Phase transition of an S = 1 quasi-one-dimensional Heisenberg antiferromagnet in applied magnetic fields

The phase transition of the S=1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C5H14N2)(2)N-3(PF6) in an applied magnetic field (H) is studied from the measurement of the heat capacity (C-p). There is no anomaly in C-p below H=4 T (H parallel to the chain) or H=6 T (H perpendicular to the chain). A sharp anomaly in C-p is observed above H=4 T (H parallel to the chain) or H=6 T (H perpendicular to the chain) indicating the occurrence of long-range ordering. The H dependence of the critical temperature is studied and a temperature versus magnetic field phase diagram is constructed. (C) 1999 American Institute of Physics. [S0021-8979(99)28508-8].