  The role of self-intercalation in 2D van der Waals materials is key to the
understanding of many of their properties. Here we show that the magnetic
ordering temperature of thin films of the 2D ferromagnet Fe5GeTe2 is
substantially increased by self-intercalated Fe that resides in the van der
Waals gaps. The epitaxial films were prepared by molecular beam epitaxy and
their magnetic properties explored by element-specific x-ray magnetic circular
dichroism that showed ferromagnetic ordering up to 375 K. Both surface and bulk
sensitive x-ray absorption modes were used to confirm that the magnetic signal
is of an intrinsic nature. Fe occupation within the van der Waals gaps was
determined by high-precision x-ray diffraction analysis which showed a notably
higher Fe occupation than in bulk crystals. We thus infer that the higher
magnetic ordering temperature results from an increased exchange interaction
among the Fe5GeTe2 layers mediated by iron within the gaps between these
layers. Our findings establish self-intercalation during epitaxial growth as an
efficient mechanism to achieve high-temperature magnetism in a broad class of
van der Waals materials.
