Fe₆GeTe₂ Precipitation in a 478 K vdW Ferromagnet (Fe,Ni)₅GeTe₂

Above room temperature ferromagnetism reported in the metallic van der Waals (vdW) materials Fe_NGeTe₂ (N=3-7) has opened the possibility for their application in spintronic vdW devices at technologically relevant temperature scales. Specifically in Fe₅GeTe₂ (T_C ~ 300 K), dilution of magnetic moments by nickel substitution was recently found to achieve a T_C of 478 K despite an apparent violation of Vegard's law. When T_C is maximized, two mesoscopic chemical domains occur, whose impact on the anomalous T_C enhancement is yet understood. We combine scanning tunneling spectroscopy, synchrotron x-ray diffraction, and angle-resolved photoemission spectroscopy (ARPES) to reveal the precipitation of Fe₆GeTe₂ structures and the associated inter-domain strain. With spin-resolved ARPES and first principles calculation, we find the magnetization primarily coming from local moments on two innermost iron layers. The Fe₆GeTe₂ precipitates are stabilized by the large energy cost of homogeneously distributing nickel dopants. We argue that the origin of the T_C enhancement lay in this new Fe₆GeTe₂ structure, offering insight into how to reach far above room-temperature vdW ferromagnetism.