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).