Spin dynamics in the Van der Waals magnet CrCl$_3$

The magnetic nature of low dimensional compound, CrCl$_3$, was investigated by muon spin rotation, relaxation and resonance ($\mu^+$SR). The $\mu^+$SR measurements revealed three distinct phases as a function of temperature: an antiferromagnetic state (AF) for $T\leq T_{\rm N}=14.32(6)$~K, a ferromagnetic short range ordered state (FM-SRO) for $T_{\rm N}<T<\sim18$~K and a paramagnetic phase (PM) above $\sim18$~K. Moreover, the AF state exhibits appreciable spin dynamics, which increases with decreasing temperature below $T_{\rm N}$. These dynamics originate from out of plane fluctuations, which seem to settle for $9.5$~K$\leq T\leq T_{\rm N}$, evidenced from measurements in ZF and complementary local field calculations. Moreover, the presented muon Knight shift measurements just above $T_{\rm N}$ represent a clear microscopic evidence for the absence of the previously speculated long range quasi-2D FM order.