and (2) orbital current2,3 generated from the bulk of Nb and Ta which convert into the spin-current while entering in the ferromagnet due to the spin-orbit coupling of the ferromagnet which is predicted to produce an orbital-torque with the positive sign4,5. In Ta/Ni and Nb/Ni bilayers, the orbital current is dominant as Ni is predicted to be a good detector of the orbital current as compared to Py resulting in the sign reversal of the damping-like torque efficiency in Ta/Ni and Nb/Ni bilayers as compared to Ta/Py and Nb/Py samples. We further inserted a Cu spacer between the non-magnet and Ni that still shows the positive sign of the damping-like torque suggesting a generation of the orbital current from the bulk of Ta and Nb. Generation of orbital torque will open up new possibilities to manipulate the magnetization more efficiently6.<br
> References
1 L. Liu, C.-F. Pai, Y. Li, H.W. Tseng, D.C. Ralph, and R.A. Buhrman, Science (80-. ). 336, 555 (2012). 2 T. Tanaka, H. Kontani, M. Naito, T. Naito, D.S. Hirashima, K. Yamada, and J. Inoue, Phys. Rev. B 77, 165117 (2008). 3 D. Go, D. Jo, C. Kim, and H. Lee, Phys. Rev. Lett. 121, 86602 (2018). 4 D. Go, F. Freimuth, J.-P. Hanke, F. Xue, O. Gomonay, K.-J. Lee, S. Blügel, P.M. Haney, H.-W. Lee, and Y. Mokrousov, Phys. Rev. Res. 2, 033401 (2020). 5 D. Go and H.-W. Lee, Phys. Rev. Res. 2, 013177 (2020). 6 D. Go, D. Jo, H.-W. Lee, M. Kläui, and Y. Mokrousov, EPL (Europhysics Lett. 135, 37001 (2021).

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