Fe-Intercalation Dominated Ferromagnetism of van der Waals Fe₃GeTe₂

Fe3GeTe2 have proven to be of greatly intrigue. However, the underlying mechanism behind the varying Curie temperature (T-c) values remains a puzzle. This study explores the atomic structure of Fe3GeTe2 crystals exhibiting T-c values of 160, 210, and 230 K. The elemental mapping reveals a Fe-intercalation on the interstitial sites within the van der Waals gap of the high-T-c (210 and 230 K) samples, which are observed to have an exchange bias effect by electrical transport measurements, while Fe intercalation or the bias effect is absent in the low-T-c (160 K) samples. First-principles calculations further suggest that the Fe-intercalation layer may be responsible for the local antiferromagnetic coupling that gives rise to the exchange bias effect, and that the interlayer exchange paths greatly contribute to the enhancement of T-c. This discovery of the Fe-intercalation layer elucidates the mechanism behind the hidden antiferromagnetic ordering that underlies the enhancement of T-c in Fe3GeTe2.