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