“Spin transport and magnetic dynamics in the topological insulator-magnetic insulator heterostructures”

J. Raynien Kwo^1*, Y. T. Fanchiang², M. Hong², S. F. Lee³

¹physics, National Tsing Hua University, Hsinchu, Taiwan
²physics, National Taiwan University, Taipei, Taiwan
³physics, Academia Sinica, Nankang, Taiwan

* Presenter:J. Raynien Kwo, email:raynien@phys.nthu.edu.tw

Emergent quantum materials such as topological insulators (TIs) host topologically protected surface states (TSS) featured spin–momentum locking, implying dissipationless spin transport and efficient spin-charge conversion for spintronic applications. The exchange coupling at the topological insulator (TI) and ferromagnetic insulator (FI) interface is fundamental to investigate spin transport properties of TIs and interfacial ferromagnetism via magnetic proximity effect (MPE). The ferromagnetic resonance (FMR) study undertaken on Bi₂Se₃ thin films on yttrium iron garnet (YIG) showed strong interfacial exchange coupling, manifested as a large interfacial in-plane magnetic anisotropy (IMA) at room temperature.* We find that the Bi₂Se₃ thickness dependence of the interfacial IMA and damping enhancement are correlated with the evolution of the Bi₂Se₃ surface band structure, suggesting TSS-originated exchange coupling. In the meantime, Dirac energy gap arising from MPE at the TI/FI interface and its transport signatures remains elusive. With comprehensive magneto-transport measurements, we showed that Bi₂Se₃/thulium iron garnet heterostructures have fulfilled the criteria of realizing quantum anomalous Hall effect (QAHE) via MPE; namely: simultaneous presence of the exchange Dirac gap and long-range ferromagnetic order, manifested as the weak localization (WL) and square anomalous Hall resistance loops in a TI/FI system. We report the first observation of a genuine weak-localization (WL) effect due to the exchange Dirac gap. Hence our efforts have led to significant and broad impacts both in fundamental study and future spintronic application.

*Y. T. Fanchiang et al., Nat. Commun. 9, 223 (2018).

Keywords: topological insulator, magnetic proximity effect, magnetic anisotropy, weak localization, spintronics