Observation of Superparamagnetism in Coexistence with Quantum Anomalous Hall C=±1 and C=0 Chern States*

Simultaneous transport and scanning nanoSQUID-on-tip magnetic imaging studies in Cr-(Bi,Sb)₂Te₃ modulation-doped films reveal the presence of superparamagnetic (SPM) order within the quantum anomalous Hall (QAH) regime. SPM dynamics of weakly interacting nanoscale magnetic islands is observed both in the transition regions as well as within the fully quantized C=±1 Chern plateaus. Modulation doping of the topological insulator films is found to give rise to significantly larger SPM islands as compared to uniform magnetic doping, evidently leading to enhanced robustness of the QAH effect. Even in this more robust quantum state, attaining full quantization of transport coefficients requires magnetic alignment of at least 95% of the SPM islands.
The SPM order is also found within the incipient C=0 zero Hall plateau, which may host an axion state if top and bottom layers are magnetized in opposite directions. In this regime, however, a lower level of island alignment is found, hindering the formation of the axion state. Comprehension and control of SPM dynamics is thus a key factor in apprehending the fragility of the QAH state.
[arXiv:1710.06446]