Optical Kerr effect and Terahertz spectroscopy in an intrinsic ferromagnetic topological insulator*

Incorporating magnetism into topology has brought exotic properties due to time-reversal symmetry breaking. In the case of magnetic topological insulators, the exchange gap emerges at the surface and induces quantum anomalous Hall effect (QAHE). The antiferromagnetic material MnBi₂Te₄ is the first intrinsic magnetic TI confirmed by experiments. However, the van der Waals compound shows a distinct dependence on the even-odd layers and the QAHE is only realized in the uncompensated odd layers. It remains elusive about an intrinsic ferromagnetic TI. In this work, we report a newly discovered ferromagnetic topological insulator, Cr(Bi,Sb)₆Te₁₀ (CBST). The large magneto-optical Kerr effect determines the ferromagnetic order and a high Curie temperature. It also shows a large terahertz Faraday and Kerr rotation that preserve below the transition temperature. Our work shows that CBST is a good platform to search for QAHE at high temperature.