  Room temperature two-dimensional (2D) ferromagnetism is highly desired in
practical spintronics applications. Recently, 1T phase CrTe2 (1T-CrTe2)
nanosheets with five and thicker layers have been successfully synthesized,
which all exhibit the properties of ferromagnetic (FM) metals with Curie
temperatures around 305 K. However, whether the ferromagnetism therein can be
maintained when continuously reducing the nanosheet's thickness to monolayer
limit remains unknown. Here, through first-principles calculations, we explore
the evolution of magnetic properties of 1 to 6 layers CrTe2 nanosheets and
several interesting points are found: First, unexpectedly, monolayer CrTe2
prefers a zigzag antiferromagnetic (AFM) state with its energy much lower than
that of FM state. Second, in 2 to 4 layers CrTe2, both the intralayer and
interlayer magnetic coupling are AFM. Last, when the number of layers is equal
to or greater than five, the intralayer and interlayer magnetic coupling become
FM. Theoretical analysis reveals that the in-plane lattice contraction of few
layer CrTe2 compared to bulk is the main factor producing intralayer AFM-FM
magnetic transition. At the same time, as long as the intralayer coupling gets
FM, the interlayer coupling will concomitantly switch from AFM to FM. Such
highly thickness dependent magnetism provides a new perspective to control the
magnetic properties of 2D materials.
