  Topological surface states with intrinsic magnetic ordering in the
MnBi$_2$Te$_4$(Bi$_2$Te$_3$)$_n$ compounds have been predicted to host rich
topological phenomena including quantized anomalous Hall effect and axion
insulator state. Here we use scanning tunneling microscopy to image the surface
Dirac fermions in MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$. We have determined the
energy dispersion and helical spin texture of the surface states through
quasiparticle interference patterns far above Dirac energy, which confirms its
topological nature. Approaching the Dirac point, the native defects in the
MnBi$_2$Te$_4$ septuple layer give rise to resonance states which extend
spatially and potentially hinder the detection of a mass gap in the spectra.
Our results demonstrate that regulating defects is essential to realize exotic
topological states at higher temperatures in these compounds.
