High-field phase diagram of the Haldane-gap antiferromagnet $Ni (C_5 H_{14} N_2)_2 N_3 (PF_6)$

TL;DR

This study maps the magnetic phase diagram of a quasi-one-dimensional S=1 Heisenberg antiferromagnet using specific heat measurements up to 150 mK and 32 T, revealing a new phase and analyzing critical behavior near quantum phase transitions.

Contribution

The paper provides the first detailed phase diagram of Ni(C5H14N2)2N3(PF6) at ultra-low temperatures and high magnetic fields, including a newly observed phase and refined critical exponents.

Findings

Discovery of a new phase at Hc2 ≈ 14 T when field is along the spin chain.

Precise determination of power-law exponents near the critical field Hc.

Deuterium substitution slightly reduces interchain exchange interactions.

Abstract

We have determined the magnetic phase diagram of the quasi-one-dimensional $S=$ 1 Heisenberg antiferromagnet $Ni (C_5 H_{14} N_2)_2 N_3 (PF_6)$ by specific heat measurements to 150 mK in temperature and 32 T in magnetic field. When field is applied along the spin-chain direction, a new phase appears at $H_{c2}\approx 14$ T. For the previously known phases of field-induced order, accurate determination is made of the power-law exponents of the ordering temperature near the zero-temperature critical field $H_c$, owing to the four-fold improvement of the minimum temperature over the previous work. The results are compared with the predictions based on the Bose-Einstein condensation of triplet excitations. Substituting deuterium for hydrogen is found to slightly reduce the interchain exchange.