  The phase transitions from one plateau to the next plateau or to an insulator
in quantum Hall and quantum anomalous Hall (QAH) systems have revealed
universal scaling behaviors. A magnetic-field-driven quantum phase transition
from a QAH insulator to an axion insulator was recently demonstrated in
magnetic topological insulator sandwich samples. Here, we show that the
temperature dependence of the derivative of the longitudinal resistance on
magnetic field at the transition point follows a characteristic power-law that
indicates a universal scaling behavior for the QAH to axion insulator phase
transition. Similar to the quantum Hall plateau to plateau transition, the QAH
to axion insulator transition can also be understood by the Chalker-Coddington
network model. We extract a critical exponent k~ 0.38 in agreement with recent
high-precision numerical results on the correlation length exponent of the
Chalker-Coddington model at v ~ 2.6, rather than the generally-accepted value
of 2.33.
