Field-induced long-range ordering in an S = 1 quasi-one-dimensional Heisenberg antiferromagnet

Detailed results of heat capacity and magnetization measurements are reported on a single crystal sample of the spin S = 1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C₅H₁₄N₂)₂N₃(PF₆), abbreviated NDMAP. From these results, we constructed the magnetic field (H) versus temperature phase diagram of this compound which exhibits the quantum disordered Haldane, field-induced long-range-ordered (LRO) and thermally disordered paramagnetic phases. The phase boundary curve separating the paramagnetic and LRO phases is anisotropic; the increase of the Néel temperature (T_N) with H along the a and b axes is more rapid than that along the c axis. We calculated T_N as a function of H by taking into account the interchain coupling as the form of a mean field. The calculation of the staggered susceptibility of the S = 1 one-dimensional antiferromagnet with an easy-plane anisotropy shows quite different behavior in the different field directions, resulting in the anisotropic phase boundary curve. A good agreement is obtained between theory and experiment using the exchange and anisotropy constants obtained from the neutron scattering experiment.