Direct evidence of ferromagnetism in a quantum anomalous Hall system

Quantum anomalous Hall (QAH) systems are of great fundamental interest and potential application because of their dissipationless conduction without the need for an external magnetic field(1-9). The QAH effect has been realized in magnetically doped topological insulator thin films(10-14). However, full quantization requires extremely low temperature (T < 50 mK) in the earliest works, athough it has been significantly improved by modulation doping or co-doping of magnetic elements(15,16). Improved ferromagnetism has been shown in these thin films, yet direct evidence of long-range ferromagnetic order is lacking. Herein, we present direct visualization of long-range ferromagnetic order in thin films of Cr and V co-doped (Bi, Sb)(2)Te-3 using low-temperature magnetic force microscopy with in situ transport. The magnetization reversal process reveals typical ferromagnetic domain behaviour-that is, domain nucleation and possibly domain wall propagation-in contrast to much weaker magnetic signals observed in the endmembers, possibly due to superparamagnetic behaviour(17-19). The observed long-range ferromagnetic order resolves one of the major challenges in QAH systems, and paves the way towards high-temperature dissipationless conduction by exploring magnetic topological insulators.