  MnBi$_2$Te$_4$ (MBT) materials are promising antiferromagnetic topological
insulators where field driven ferromagnetism is predicted to cause a transition
between axion insulator and Weyl semimetallic states. However, the presence of
antiferromagnetic coupling between Mn/Bi antisite defects and the main Mn layer
can reduce the low-field magnetization, and it has been shown that such defects
are more prevalent in the structurally identical trivial magnetic insulator
MnSb$_2$Te$_4$ (MST). We use high-field magnetization measurements to show that
the magnetization of MBT and MST occur in stages and full saturation requires
fields of~$\sim$~60 Tesla. As a consequence, the low-field magnetization
plateau state in MBT, where many determinations of quantum anomalous Hall state
are studied, actually consists of ferrimagnetic septuple blocks containing both
a uniform and staggered magnetization component.
