Magnetic phase transitions are one of the mainstays to understand the origin of long-range spin order in condensed matter systems. The first observation of an antiferromagnetic phase transition in the metal halides paved the way for the development of the quantum theory of superexchange and for the discovery of new magnetic ground states [1,2]. Another metal halide, chromium triiodide (CrI₃), was the first free-standing system to demonstrate that long-range magnetic phase transitions in the true two-dimensional limit were possible [3]. However, there are still unknown features in its magnetism [4] that indicate the magnetic phase diagram goes beyond the classically reported single magnetic transition at 61 K [5]. Combining muon spin rotation (μSR) and superconducting quantum interference device (SQUID) measurements, we reveal the presence of multiple magnetic phase transitions of different nature, suggesting that the ferromagnetic transition transcends the classical concept of long-range order.

References
[1] H. Kamerlingh Onnes et al., Leiden Comm., 15 (192b), 1912.
[2] P. Day et al., J. Phys. C., 9, 1976.
[3] B. Huang et al., Nature, 546, 2017.
[4] Z. Wang et al., Nat. Comm., 9, 2018.
[5] M. A. McGuire et al., ACS, 27, 2015.