  Magnetic phase transitions often occur spontaneously at specific critical
temperatures. The presence of more than one critical temperature (Tc) has been
observed in several compounds where the coexistence of competing magnetic
orders highlights the importance of phase separation driven by different
factors such as pressure, temperature and chemical composition. However, it is
unknown whether recently discovered two-dimensional (2D) van der Walls (vdW)
magnetic materials show such intriguing phenomena that can result in rich phase
diagrams with novel magnetic features to be explored. Here we show the
existence of three magnetic phase transitions at different Tc's in 2D vdW
magnet CrI3 revealed by a complementary suite of muon spin relaxation-rotation,
superconducting quantum interference device magnetometry, and large-scale
atomistic simulations including higher-order exchange interactions. We find
that the traditionally identified Curie temperature of bulk CrI3 at 61 K does
not correspond to the long-range order in the full volume (VM) of the crystal
but rather a partial transition with less than 25% of VM being magnetically
spin-ordered. This transition is composed of highly disordered domains with the
easy-axis component of the magnetization Sz not being fully spin-polarized but
disordered by in-plane components (Sx, Sy) over the entire layer. As the system
cools down, two additional phase transitions at 50 K and 25 K drive the system
to 80% and nearly 100% of the magnetically ordered volume, respectively, where
the ferromagnetic ground state has a marked Sz character yet also displaying
finite contributions of Sx and Sy to the total magnetization. Our results
indicate that volume-wise competing electronic phases play an important role in
the magnetic properties of CrI3 which set a much lower threshold temperature
for exploitation in magnetic device-platforms than initially considered.

Mesoscale and Nanoscale Physics;Materials Science;Strongly Correlated Electrons