Spin fluctuations in a random one-dimensional Heisenberg antiferromagnet:(CD3)4NMncCu1−cCl3

Abstract

In this paper we report a detailed neutron scattering study of the spin fluctuations in the random one-dimensional Heisenberg antiferromagnet ${\left(\mathrm{C}{\mathrm{D}}_3\right)}_4\mathrm{N}{\mathrm{Mn}}_c{\mathrm{Cu}}_{1-c}{\mathrm{Cl}}_3$ with $c=0.93\mathrm{}$ and $c=0.85\mathrm{}$. In this material the Mn-Cu exchange is only about 4% of the Mn-Mn exchange so that this system corresponds to a "weak-link" impure chain. For temperatures such that $k_{B}T>\mathrm{}{J}_{\mathrm{M}\mathrm{n}-\mathrm{C}\mathrm{u}}$ percolationlike behavior is observed, while for $k_{B}T<\mathrm{}{J}_{\mathrm{M}\mathrm{n}-\mathrm{C}\mathrm{u}}$ "weak-link" behavior obtains. The instantaneous magnetic correlations are found to be in good quantitative accord with the exact results for the random classical Heisenberg model. We also discuss briefly the spin-wave excitations at large $q$'s and the spin dynamics for $q\sim \kappa$, the inverse correlation length.