  Two-dimensional van der Waals (vdW) heterostructures are an attractive
platform for studying exchange bias due to their defect free and atomically
flat interfaces. Chromium thiophosphate (CrPS4), an antiferromagnetic material,
possesses uncompensated magnetic spins in a single layer, rendering it a
promising candidate for exploring exchange bias phenomena. Recent findings have
highlighted that naturally oxidized vdW ferromagnetic Fe3GeTe2 exhibits
exchange bias, attributed to the antiferromagnetic coupling of its ultrathin
surface oxide layer (O-FGT) with the underlying unoxidized Fe3GeTe2. Anomalous
Hall measurements are employed to scrutinize the exchange bias within the
CrPS4/(O-FGT)/Fe3GeTe2 heterostructure. This analysis takes into account the
contributions from both the perfectly uncompensated interfacial CrPS4 layer and
the interfacial oxide layer. Intriguingly, a distinct and non-monotonic
exchange bias trend is observed as a function of temperature below 140 K. The
occurrence of exchange bias induced by a 'pre-set field' implies that the
prevailing phase in the polycrystalline surface oxide is ferrimagnetic Fe3O4.
Moreover, the exchange bias induced by the ferrimagnetic Fe3O4 is significantly
modulated by the presence of the van der Waals antiferromagnetic CrPS4 layer,
forming a heterostructure, along with additional iron oxide phases within the
oxide layer. These findings underscore the intricate and complex nature of
exchange bias in van der Waals heterostructures, highlighting their potential
for tailored manipulation and control.
