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 1.89 V 0.11 Te 3 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 Sb 2 Te 3 , 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.