  It has been long hoped that the realization of long-range ferromagnetic order
in two-dimensional (2D) van der Waals (vdW) crystals, combined with their rich
electronic and optical properties, would open up new possibilities for
magnetic, magnetoelectric and magneto-optic applications. However, in 2D
systems, the long-range magnetic order is strongly hampered by thermal
fluctuations which may be counteracted by magnetic anisotropy, according to the
Mermin-Wagner theorem. Prior efforts via defect and composition engineering,
and proximity effect only locally or extrinsically introduce magnetic
responses. Here we report the first experimental discovery of intrinsic
long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers by scanning
magneto-optic Kerr microscopy. In such a 2D vdW soft ferromagnet, for the first
time, an unprecedented control of transition temperature of ~ 35% - 57%
enhancement is realized via surprisingly small fields (<= 0.3 Tesla in this
work), in stark contrast to the stiffness of the transition temperature to
magnetic fields in the three-dimensional regime. We found that the small
applied field enables an effective anisotropy far surpassing the tiny
magnetocrystalline anisotropy, opening up a sizable spin wave excitation gap.
Confirmed by renormalized spin wave theory, we explain the phenomenon and
conclude that the unusual field dependence of transition temperature
constitutes a hallmark of 2D soft ferromagnetic vdW crystals. Our discovery of
2D soft ferromagnetic Cr2Ge2Te6 presents a close-to-ideal 2D Heisenberg
ferromagnet for studying fundamental spin behaviors, and opens the door for
exploring new applications such as ultra-compact spintronics.
