Photoemission study of topological insulator grown on ferromagnetic insulator heterostructures

Sheng-Wen Huang^1*, Keng-Yung Lin², Chun-Chia Chen¹, Meng-Xin Guo¹, Chao-Kai Cheng², Cheng-Maw Cheng³, Minghwei Hong², J. Raynien Kwo¹

¹Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
²Grad. Inst. of Appl. Phys. and Dept. of Phys., National Taiwan University, Taipei, Taiwan
³National Synchrotron Radiation Research Center, Hsinchu, Taiwan

* Presenter:Sheng-Wen Huang, email:wendy3407@gmail.com

Since the theoretical prediction of topological insulators (TI), there have been intense interest worldwide in identifying new topological systems and studying the Dirac fermion physics of topological surface states. Breaking the time-reversal symmetry in TIs leads to numerous exotic phenomena such as quantum anomalous Hall effect (QAHE) and neutral Majorana fermions. A central cause responsible for these novel phenomena is the existence of an energy gap opening at Dirac surface states induced by exchange interaction with magnetic elements. For example, magnetically doped TI was firstly reported to find the exchange gap opening, but the observation temperature of QAHE was less than 2 K because of the disorder created by magnetic dopants. Another way to break the time-reversal symmetry is through the interfacial magnetic proximity effect (MPE) in TI/ferromagnetic insulator (FI) heterostructures with a higher observation temperature and a uniform interfacial magnetization. Previously, Bi₂Se₃/EuS bilayer has been reported to observe a room-temperature ferromagnetism in TI induced by MPE.
In this work, TI Bi₂Se₃ thin films were grown by molecular beam epitaxy (MBE) on sputtered growth thulium iron garnet (TmIG) films on gadolinium gallium garnet (GGG) substrates. The TmIG films are ferromagnetic insulators with perpendicular magnetic anisotropy (PMA). The electronic band structures of Bi₂Se₃/TmIG bilayers were investigated by in-situ angle-resolved photoemission spectroscopy (ARPES). Because the limit of the probing depth of the He I photon energy, only the top surface states of Bi₂Se₃ thin films could be observed. Thickness dependence study in Bi₂Se₃ suggests that the interfacial exchange coupling length of Bi₂Se₃/TmIG is less than the topological thickness limit of Bi₂Se₃. Furthermore, the energy gap opening at Dirac surface states of 2QL Bi₂Se₃/TmIG/GGG is larger than that of 2QL Bi₂Se₃/sapphire(0001) by about 24 meV. It might be induced by hybridization of the upper and lower surface states, but could also be due to the MPE, thus requiring further investigations underway. In our Bi₂Se₃/TmIG heterostructures, MPE induced exchange gap was directly investigated by the photoemission study, and may provide an opportunity to understand the zero magnetic field spintronics and to develop its applications.

Keywords: Topological Insulator, Ferromagnetic Insulator, Angle-Resolved Photoemission Spectroscopy