  Interlayer exchange coupling (IEC) between two magnetic layers sandwiched by
a nonmagnetic spacer layer plays a critical role in shaping the magnetic
properties of such heterostructures. The quantum anomalous Hall (QAH) effect
has been realized in a structure composed of two magnetically doped topological
insulator (TI) layers separated by an undoped TI layer. The quantized Hall
conductance observed in this sandwich heterostructure originates from the
combined contribution of the top and bottom surface states. In this work, we
employ molecular beam epitaxy to synthesize a series of magnetic TI sandwiches
with varying thicknesses of the middle undoped TI layer. The well-quantized QAH
effect is observed in all these samples and its critical behavior is modulated
by the IEC between the top and bottom magnetic TI layers. Near the plateau
phase transition (PPT), we find that thinner QAH samples exhibit a
two-dimensional critical metal behavior with nearly temperature-independent
longitudinal resistance, whereas thicker QAH samples behave as a
three-dimensional insulator with reduced longitudinal resistance at higher
temperatures. The IEC-induced critical-metal-to-insulator transition in the QAH
PPT regime can be understood through a two-channel Chalker-Coddington network
model by tuning inter-channel tunneling. The agreement between experiment and
theory strongly supports the QAH PPT within the Kosterlitz-Thouless framework,
where the critical metal and disordered insulator phases exist in bound and
unbound states of vortex-antivortex pairs, respectively.
