Antiferromagnetic resonance in a spin-gap magnet with strong single-ion anisotropy

TL;DR

This paper models antiferromagnetic resonance in a quasi-one-dimensional spin-gap magnet with strong single-ion anisotropy, explaining experimental observations of excitation spectra and Goldstone mode behavior.

Contribution

It introduces a simple theoretical description combining strong coupling and mean-field models to accurately fit experimental data on antiferromagnetic resonance in DTN.

Findings

Model successfully fits experimental excitation spectra.

Goldstone mode becomes gapped when magnetic field deviates from symmetry axis.

Provides insight into field-induced antiferromagnetic order in spin-gap magnets.

Abstract

Quasi-one-dimensional magnet NiCl$_2\cdot$4SC(NH$_2$)$_2$, usually abbreviated as DTN, does not order at zero field down to $T=0$: due to the strong single-ion anisotropy of the "easy plane" type acting on $S=1$ Ni$^{2+}$ ions, the $S_z=0$ ground state is separated from $S_z=\pm 1$ excitations by an energy gap. Once the magnetic field is applied along the main anisotropy axis, the gap closes at $B_{c1}=2.18$ T and the field-induced antiferromagnetic order arises. The low-energy excitations spectrum of this field-induced ordered state includes two branches of excitations, one of them have to be a gapless Goldstone mode. Recent studies of excitations spectrum in a field-induced ordered state of DTN (T.Soldatov et.al, Phys.Rev.B 101, 104410 (2020)) have revealed that Goldstone mode became gapped as magnetic field deviates from the main symmetry axis. This paper proposes simple description of antiferromagnetic resonance modes of quasi-one-dimensional quantum $S=1$ magnet with strong single-ion anisotropy. The approach used is based on a combination of the strong coupling model for the anisotropic spin chain with the conventional mean-field model of antiferromagnetic resonance. The resulting model fits to the known experimental results without additional tuning parameters.