Microscopic evidence of enhanced ferromagnetism in Cr/V doped (Bi,Sb)2Te3 thin films

Quantum anomalous Hall effect (QAHE) was experimentally realized in a ferromagnetic topological insulator Cr-doped (Bi,Sb)$_2$Te$_3$ (BST) thin film for the first time[1]. Recently, a more robust QAHE has been observed in V-doped BST thin films, which has a much larger coercive field and higher Curie temperature at the same doping level[2]. However, the quantization of Hall resistance in V-doped BST still occurs at extremely low temperature (< 150 mK). In order to further enhance the QAHE temperature, we co-doped BST thin films by both Cr and V. The co-doped samples show enhanced QAHE temperature, comparing to the end members. Our magnetic force microscopy (MFM) results reveal an improved ferromagnetism in the co-doped samples. This is consistent with the in-situ Hall measurements, which show a steeper magnetization reversal with enhanced Hall resistance. [1] C.-Z. Chang et al., Science 340, 167 (2013). [2] C.-Z. Chang et al., Nature Materials 14, 473–477(2015).