  We use magnetic force microscopy (MFM) to study spatial uniformity of
magnetization of epitaxially grown MnBi2Te4 thin films. Compared to films which
exhibit no quantum anomalous Hall effect (QAH), films with QAH are observed to
have more spatial uniformity of magnetization with larger domain size. The
domain evolution upon magnetic field sweeping indicates that the magnetic
domains or the spatial nonuniformity of magnetization originates from the
strong pinning of the inherent sample inhomogeneity. A direct correlation
between the Hall resistivity and the domain size has been established by
analyzing a series of thin films with and without QAH. Our observation shows
that one has to suppress the spatial nonuniformity of magnetization to allow
the Hall resistivity to be quantized. The fact that a sizable longitudinal
resistivity remains even for the QAH sample suggests a quantized Hall insulator
scenario. Our work provides important insights to the understanding of the
quantization mechanism and the dissipation of the QAH state in MnBi2Te4 system.
