Observation of ferromagnetic domains in magnetic topological insulator

The breaking of time reversal symmetries in a topological insulator can lead to exotic quantum effects, such as magnetic monopoles, quantum anomalous hall effect(QAHE), and so on. Recently QAHE has been experimentally observed in a ferromagnetic topological insulator Cr doped Bi$_x$Sb$_{2-x}$Te$_3$[1]. Although a lot of studies have conducted on characterizing the ferromagnetic properties in these fascinating materials, direct observation of ferromagnetic domains is still lacking. Here we report variable temperature magnetic force microscopy (VT-MFM) studies on thin film of Cr doped Bi$_x$Sb$_{2-x}$Te$_3$ synthesized by molecular beam epitaxy (MBE). Small bubble-like ferromagnetic domains were observed below T$_C\approx$ 30 K. The evolution of these domains at various temperatures and magnetic fields will be presented.\\[4pt] [1] C.-Z. Chang et al., Science 340, 167 (2013).