Universal fluctuating regime in triangular chromate antiferromagnets

Abstract

We report x-ray diffraction, magnetic susceptibility, heat capacity, ${}^1$H nuclear magnetic resonance (NMR), and muon spin relaxation ($\mu$SR) measurements, as well as density-functional band-structure calculations for the frustrated $S=3/2$ triangular lattice Heisenberg antiferromagnet (TLHAF) $\alpha$-HCrO${}_2$ (trigonal, space group: $R\bar{3}m$). 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. Local probes (NMR and $\mu$SR) reveal a broad regime with slow fluctuations down to $0.7T_{\mathrm{N}}$, this temperature corresponding to the maximum in the $\mu$SR relaxation rate and in the NMR wipe-out. From the comparison with NaCrO${}_2$ and $\alpha$-KCrO${}_2$, the fluctuating regime and slow dynamics below $T_{\mathrm{N}}$ appear to be hallmarks of the TLHAF with $ABC$ stacking that leads to a frustration of interlayer couplings between the triangular planes. This interlayer frustration is a powerful lever to generate spin states with persistent dynamics and may bear implications to spin-liquid candidates with the triangular geometry.

14 pages, 11 figures, 2 tables

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