Fe‐Intercalation Dominated Ferromagnetism of van der Waals Fe₃GeTe₂

Fe₃ GeTe₂ 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 Fe₃ GeTe₂ 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 Fe₃ GeTe₂ .