Interfacial Oxide Modulated unique Exchange Bias in CrPS4/Fe3GeTe2 van der Waals heterostructures

Two-dimensional van der Waals heterostructures are an attractive platform for studying exchange bias due to their defect free and atomically flat interfaces. Chromium thiophosphate (CrPS4), an antiferromagnet, has uncompensated magnetic spins in a single layer that make it an excellent candidate for studying exchange bias. In this study, we examined the exchange bias in CrPS4/Fe3GeTe2 van der Waals heterostructures using anomalous Hall measurements. Our results show that the exchange bias strength is robust for clean interfaces, with a hysteresis loop shift of about 55 mT at 5 K for few-layer Fe3GeTe2, which is larger than that obtained in most van der Waals AFM/FM heterostructures. However, when exposed to air, the ferromagnetic Fe3GeTe2 layer develops a thin surface oxide layer that significantly modulates the exchange bias. Remarkably, this surface oxide layer can induce exchange bias without any field-cooling, but merely by applying a 'preset field' from 5 K up to 140K due to the presence of ferrimagnetic magnetite. We also observed exchange bias beyond the N\'eel temperature of CrPS4, with two local minima due to contributions from CrPS4 and surface oxides. These results illustrate the complex behaviour of exchange bias in van der Waals heterostructures and its potential for tailored control.