Magnetic Field-Induced Second Transition of the Haldane-Gap Antiferromagnet Ni(C_(5)H_(14)N_(2))_(2)N_(3)(PF_(6))

摘要

We have used the magnetocaloric effect to investigate the new transition that was recently discovered in the Haldane-gap antiferromagnet Ni(C_(5)H_(14)N_(2))_(2)N_(3)(PF_(6)) at H_(c2) approx= 14 T and at temperatures below 0.886 K. According to the specific-heat data in which this transition was observed in a magnetic field applied in the spin-chain direction, H_(c2) is nearly temperature-independent. Measured as a function of magnetic field, the variation of the sample temperature due to the magnetocaloric effect indicates that the transition at H_(c2) is second order. It also shows that the magnetization of the new phase above H_(c2) decreases with increasing temperature, whereas it decreases with increasing temperature in the previously known, commensurate phase below H_(c2). We propose the relevance of the Tomonaga-Luttinger spin liquid or the weak in-plane anisotropy of Ni(C_(5)H_(14)N_(2))_(2)N_(3)(PF_(6)) to the new phase.