Significantly enhanced interlayer ferromagnetic coupling in van der Waals Fe3GeTe2 bilayer by Be-ion intercalation

Two-dimensional (2D) van der Waals (vdW) ferromagnetic (FM) materials have recently received attention due to their potential applications in next-generation spintronic devices. However, the reduced dimensionality and weak interlayer vdW interaction seriously suppress the magnetic Curie temperature Tc, raising the concern with enhancing the interlayer FM coupling. It is argued that proper ion intercalation may enhance the interlayer coupling by establishing strong chemical bonding. In this work, this issue in a 2D vdW FM Fe3GeTe2 (FGT) bilayer as an example is addressed, and our first-principles calculations predict that beryllium (Be) can be a promising intercalant for such enhancement. It is revealed that the Be-ion migration in-between the vdW gap has only moderate energy barriers owing to its small ionic radius, suggesting the feasibility of reversible intercalation. Particularly, Be-ion intercalation can significantly enhance the interlayer FM coupling by reducing the interlayer distance. The strong bonding that pulls two FGT monolayers closer is ascribed to orbital hybridization between Be-ions and interfacial Te-FeI sites. Be-ion intercalation also contributes to electron doping via charge transfer, favoring the enhanced intralayer FM coupling. This work suggests an alternative scheme for reversibly controlled ferromagnetism enhancement in 2D vdW ferromagnets using ion intercalation.