Spin dynamics of the S = 5/2 2D triangular antiferromagnet Ba₃NbFe₃Si₂O₁₄

We report pulse-field magnetization, ac susceptibility, and 100 GHz electron spin resonance (ESR) measurements on the S = 5/2 two-dimensional triangular compound Ba₃NbFe₃Si₂O₁₄ with the N´eel temperature T_N = 26 K . The magnetization curve shows an almost linear increase up to 60 T with no indication of a one-third magnetization plateau. An unusually large frequency dependence of the ac susceptibility in the temperature range of T = 20–100 K reveals a spin-glass behavior or superparamagnetism, signaling the presence of frustration-related slow magnetic fluctuations. The temperature dependence of the ESR linewidth exhibits two distinct critical regimes; (i) ΔH_pp(T) α(T-T_N)^-p with the exponent p = 0.2(1)–0.2(3) for temperatures above 27 K, and (ii) ΔH_pp(T) α(T- T)^-p with T=12 K and p = 0.8.(1)–0.8(4) for temperatures between 12 and 27 K. This is interpreted as indicating a dimensional crossover of magnetic interactions and the persistence of short-range correlations with a helically ordered state.