Magnetic behavior of the ferromagnetic quantum chain systems (${\mathrm{C}}_6$${\mathrm{H}}_{11}$N${\mathrm{H}}_3$)Cu${\mathrm{Cl}}_3$ (CHAC) and (${\mathrm{C}}_6$${\mathrm{H}}_{11}$N${\mathrm{H}}_3$)Cu${\mathrm{Br}}_3$ (CHAB)

Heat-capacity measurements on (${\mathrm{C}}_6$ ${\mathrm{H}}_{11}$N${\mathrm{H}}_3$)Cu${\mathrm{Br}}_3$ (CHAB) in the region 0.45 < $T<\mathrm{}55\mathrm{}$ K are presented. Three-dimensional ordering is observed at $T_c=1.50\mathrm{}$ K. The data are analyzed together with previously reported measurements on (${\mathrm{C}}_6$ ${\mathrm{H}}_{11}$N${\mathrm{H}}_3$)Cu${\mathrm{Cl}}_3$ (CHAC). Both compounds appear to be very good approximations of a ferromagnetic $S=\frac{1}{2}$ Heisenberg linearchain system with $\frac{J}{k}\approx 50$ K. From the data it is inferred that the intrachain interaction of CHAC contains about 2% uniaxial anisotropy, whereas CHAB contains about 5% easy-plane anisotropy. The results are corroborated by magnetization measurements on CHAB below $T_c$, which also reveal a metamagnetic transition for $\overrightarrow{\mathrm{H}}\parallel b$ ($H_c=210\mathrm{}$Oe). From the value of $H_c$ and the location of the tricitical point the interchain interactions in CHAB were found as $\frac{z_{\mathrm{AF}}{J}_{\mathrm{AF}}}{k}=-0.03\mathrm{}$ K, $\frac{z_{\mathrm{F}}{J}_{\mathrm{F}}}{k}\approx 0.08$ K.