Universal fluctuating regime in triangular chromate antiferromagnets

We report x-ray diffraction, magnetic susceptibility, heat capacity, ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR), and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements, as well as density-functional band-structure calculations for the frustrated $S=3/2$ triangular lattice Heisenberg antiferromagnet (TLHAF) $\alpha \text{−}{\mathrm{HCrO}}_2$ (trigonal, space group: $R\overline{3}m$). This compound undergoes a clear magnetic transition at $T_{\mathrm{N}}\simeq 22.5$ K, as seen from the drop in the muon paramagnetic fraction and concurrent anomalies in the magnetic susceptibility and specific heat capacity. Local probes (NMR and $\mu \mathrm{SR}$) reveal a broad regime with slow fluctuations down to $0.7T_{\mathrm{N}}$, this temperature corresponding to the maximum in the $\mu \mathrm{SR}$ relaxation rate and in the NMR wipe-out. From the comparison with ${\mathrm{NaCrO}}_2$ and $\alpha \text{−}{\mathrm{KCrO}}_2$, the fluctuating regime and slow dynamics below $T_{\mathrm{N}}$ appear to be hallmarks of the TLHAF with $ABC$ stacking. We discuss the role of interlayer frustration, which may have impacted recent spin-liquid candidates with triangular geometry.