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Intrinsic magnetic topological insulators offer low disorder and largemagnetic bandgaps for robust magnetic topological phases operating at highertemperatures. By controlling the layer thickness, emergent phenomena such asthe Quantum Anomalous Hall (QAH) effect and axion insulator phases have beenrealised. These observations occur at temperatures significantly lower than theNeel temperature of bulk MnBi2Te4, and measurement of the magnetic energy gapat the Dirac point in ultra-thin MnBi2Te4 has yet to be achieved. Critical toachieving the promise of this system is a direct measurement of thelayer-dependent energy gap and verifying whether the gap is magnetic in the QAHphase. Here we utilise temperature dependent angle-resolved photoemissionspectroscopy to study epitaxial ultra-thin MnBi2Te4. We directly observe alayer dependent crossover from a 2D ferromagnetic insulator with a bandgapgreater than 780 meV in one septuple layer (1 SL) to a QAH insulator with alarge energy gap (>100 meV) at 8 K in 3 and 5 SL MnBi2Te4. The QAH gap isconfirmed to be magnetic in origin, as it abruptly diminishes with increasingtemperature above 8 K. The direct observation of a large magnetic energy gap inthe QAH phase of few-SL MnBi2Te4 is promising for further increasing theoperating temperature of QAH materials.

Crossover from 2D ferromagnetic insulator to wide bandgap quantum anomalous Hall insulator in ultra-thin MnBi2Te4