Measurements of the magnetic susceptibility ($\chi$) using low-field electron spin resonance over the temperature range $30 \mathrm{mK}<T<\mathrm{}300\mathrm{} \mathrm{K}$ are reported for a polycrystalline sample of quinolinium-di-tetracyanoquinodimethanide, as well as over a more limited range in $T$ for two other polycrystalline samples and one single-platelet sample. In all cases, below ∼20 K, $\chi \propto T^{-\alpha }$, where for each preparation batch $\alpha$ is constant. The temperature dependence of $\chi$ is interpreted as evidence that quinolinium-di-tetracyanoquinodimethanide is a random-exchange Heisenberg antiferromagnetic chain with a high concentration of spins. It is speculated that the variation in $\alpha$ between different preparation batches may be due to variations of short-range order introduced by different conditions of crystal growth. The relation of the observed $\chi$ to the microscopic exchange coupling is discussed in terms of recent renormalization calculations and the quasiuniversal behavior predicted by them. The absence of a sharp magnetic phase transition is interpreted as the suppression of an ordering transition by disorder.

• Received 8 July 1980

DOI:https://doi.org/10.1103/PhysRevB.23.5846