Harnessing Van der Waals CrPS and Surface Oxides for Nonmonotonic Preset Field Induced Exchange Bias in FeGeTe.

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 (CrPS), 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 FeGeTe exhibits exchange bias, attributed to the antiferromagnetic coupling of its ultrathin surface oxide layer (O-FGT) with the underlying unoxidized FeGeTe. Anomalous Hall measurements are employed to scrutinize the exchange bias within the CrPS/(O-FGT)/FeGeTe heterostructure. This analysis takes into account the contributions from both the perfectly uncompensated interfacial CrPS 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 FeO. Moreover, the exchange bias induced by the ferrimagnetic FeO is significantly modulated by the presence of the van der Waals antiferromagnetic CrPS 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.