Nuclear-quadrupole-resonance (NQR) measurements of the spin-lattice relaxation rate (${\mathit{T}}_1^{\mathrm{-}1}$) for the halide nuclei in the praseodymium trihalides ${\mathrm{PrCl}}_3$ and ${\mathrm{PrBr}}_3$ are reported. They provide a significant test of the dynamical properties of the one-dimensional (1D) XY model as predicted within the Kubo linear-response regime. Measurements of ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ ${\mathit{T}}_1^{\mathrm{-}1}$ values in ${\mathrm{PrCl}}_3$ and ${}_{}{}^{79}{}_{}{}^{}\mathrm{Br}$ and ${}_{}{}^{81}{}_{}{}^{}\mathrm{Br}$ ${\mathit{T}}_1^{\mathrm{-}1}$ values in ${\mathrm{PrBr}}_3$ have been made in the millikelvin temperature range where the 1D dynamical effects are enhanced. The data are shown to be in agreement with the predictions of a relaxation theory for a magnetic interaction, the theory is based on a rigorous treatment of the longitudinal dynamical correlation function 〈${\mathit{S}}_{\mathit{z}}^{\mathit{m}}$(t)${\mathit{S}}_{\mathit{z}}^{\mathit{n}}$(0)〉 of the electronic pseudospins ${\mathit{S}}^{\mathit{m}}$ associated with the crystalline electric-field ground states. The fits to the data yield reasonable values for the hyperfine-interaction parameters A and the exchange integrals J/${\mathit{k}}_{\mathit{B}}$.

• Received 27 August 1990

DOI:https://doi.org/10.1103/PhysRevB.43.7565