  Heterostructures of two-dimensional (2D) van der Waals (vdW) magnets and
topological insulators (TI) are of substantial interest as candidate materials
for efficient spin-torque switching, quantum anomalous Hall effect, and chiral
spin textures. However, since many of the vdW magnets have Curie temperatures
below room temperature, we want to understand how materials can be modified to
stabilize their magnetic ordering to higher temperatures. In this work, we
utilize molecular beam epitaxy to systematically tune the Curie temperature
($T_C$) in thin film Fe$_3$GeTe$_2$/Bi$_2$Te$_3$ from bulk-like values
($\sim$220 K) to above room temperature by increasing the growth temperature
from 300 $^\circ$C to 375 $^\circ$C. For samples grown at 375 $^\circ$C,
cross-sectional scanning transmission electron microscopy (STEM) reveals the
spontaneous formation of different Fe$_m$Ge$_n$Te$_2$ compositions (e.g.
Fe$_5$Ge$_2$Te$_2$ and Fe$_7$Ge$_6$Te$_2$) as well as intercalation in the vdW
gaps, which are possible origins of the enhanced Curie temperature. This
observation paves the way for developing various Fe$_m$Ge$_n$Te$_2$/TI
heterostructures with novel properties.
