Quantum anomalous Hall (QAH) systems are of great fundamental interest and of potential application (e.g. quantum computing) because of dissipationless conduction without external magnetic field.1–3 The QAH effect has been realized in magnetically doped topological insulator (TI) thin films.4–7 However, full quantization requires extremely low temperature (<50 mK) in the initial works, though it was significantly improved with either modulation doping or co-doping of magnetic elements.8,9 Improved ferromagnetism was indicated in the Cr and V coped TI thin films, yet a direct evidence of long-range ferromagnetic order is lacking. In this talk, I will present direct evidence of long-range ferromagnetic order in thin films of Cr and V co-doped (Bi,Sb)2Te3 using low-temperature magnetic force microscopy (MFM) with in-situ transport. The magnetization reversal process reveals a typical ferromagnetic domain behavior, i.e., domain nucleation and domain wall propagation, in contrast to much weaker magnetic signals observed in the end members, possibly due to superparamagnetic behavior observed in Cr doped TI films.10,11 If time allows, preliminary results of spectroscopy imaging of Cr and V defects in Sb2Te3 will also be presented. The observed long-range ferromagnetic order resolves one of the major challenges in QAH systems, and paves the way to high-temperature dissipationless conduction and exotic phenomena such as Axion magnetoelectric effect12.
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