Specific Heat of Three Magnetic Linear-Chain Antiferromagnets

Abstract

The specific heat of ${\mathrm{C}\mathrm{u}\left[\mathrm{N}{\mathrm{H}}_2{\left(\mathrm{C}{\mathrm{H}}_2\right)}_3\mathrm{N}{\mathrm{H}}_2\right]}_2$S${\mathrm{O}}_4$ · ${\mathrm{H}}_2$O ("CPDS·${\mathrm{H}}_2$O") and of a mixed sample of Cu${\left(\mathrm{N}{\mathrm{H}}_3\right)}_4$-Se${\mathrm{O}}_4$ ("CTSel") and Cu${\left(\mathrm{N}{\mathrm{H}}_3\right)}_4$Se${\mathrm{O}}_4$·${\mathrm{H}}_2$O ("CTSel·${\mathrm{H}}_2$O") have been measured from 2 to 20 K, and excellent agreement has been obtained with the Bonner-Fisher theory for infinite magnetic linear chains of spin ½, with an isotropic Heisenberg interaction between nearest neighbors. From the fit between experiment and theory, the following exchange constants $J$ are obtained: CPDS·${\mathrm{H}}_2$O, (${1.09}_{-0.08\mathrm{}}^{+0.04\mathrm{}}$) K; CTSel, (${0.24}_5$ ±0.03) K; and CTSel·${\mathrm{H}}_2$O, (2.36±0.03) K. A preliminary description of the crystal structures of these compounds is also given. CTSel·${\mathrm{H}}_2$O is isomorphous with Cu${\left(\mathrm{N}{\mathrm{H}}_3\right)}_4$S${\mathrm{O}}_4$·${\mathrm{H}}_2$O; in addition, a close structural relationship is found between these salts and CPDS·${\mathrm{H}}_2$O. Thus, the variation of $J$ with interionic separation $\mathcal{r}$ may be estimated for these simple linear systems. This estimate gives $N=10.9\mathrm{}$ where $J=a{r}^{-n}$.