The influence of the small (< 2% of the Heisenberg exchange), but anisotropic, dipolar interaction on the spin dynamics of the quasi-one-dimensional antiferromagnet, ${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$NMn${\mathrm{Cl}}_3$, is explored. The proton spin-lattice relaxation time at low temperatures is calculated, and a crossover from isotropic Heisenberg (order-parameter dimensionality $n=3\mathrm{}$) to $\mathrm{XY} \mathrm{}(n=2\mathrm{})\mathrm{}$ behavior is found, with a crossover temperature $T_x\approx 20$ °K. Previous qualitative discrepancies between theory and experiment at low temperatures are resolved by inclusion of the effects of dipolar anisotropy.

• Received 24 June 1976

DOI:https://doi.org/10.1103/PhysRevB.15.323