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

TL;DR

This study uses neutron spin-echo techniques to investigate the critical dynamics of spin-5/2 antiferromagnets in 2D and 3D, revealing both expected and novel behaviors near the Néel temperature.

Contribution

It provides experimental measurements of dynamical critical exponents in 2D and 3D antiferromagnets, including a new observed exponent in 2D not predicted by existing theories.

Findings

In MnF₂, critical exponents match dynamical scaling theory.

In Rb₂MnF₄, a new scaling regime with an unexplained exponent was observed.

The amplitude ratio of fluctuations agrees with theoretical predictions.

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 N\'eel 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.