Critical behavior of the electron-paramagnetic-resonance linewidth of a spin-1/2 two-dimensional antiferromagnet

The 9-GHz data on the electron-paramagentic-resonance linewidth of a two-dimensional spin-1/2 antiferromagnet [Cu(HCOO${)}_2$⋅${4\mathrm{H}}_2$O] have been reanalyzed just above the Néel temperature ${\mathit{T}}_{\mathit{N}}$. The temperature dependence of the critical broadening contribution observed between 2${\mathit{T}}_{\mathit{N}}$ and 1.5${\mathit{T}}_{\mathit{N}}$ [${\mathit{T}}_{\mathit{N}}$=17 K] is exponential in 1/T with an isotropic characteristic energy and is in very good agreement with the ${\xi }^3$ dependence (ξ=spin-correlation length) predicted by theory. The data suggest the critical spin fluctuations depend only on a single parameter, the spin-stiffness constant.