Higher‐order exchange interactions and quantum effects are widely known to play an important role in describing the properties of low‐dimensional magnetic compounds. Here, the recently discovered 2D van der Waals (vdW) CrI3 is identified as a quantum non‐Heisenberg material with properties far beyond an Ising magnet as initially assumed. It is found that biquadratic exchange interactions are essential to quantitatively describe the magnetism of CrI3 but quantum rescaling corrections are required to reproduce its thermal properties. The quantum nature of the heat bath represented by discrete electron–spin and phonon–spin scattering processes induces the formation of spin fluctuations in the low‐temperature regime. These fluctuations induce the formation of metastable magnetic domains evolving into a single macroscopic magnetization or even a monodomain over surface areas of a few micrometers. Such domains display hybrid characteristics of Néel and Bloch types with a narrow domain wall width in the range of 3–5 nm. Similar behavior is expected for the majority of 2D vdW magnets where higher‐order exchange interactions are appreciable.

Higher‐order exchange and quantum effects are widely known to play an important role in the properties of nanomagnets. In article number 2004138, Elton J. G. Santos and co‐workers show that recently discovered two‐dimensional CrI3 magnets require biquadratic exchange interactions and quantum‐rescaling to describe their long‐range ordering. Metastability of the magnetic domains induces domain walls with different chirality, which is important for the new generation of quantum‐materials‐based technologies.

Magnetic phase transitions often occur spontaneously at specific critical temperatures and are instrumental to understand the origin of long-range spin order in condensed matter systems. The presence of more than one critical temperature (T c ) has been observed in several compounds [1][2][3][4][5] 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 6,7 show such intriguing phenomena that can result in

Magnetic phase transitions often occur spontaneously at specific critical temperatures and are instrumental to understand the origin of long-range spin order in condensed matter systems. 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 (μSR), superconducting quantum interference device (SQUID) magnetometry, and large-scale micromagnetic simulations including higher order exchange interactions and dipolar fields. 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.