Universal fluctuating regime in triangular chromate pure Heisenberg S=3/2 antiferromagnets

We report x-ray diffraction, magnetic susceptibility, heat capacity, ¹H nuclear magnetic resonance (NMR), and muon spin relaxation (µSR) measurements, as well as band-structure calculations for the frustrated S = 3/2 triangular lattice Heisenberg antiferromagnet (TLHAF) α-HCrO₂ (trigonal, space group: R-3m). The intra-layer and inter-layer couplings are estimated to be J/k_B ≈ 24 K and J’/k_B ≈ 2.8 K, respectively. This compound undergoes a clear magnetic transition at T_N ≈ 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 µSR) reveal a broad regime with slow fluctuations down to 0.7 T_N, this temperature corresponding to the maximum in the µSR relaxation rate and in the NMR wipe-out. The scaling of the NMR and µSR data with respect to J or T_N supports a scenario where a crossover from 2D to 3D correlations sets in around 0.7 T_N preceded by a typical 2D regime of the TLHAF. From the comparison with NaCrO₂ and α-KCrO₂, the fluctuating regime and slow dynamics below T_N appear to be hallmarks of the TLHAF with ABC stacking. We discuss the role of interlayer frustration which may bear implications to recent spin-liquid candidates with the triangular geometry.