Noble-metal free spintronic system with proximity-enhanced ferromagnetic topological surface state of FeSi above room temperature

 A nonmagnetic insulator FeSi has been shown to host a two-dimensional ferromagnetic metal state at the surface [1]. Unlike the topological insulators, the electronic state of FeSi is characterized by a quantum geometric phase, so-called Zak phase. As a consequence of the nearly quantized Zak phase of bulk electronic state, surface polarization charges with high density of states emerge, leading to strong Rashba-type spin-orbit coupling (SOC) as well as good electrical conduction and high ferromagnetic-ordering temperature (T_C). Owing to the coexistence of ferromagnetism and strong SOC at the surface, the magnetization switching through spin-orbit torque has been realized with high efficiency. However, the operating temperature of such SOC-related properties is still limited to low temperatures below 200 K [1].

 In this study, we enhance T_C and the size of SOC by using proximity effects from wide bandgap insulators. We found that electrical-conduction and magnetic properties of the FeSi interface could be dramatically modulated by the chemical species of insulator material. This is attributed to the degree of hybridization at the interface between FeSi and the adjacent insulator. In this presentation, we will show the detailed SOC-performance of FeSi heterostructures and discuss the method to control topological surface state of Zak-phase origin from the viewpoint of first-principles calculations.

[1] Y. Ohtsuka, N. Kanazawa, M. Hirayama et al., Sci. Adv. 7, eabj0498 (2021).