  Two-dimensional (2D) magnetic transition metal compounds with atomic
thickness exhibit intriguing physics in fundamental research and great
potential for device applications. Understanding the correlations between their
macrosopic magnetic properties and the dimensionality of microscopic magnetic
exchange interactions are valuable for the designing and applications of 2D
magnetic crystals. Here, using spin-polarized scanning tunneling microscopy,
magnetization and magneto-transport measurements, we identify the
zigzag-antiferromagnetism in monolayer CrTe2, incipient ferromagnetism in
bilayer CrTe2, and robust ferromagnetism in bilayer Cr3Te4 films. Our density
functional theory calculations unravel that the magnetic ordering in ultrathin
CrTe2 is sensitive to the lattice parameters, while robust ferromagnetism with
large perpendicular magnetic anisotropy in Cr3Te4 is stabilized through its
anisotropic 3D magnetic exchange interactions.
