  The recent advent of topological states of matter spawned many significant
discoveries. The quantum anomalous Hall effect[1-3] is a prime example due to
its potential for applications in quantum metrology[4, 5] as well as its
influence on fundamental research into the underlying topological and magnetic
states[6-11] and axion electrodynamics[2, 12-14]. Here, we perform electronic
transport studies on a (V,Bi,Sb)2Te3 ferromagnetic topological insulator
nanostructure in the quantum anomalous Hall regime. This allows us access to
the dynamics of an individual ferromagnetic domain. The volume of the domain is
estimated to be about 85 000 nm3, containing some 50 000 vanadium atoms, spread
over a macroscopic distance of 115 nm. Telegraph noise resulting from the
magnetization fluctuations of this domain is observed in the Hall signal.
Careful analysis of the influence of temperature and external magnetic field on
the domain switching statistics provides evidence for quantum tunneling of
magnetization[15-22] in a macrospin state. This ferromagnetic macrospin is not
only the largest magnetic object in which quantum tunneling has been observed,
but also the first observation of the effect in a topological state of matter.
