Exchange bias effect in thin Fe3GeTe2 and Fe5GeTe2 flakes sensitive to oxidation and annealing methods

The exchange bias (EB) effect—an asymmetric shift in the ferromagnetic (FM) hysteresis loop arising from the coupling between bulk FM and surface antiferromagnetic (AFM) spin alignments in FM materials—has attracted considerable attention. Owing to their weak interlayer FM spin-coupling and facile surface oxidation that promotes AFM spin formation, thin van der Waals (vdW) magnetic materials provide an ideal platform for investigating the EB effect. In this study, we fabricate thin vdW magnetic flakes of Fe3GeTe2 (F3GT) and Fe5GeTe2 (F5GT) and subject them to both natural and intentional oxidation under ambient air. It is found that intentionally oxidized F3GT flakes exhibit an enhanced EB effect, approximately 2.5 times larger than that of naturally oxidized flakes, owing to homogeneous surface oxidation. In contrast, F5GT flakes display an AFM spin phase with the negligible EB effect in both naturally and intentionally oxidized samples. However, high-temperature annealing of the intentionally oxidized F5GT flakes under vacuum eliminates the AFM phase, leading to the emergence of diamagnetism superimposed with a pronounced EB-like effect, which is approximately 1.5–2.5 times greater than that observed in intentionally oxidized F3GT. These findings demonstrate innovative strategies for controlling and inducing the EB effect, paving the way for its potential spintronic applications.