Critical region phase transitions in the quasi-2D magnet CrCl3

CrCl3 has been known for a long time to be a quasi-2D ferromagnet (FM) with a specific-heat peak at 16.8K. Its 3D ordering is antiferromagnetic (AFM) with alternating 2D FM sheets perpendicular to the c-axis. Kuhlow [Phys. Stat. Sol. A 72, 161 (1982)] carried out Faraday rotation measurements on CrCl3 and concluded, on fairly tenuous grounds, that there are actually two closely spaced magnetic transitions, one at “16.7K” at which 2D FM sheets of in-plane spins start ordering, but do so with random alignment relative to the c-axis (the latter being perpendicular to the 2D FM sheets), and one in which a 3D AFM alignment of the 2D sheets of spins takes place, that being at TN ∼15.5K. We have carried out an extensive series of precision SQUID magnetometer measurements as a function of temperature, from very low fields (≥2.5G) and up to ∼2 kG on a high-purity single-crystal platelet, with the magnetic field both parallel and perpendicular to the c axis. The measurements provide more definitive evidence that there are two transitions, possibly three (at ∼16.8K, 16.0 and ∼14.3K, in our measurements). These data provide some support to Kuhlow’s conjecture that the initial phase transition is to a phase consisting of 3D-disordered, 2D-ordered, planes of spins. The very low field measurements provide the strongest support for this picture. To clarify the situation further neutron scattering (NS) measurements were carried out on a batch of the same material. The NS temperature-dependence measurements appear to provide additional support by showing a change in curvature in the magnetization (M) curve near 14K, and then an apparent linear decrease in M above 14K. Additionally, the NS data show a possible hysteresis near 16K during cooling. Possibly (though unlikely) the critical-region phase-transition behaviors observed in CrCl3 are unique to this quasi-2D system.