Proximal Anomalous Hall Effect in Ferromagnetic Insulator/Bulk-insulating BiSbTeSe₂ Heterostructures

Realization of Quantized Anomalous Hall Effect in topological insulators (TIs) thus far has been limited to ultra-thin MBE films of Cr- or V-doped (Bi,Sb)₂Te₃. In this system the disorder of the dopant landscape may limit the quantization temperature range in which the chiral channels can be observed. The alternative is to obtain proximal ferromagnetism in a 3D TI by creating a trilayer sandwich comprising ferromagnetic insulator (FMI) on the top and bottom surfaces of a TI. Here we report on the trilayer structure comprising bulk-insulating TI BiSbTeSe₂ (BSTS) sandwiched between a two-dimensional van der Waals ferromagnetic insulator Cr₂Ge₂Te₆ (CGT) grown using Bridgeman method. The structure was assembled in the inert-gas glovebox to prevent oxidation and contamination of the interface. h-BN capping was used to protect CGT against degradation in air. The out-of-plane magnetization of CGT down to thicknesses of 18 nm was confirmed by exfoliating it directly atop our custom-designed Hall microsensor chips. The enhancement of proximal magnetism obtained by (1) the optimal thickness of BSTS, (2) using pressure to obtain a better interfacial coupling, and (3) by electrostatic gating to the Dirac gap will be discussed.