Evolution of the carrier concentration and its effects on the Currie Temperature in ultra-thin Fe₃GeTe₂ films.

Recently, Fe₃GeTe₂ (FGT) has generated much interest due to its

2-dimensional itinerant magnetism and correlated behavior. It is well

documented in experiment that the Curie temperature (T_C) of FGT can be

tuned with both layer number and carrier concentration, yet the

theoretical origin of these observations remains unclear. In this work,

we explore the relationship between carrier concentration and T_C for 1

to 10 QL films grown via molecular beam epitaxy (MBE). Samples are

structurally and stoichiometrically characterized through RHEED

diffraction analysis where stoichiometries are explored though

additional Fe and Ge depositions. Further structural parameters are

supported by CTR measurements where the observed lattice parameters

are in good agreement with those of stoichiometric FGT. However, in

transport experiments, we observe the carrier concentration increases

with decreasing temperature and is constant between all layer numbers

at TC. This suggests the carriers exhibit correlated effects which may

be responsible for the observed magnetic phase changes: The

ferromagnetic state requires a minimum carrier concentration which

vanishes with increasing temperature. This could also explain why

increasing carrier density leads to an increase of T_C, instigating

further theoretical and experimental studies of the relationship

between carrier density, layer number, and T_C.