Hybridization-induced interface states in a topological-insulator–ferromagnetic-metal heterostructure

Abstract

Recent experiments demonstrating large spin-transfer torques in topological insulator (TI)-ferromagnetic metal (FM) bilayers have generated a great deal of excitement due to their potential applications in spintronics. The source of the observed spin-transfer torque, however, remains unclear. This is because the large charge transfer from the FM to TI layer would prevent the Dirac cone at the interface from being anywhere near the Fermi level to contribute to the observed spintransfer torque. Moreover, there is yet little understanding of the impact on the Dirac cone at the interface from the metallic bands overlapping in energy and momentum, where strong hybridization could take place. Here, we build a simple microscopic model and perform first-principles-based simulations for such a TI-FM heterostructure, considering the strong hybridization and charge transfer effects. We find that the original Dirac cone is destroyed by the hybridization as expected. Instead, we find a new interface state which we dub 'descendent state' to form near the Fermi level due to the strong hybridization with the FM states at the same momentum. Such a 'descendent state' carries a sizable weight of the original Dirac interface state, and thus inherits the localization at the interface and the same Rashba-type spin-momentum locking. We propose that the 'descendent state' may be an important source of the experimentally observed large spin-transfer torque in the TI-FM heterostructure.

Magnetization switching using topological surface states

“…3 (D and E) and 4A represent the first unambiguous demonstration of TSS-driven magnetization switching. As mentioned above, the SOT switching in recent studies on TI/FM systems ( 6 – 12 ) may not be TSS-driven because interfacing a TI layer with a conductive FM film can significantly modify or even completely suppress the TSSs in the TI layer ( 14 – 18 ). In contrast, the BaFe 12 O 19 layer in this work is an MI, which is expected to produce negligible effects on the TSSs ( 16 ).…”

Proximity effect in a superconductor–topological insulator heterostructure based on first principles

“…2(a)-2(f) with Fig. 1(b) [40,41]. For thin films (<5 QLs) we also observe an energy gap in the vicinity of the two Dirac points which decreases with increasing TI overlayer thickness.…”

Large Damping Enhancement in Dirac‐Semimetal–Ferromagnetic‐Metal Layered Structures Caused by Topological Surface States

“…[ 42–45 ] 2)Previous work has shown that interfacing a TI with a ferromagnet can damage or suppress the TSS in the TI. [ 10,35–38 ] The comparison between the damping of the InSb/α‐Sn/Ag/NiFe and InSb/α‐Sn/NiFe samples in Figure 4 evidently suggests that this is also the case for the TSS in the TDS α‐Sn. 3)TDSs should also host Fermi arc surface states; [ 46,47 ] in the case of TDS α‐Sn, such states are expected to exist on surfaces parallel to the uniaxial tensile axis, [ 16 ] namely, the four side surfaces of the α‐Sn film in this work. Future studies on spin pumping effects associated with these surface states are very interesting. 4)This work suggests that SOC in the bulk of α‐Sn and β‐Sn films is no stronger than that in Ag.…”

“…[ 10 ] Previous experimental and theoretical works have shown that the magnetism in a ferromagnetic metal can significantly modify and even completely suppress TSS in a neighboring TI or α‐Sn thin film. [ 10,35–38 ]…”