Recently, the issue of ferromagnetism enhancement in two-dimensional (2D) van der Waals (vdW) layered magnetic systems has been highly concerned. It is believed that ion intercalation in vdW layered ferromagnets, targeting either enhanced interlayer spin exchanges or intralayer ones, can be an efficient scheme. In this work, by means of the first-principles calculations, we investigate the Li-ion intercalation between the two monolayers of the ferromagnetic (FM) vdW Fe3GeTe2 (FGT) bilayer and its impact on the ferromagnetism. It is revealed that the Li-ion intercalation provides hopping carriers between the two interfacial Te sublayers, beneficial for the enhancement of the interlayer FM coupling at a relatively low intercalation level. On the other hand, the Li-ion intercalation lifted Fermi level promotes the electron transfer from the minority spin channel to the majority one for the Fe-3d bands, favoring the stronger intralayer FM coupling. However, the over-intercalation generated carriers may fill up the majority spin channel, reversely leading to the reduced interlayer FM coupling. Consequently, an optimized intercalation level is expected in terms of ferromagnetism enhancement. This work not only helps to explain the recent experimental finding on the gate-controlled Li-ion intercalation in vdW FGT few-layers but also suggests a general scheme for ferromagnetism enhancement in 2D vdW layered ferromagnets using the ion intercalation approach.

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