High-field multifrequency electron-spin-resonance study of the Haldane magnetNi(C5H14N2)

Abstract

In order to understand physical properties of the field-induced phase of a spin-gap system, we performed high-field and multifrequency electron-spin-resonance ͑ESR͒ measurements on single crystals of the S =1 quasi-one-dimensional Heisenberg antiferromagnet, namely, the Haldane magnet, Ni͑C 5 H 14 N 2 ͒ 2 N 3 ͑PF 6 ͒, abbreviated as NDMAP. This compound has an energy gap ͑Haldane gap͒ at zero field and one of the excited triplet branches goes down on applying magnetic fields, resulting in the gap closing at a critical field H c around 5 T that is slightly different depending on the field direction. First, we studied the angular dependence of spin excitations below 14 T. Two sets of resonance modes caused by two types of Ni 2+ chains in NDMAP are observed. These data are analyzed by comparing with a phenomenological field theory ͑PFT͒. The experimental results between H c and about 12 T are well fitted with the calculated ones by the PFT, but the fitting above 12 T is not satisfactory. Therefore, we studied spin excitations at much higher magnetic fields up to about 55 T. Several ESR signals are observed above H c for each crystallographic axis, and one or two of them survive in the high-field region above about 15 T. One mode approaches a paramagnetic resonance line at high fields and the other mode broadly changes with magnetic fields. These modes fit well with the conventional antiferromagnetic resonance modes with biaxial anisotropy. This result suggests that the quantum fluctuations are suppressed by strong magnetic field and the spin excitations change from a quantum nature to a classical one in high magnetic fields.

Electron spin resonance shifts inS=1antiferromagnetic chains

“…In order to see the field dependence explicitly, (54) with the experimental data on NDMAP at a temperature T = 0.05J. 5 The solid curve is the free fermion result (54) near Hc1 and the solid circles denote the experimental data. We used the parameters J = 30.0 K, D/J = 0.25, and ge = 2.10.…”

“…(8) is equivalent to (5). Therefore, as long as we are concerned with the first order perturbation theory around (6), it does not matter whether we change ω or H. We apply the formula (5) to our model (1), namely,…”

“…We apply our theory of ESR shifts to an S = 1 HAF chain compound Ni(C 5 H 14 N 2 ) 2 N 3 (PF 6 ) (abbreviated to NDMAP 5,[33][34][35] ). There are several S = 1 HAF chain compounds, for instance, Ni(C 2 H 8 N 2 ) 2 (NO 2 )ClO 4 (abbreviated to NENP 36 ), Ni(C 9 H 24 N 4 )(NO 2 )ClO 4 (abbreviated to NTENP 37 ) and Ni(C 5 H 14 N 2 ) 2 N 3 (ClO 4 ) (abbreviated to NDMAZ 38 ).…”

“…In fact, thanks to this advantage of ESR, many interesting ESR experiments have been performed in onedimensional quantum spin systems under high magnetic field. [4][5][6] These recent precise ESR experiments highlights necessity of reliable quantitative theory of ESR in the field-induced critical phase.…”

“…Recently, dynamics of electron spins in the field-induced critical phases are actively investigated by various experimental techniques. [3][4][5] Among these experimental techniques, electron spin resonance (ESR) occupies a unique position in its sensitivity to interactions between electron spins. In fact, thanks to this advantage of ESR, many interesting ESR experiments have been performed in onedimensional quantum spin systems under high magnetic field.…”