We have studied the λ transition in ammonium chloride (${\mathrm{NH}}_4$Cl) by means of ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ NMR. In previous reports, the ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ NMR spin-lattice relaxation time (${\mathit{T}}_1$) did not show the critical slowing down, whereas the ${\mathrm{Cl}}^{35}$ NMR spin-lattice relaxation time revealed a sharp λ transition at the critical temperature. Our measurements of the ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ solid echo decay constant (${\mathit{T}}_{2\mathit{E}}$) and the second moment near the critical temperature manifest the critical slowing down, and the temperature dependence of the ${\mathit{T}}_{2\mathit{E}}$ is well explained by the dynamic Ising model.

• Received 21 August 1995

DOI:https://doi.org/10.1103/PhysRevB.52.16028