  The study of the mechanism of exchange bias phenomena and the achievement of
its efficient control are of great importance, as it promotes the revelation of
unique exchange interactions and the development of exotic applications.
However, it is challenging due to the elusive interface between magnetic
phases. In this study, we report an unprecedented exchange bias phenomenon
observed in ultrathin uncompensated antiferromagnetic MnBi$_2$Te$_4$. The
magnitude and direction of the exchange field can be intentionally controlled
by designing a magnetic field sweep protocol without a field cooling process.
The combined experimental and theoretical simulation results indicate that the
spin-flip process assisted by the ubiquitous defect-induced pinning domain
sites with varying inner exchange interactions might give rise to the emergence
and robustness of this peculiar exchange bias. The temperature and thickness
dependence of the exchange bias phenomena are systematically investigated for
further study and exploitation of its unique properties. This mechanism hold
promise for highly tunable exchange bias in prevalent magnetic systems by
engineering the properties of domain structures, and also offers promising
avenues for the design of spintronic devices combing its topology based on
MnBi$_2$Te$_4$.
