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 antiferro magnetic 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 T N similar to 15.5K. We have carried out an extensive series of precision SQUID magnetometer measurements as a function of temperature, from very low fields gt; 2.5G and up to similar 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 similar to 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 syste