Fe-Intercalation Dominated Ferromagnetism of van der Waals Fe GeTe.

Fe GeTe have proven to be of greatly intrigue. However, the underlying mechanism behind the varying Curie temperature (T ) values remains a puzzle. This study explores the atomic structure of Fe GeTe crystals exhibiting T 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 (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 (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 . This discovery of the Fe-intercalation layer elucidates the mechanism behind the hidden antiferromagnetic ordering that underlies the enhancement of T in Fe GeTe .