  Magnetization reversal in ferro- and ferrimagnets is a well-known archetype
of non-equilibrium processes, where the volume fractions of the oppositely
magnetized domains vary and perfectly compensate each other at the coercive
magnetic field. Here, we report on a fundamentally new pathway for
magnetization reversal that is mediated by an antiferromagnetic state.
Consequently, an atomic-scale compensation of the magnetization is realized at
the coercive field, instead of the mesoscopic or macroscopic domain
cancellation in canonical reversal processes. We demonstrate this unusual
magnetization reversal on the Zn-doped polar magnet Fe$_2$Mo$_3$O$_8$. Hidden
behind the conventional ferrimagnetic hysteresis loop, the surprising emergence
of the antiferromagnetic phase at the coercive fields is disclosed by a sharp
peak in the field-dependence of the electric polarization. In addition, at the
magnetization reversal our THz spectroscopy studies reveal the reappearance of
the magnon mode that is only present in the pristine antiferromagnetic state.
According to our microscopic calculations, this unusual process is governed by
the dominant intralayer coupling, strong easy-axis anisotropy and spin
fluctuations, which result in a complex interplay between the ferrimagnetic and
antiferromagnetic phases. Such antiferro-state-mediated reversal processes
offer novel concepts for magnetization control, and may also emerge for other
ferroic orders.
