Harnessing Van der Waals CrPS₄ and Surface Oxides for Nonmonotonic Preset Field Induced Exchange Bias in Fe₃GeTe₂

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 nonmonotonic exchange bias trend is observed as a function of temperature below 140 K. The occurrence of exchange bias induced by a "preset 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.