Crossover Behavior of a Random-Exchange Heisenberg Antiferromagnetic Chain at Ultralow Temperatures

Abstract

Measurements of the magnetic susceptibility ($\chi$) of quinolinium ${\mathrm{}\left(\mathrm{tetracyanoquinodimethane}\right)\mathrm{}}_2$ are reported over the temperature range $0.45 \mathrm{mK}<T<\mathrm{}40\mathrm{} \mathrm{mK}$ with use of low-field ESR. Down to 7 mK, it is found that $\chi \propto T^{-\alpha }$ with $\alpha =0.81\mathrm{}$ characteristic of a random-exchange Heisenberg antiferromagnetic chain (REHAC). At lower temperatures, the susceptibility peaks, the ESR linewidth greatly increases, and an internal field rapidly builds up. These changes may be due to either hyperfine or dipolar interactions, signalling a crossover from one-dimensional REHAC to a three-dimensional behavior-possibly a dipolar spin-glass.