The field dependence of the magnetic heat capacity of (${\mathrm{C}}_6$${\mathrm{H}}_{11}$${\mathrm{NH}}_3$)${\mathrm{CuBr}}_3$ (CHAB), which is an ideal ferromagnetic S=(1/2) chain system with XY anisotropy, is compared with predictions based on the sine-Gordon (sG) model and with transfer-matrix calculations on a chain of classical spins. A detailed analysis of the excess heat capacity in the region 1.5<T<7 K and B<7 kG reveals systematic deviations from sG behavior. It is demonstrated that the spin components out of the XY plane have a rather pronounced effect. The importance of quantum effects in this S=(1/2) system seems less than for ${\mathrm{CsNiF}}_3$ and (${\mathrm{CH}}_3$${)}_4$${\mathrm{NMnCl}}_3$ (TMMC), which is related to the relatively small (5%) XY anisotropy, together with the ferromagnetic intrachain interaction.

• Received 15 April 1985

DOI:https://doi.org/10.1103/PhysRevB.32.3154