Unconventional Dynamics in Triangular Heisenberg AntiferromagnetNaCrO2

We report magnetization, specific heat, muon spin rotation, and Na NMR measurements on the $S=3/2$ rhombohedrally stacked Heisenberg antiferromagnet ${\mathrm{NaCrO}}_2$. This compound appears to be a good candidate for the study of isotropic triangular Heisenberg antiferromagnets with very weak interlayer coupling. While specific heat and magnetization measurements indicate the onset of a transition in the range $T_c\sim 40–50\mathrm{K}$, both muon spin rotation and NMR reveal a fluctuating crossover regime extending well below $T_c$, with a peak of relaxation rate $T_1^{-1}$ around $T\approx 25\mathrm{K}$. This novel finding is discussed within the context of excitations in the triangular Heisenberg antiferromagnets.