  Quantized Hall conductance is a generic feature of two dimensional electronic
systems with broken time reversal symmetry. In the quantum anomalous Hall state
recently discovered in magnetic topological insulators, time reversal symmetry
is believed to be broken by long-range ferromagnetic order, with quantized
resistance observed even at zero external magnetic field. Here, we use scanning
nanoSQUID magnetic imaging to provide a direct visualization of the dynamics of
the quantum phase transition between the two anomalous Hall plateaus in a
Cr-doped (Bi,Sb)$_2$Te$_3$ thin film. Contrary to naive expectations based upon
macroscopic magnetometry, our measurements reveal a superparamagnetic state
formed by weakly interacting magnetic domains with a characteristic size of few
tens of nanometers. The magnetic phase transition occurs through random
reversals of these local moments, which drive the electronic Hall plateau
transition. Surprisingly, we find that the electronic system can in turn drive
the dynamics of the magnetic system, revealing a subtle interplay between the
two coupled quantum phase transitions.
