  A striking prediction in topological insulators is the appearance of the
quantized Hall resistance when the surface states are magnetized. The surface
Dirac states become gapped everywhere on the surface, but chiral edge states
remain on the edges. In an applied current, the edge states produce a quantized
Hall resistance that equals the Chern number C = +1 or -1 (in natural units),
even in zero magnetic field. This quantum anomalous Hall effect was observed by
Chang et al. With reversal of the magnetic field, the system is trapped in a
metastable state because of magnetic anisotropy. Here we investigate how the
system escapes the metastable state at low temperatures (10-200 mK). When the
dissipation (measured by the longitudinal resistance) is ultralow, we find that
the system escapes by making a few, very rapid transitions, as detected by
large jumps in the Hall and longitudinal resistances. Using the field at which
the initial jump occurs to estimate the escape rate, we find that raising the
temperature strongly suppresses the rate. From a detailed map of the resistance
vs. gate voltage and temperature, we show that dissipation strongly affects the
escape rate. We compare the observations with dissipative quantum tunneling
predictions. In the ultralow dissipation regime, there exist two temperature
scales T1 = 70 mK and T2 = 145 mK between which jumps can be observed. The
jumps display a spatial correlation that extends over a large fraction of the
sample.
