Hyunsoo Yang

Affiliations

• Department of Electrical and Computer Engineering, National University of Singapore, 117576, Singapore

Title

• Spin dynamics in Bi₂Se₃/ferromagnet heterostructures

Abstract

Bi₂Se₃ is a prototypical three dimensional topological insulator (TI), which is expected to exhibit a high spin orbit torque (SOT) efficiency due to the spin-textured topological surface states. However, very few studies have reported the SOT phenomena in ferromagnet/TI by electrical measurements [1,2]. Here we investigate the SOT efficiency in ferromagnet/Bi₂Se₃ structures at different temperatures by two different techniques such as spin torque ferromagnetic resonance (ST-FMR) and spin pumping. In the ST-FMR measurements on Co₄₀Fe₄₀B₂₀ (5 nm)/Bi₂Se₃ (20 nm), where the charge currents directly flow through devices, we find that the SOTs show a steep increase as the temperature decreases to 50 K, and the SOT efficiency increases abruptly from 0.047 at 300 K to 0.42 below 50 K [3]. Moreover, we observe a significant out-of-plane SOT efficiency in the low temperature range. We exclude the possible mechanisms from the spin Hall effect in Bi₂Se₃ bulk and Rashba-split states at the interface. Finally, we conclude that our results clearly demonstrate that the topological surface states are the origin of SOTs in CoFeB/Bi₂Se₃ systems. We also report the SOT efficiency and spin diffusion length at different temperatures in Ni₈₁Fe₁₉/Bi₂Se₃ by the spin pumping technique, where no direct charge current flows thru the device [4]. The SOT efficiency of Bi2Se3 is found to be ~0.01 at room temperature. In addition, the spin diffusion length in Bi₂Se₃ is evaluated to be 6.2 nm at room temperature. Both the SOT efficiency and spin diffusion length in Bi₂Se₃ increase at low temperatures. We will discuss possible reasons for the difference between the SOT efficiency from ST-FMR and that from spin pumping measurements.

References

• [1] A. R. Mellnik et al., Spin-transfer torque generated by a topological insulator, Nature 511, 449(2014).
• [2] Y. Fan et al., Magnetization switching through giant spin–orbit torque in a magnetically doped topological insulator heterostructure, Nat. Mater. 13, 699 (2014).
• [3] Y. Wang, P. Deorani, K. Banerjee, N. Koirala, M. Brahlek, S. Oh, and H. Yang, Topological surface states originated spin-orbit torques in Bi₂Se₃, Phys. Rev. Lett. 114, 257202 (2015).
• [4] P. Deorani, J. Son, K. Banerjee, N. Koirala, M. Brahlek, S. Oh, and H. Yang, Observation of inverse spin Hall effect in bismuth selenide, Phys. Rev. B 90, 094403 (2014).