  Monolayer 1T-VSe2 has been reported as a room-temperature ferromagnet. In
this work, by using first-principles calculations, we unveil that the
ferromagnetism in monolayer 1T-VSe2 is originated from its intrinsic huge
Stoner instability enhanced by the confinement effect, which can eliminate the
interlayer coupling, and lead to a drastic increase of the density of states at
the Fermi level due to the presence of Van Hove singularity. Our calculations
also demonstrate that the Stoner instability is very sensitive to the
interlayer distance. These results provide a useful route to modulate the
nonmagnetic to ferromagnetic transition in few-layers or bulk 1T-VSe2, which
also shed light on the enhancement of its Curie temperature by enlarging the
interlayer distance.
