  Exploring new parameter regimes to realize and control novel phases of matter
has been a main theme in modern condensed matter physics research. The recent
discovery of 2D magnetism in nearly freestanding monolayer atomic crystals has
already led to observations of a number of novel magnetic phenomena absent in
bulk counterparts. Such intricate interplays between magnetism and crystalline
structures provide ample opportunities for exploring quantum phase transitions
in this new 2D parameter regime. Here, using magnetic field and temperature
dependent circularly polarized Raman spectroscopy of phonons and magnons, we
map out the phase diagram of CrI3 that has been known to be a layered AFM in
its 2D films and a FM in its 3D bulk. We, however, reveal a novel mixed state
of layered AFM and FM in 3D CrI3 bulk crystals where the layered AFM survives
in the surface layers and the FM appears in deeper bulk layers. We then show
that the surface layered AFM transits into the FM at a critical magnetic field
of 2 T, similar to what was found in the few layer case. Interestingly,
concurrent with this magnetic phase transition, we discover a first-order
structural phase transition that alters the crystallographic point group from
C3i to C2h and thus, from a symmetry perspective, this monoclinic structural
phase belongs to the 3D nematic order universality class. Our result not only
unveils the complex single magnon behavior in 3D CrI3, but also settles down
the puzzle of how CrI3 transits from a bulk FM to a thin layered AFM
semiconductor, despite recent efforts in understanding the origin of layered
AFM in CrI3 thin layer, and reveals the intimate relationship between the
layered AFM-to-FM and the crystalline rhombohedral-to-monoclinic phase
transitions. These findings further open up opportunities for future 2D
magnet-based magneto-mechanical devices.
