Neutron spin-echo study of the critical dynamics of spin-5/2 antiferromagnets in two and three dimensions

Abstract

We report a neutron spin-echo study of the critical dynamics in the $S=5/2$ antiferromagnets MnF${}_2$ and Rb${}_2$MnF${}_4$ with three-dimensional (3D) and two-dimensional (2D) spin systems, respectively, in zero external field. Both compounds are Heisenberg antiferromagnets with a small uniaxial anisotropy resulting from dipolar spin-spin interactions, which leads to a crossover in the critical dynamics close to the Néel temperature, $T_N$. By taking advantage of the $\mu \text{eV}$ energy resolution of the spin-echo spectrometer, we have determined the dynamical critical exponents $z$ for both longitudinal and transverse fluctuations. In MnF${}_2$, both the characteristic temperature for crossover from 3D Heisenberg to 3D Ising behavior and the exponents $z$ in both regimes are consistent with predictions from the dynamical scaling theory. The amplitude ratio of longitudinal and transverse fluctuations also agrees with predictions. In Rb${}_2$MnF${}_4$, the critical dynamics crosses over from the expected 2D Heisenberg behavior for $T\gg T_N$ to a scaling regime with exponent $z=1.387(4)$, which has not been predicted by theory and may indicate the influence of long-range dipolar interactions.

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