3D-XY critical behavior of CsMnF₃ from static and dynamic thermal properties.

Static and dynamic critical behavior of the easy-plane antiferromagnet CsMnF3 have been studied by means of a high-resolution ac photopyroelectric calorimeter. Thermal diffusivity, thermal conductivity and specific heat have been carefully measured in the near vicinity of the antiferromagnetic to paramagnetic transition (51.1 K). Specific heat and thermal diffusivity show singularities at the Néel temperature while thermal conductivity does not. Both the static and dynamic critical parameters agree with the standard 3D-XY universality class (α = -0.014, A⁺/A⁻ = 1.06): for specific heat α = -0.016, A⁺/A⁻ = 1.09 and for thermal diffusivity b = -0.010, U⁺/U⁻ = 1.09. As the dynamic critical behavior of thermal diffusivity has not yet been theoretically established for the 3D-XY universality class, an approximate equation relating static and dynamic critical parameters has been obtained for it, leading to b ≈ α and A⁺/A⁻ ≈ U⁺/U⁻ by studying the asymptotic behavior of the functions. This equation has also been experimentally verified for another XY antiferromagnet (SmMnO₃). As an easy-plane antiferromagnet with a hexagonal structure, CsMnF₃ could have been expected to comply with the 3D-XY chiral class (α = +0.34, A⁺/A⁻ = 0.36) (as is the case of CsMnBr₃), but the experimental results rule out that possibility. This is attributed to the presence of a small in-plane anisotropy of the spins in CsMnF₃, which breaks the chiral degeneracy of the 120° spin structure.