Magnetic Field versus Temperature Phase Diagram of a Quasi-One-Dimensional $\mathit{S}=1\mathrm{}$ Heisenberg Antiferromagnet

From a heat capacity ( $C_p$) measurement on a single crystal sample of the $S=1\mathrm{}$ quasi-one-dimensional (Q1D) Heisenberg antiferromagnet (HAF) $\mathrm{Ni}\left({\mathrm{C}}_5{\mathrm{H}}_{14}{\mathrm{N}}_2{)}_2{\mathrm{N}}_3\right({\mathrm{PF}}_6)$ in applied magnetic fields, we found an anomaly which is indicative of a magnetic long-range ordering. We were able to follow how the position of the anomaly in $C_p$ changes with temperature $\left(T\right)$ and magnetic field ( $H$). An experimental $H-T$ phase diagram of an $S=1\mathrm{}$ Q1D HAF is obtained and compared with that of the corresponding classical system.