Visualizing ferromagnetic domain behavior of magnetic topological insulator thin films

A systematic magnetic force microscopy (MFM) study of domain behavior in thin films of the magnetic topological insulator Sb_1.89V_0.11Te₃ reveals that in the virgin domain state, after zero-field cooling, an equal population of up and down domains occurs. Interestingly, the cooling field dependence of MFM images demonstrates that a small cooling magnetic field (approximately 5-10 Oe) is sufficient to significantly polarize the film despite the coercive field (H_C) for these films being on the order of a tesla at low temperature. By visualizing the magnetization reversal process around H_C of V-doped Sb₂Te₃, we observed a typical domain behavior of a ferromagnet, i.e., domain nucleation and domain wall propagation. Our results provide direct evidence of ferromagnetic behavior of the magnetic topological insulator, a necessary condition for a robust quantum anomalous Hall effect.