Microwave dynamics of the stoichiometric and bond-disordered anisotropic S=1 chain antiferromagnet NiCl2−4SC(NH2

Abstract

We studied electron spin resonance in a quantum magnet NiCl2-4SC(NH2)2, demonstrating a field-induced quantum phase transition from a quantum-disordered phase to an antiferromagnet. We observe two branches of the antiferromagnetic resonance of the ordered phase, one of them has a gap and the other is a Goldstone mode with zero frequency at a magnetic field along the four-fold axis. This zero frequency mode acquires a gap at a small tilting of the magnetic field with respect to this direction. The upper gap was found to be reduced in the doped compound Ni(Cl1−xBrx)2-4SC(NH2)2 with x = 0.21. This reduction is unexpected because of the previously reported rise of the main exchange constant in a doped compound. Further, a nonresonant diamagnetic susceptibility χ ′ was found for the ordered phase in a wide frequency range above the quasi-Goldstone mode. This dynamic diamagnetism is as large as the dynamic susceptibility of the paramagnetic resonance. We speculate that it originates from a two-magnon absorption band of low-frequency dispersive magnon branch.

Magnon spectrum and electron spin resonance in antiferromagnet with large single-ion easy plane anisotropy

“…We showed that the dependence ∆(h) in the linear spin-wave approximation is drastically different from the one which includes quantum corrections of the order 1/S. The latter are responsible for nonmonotonic dependence of ∆(h) on the external magnetic field which was observed experimentally in both DTN 21 and DTNX 22 . Furthermore, according to our analysis quantum corrections give rise to nontrivial ∆(h) dependence on the system parameters in contrast to the simple classical behaviour.…”

“…One can see that at fixed J a = 0.2 K ∆ min increases with J c increase and decreases with D increase. Finally, we want to stress that our analysis explains the fact that ESR spectrum of DTN can remain almost unchanged 22 when small concentrations of Br are introduced, while according to the neutron experiment exchange couplings and single-ion anisotropy constant increase with the increase of Br concentration 18 . From the point of view of ∆ (1) (h) in our theory different system parameters variation can compensate each other, which can not occur in the linear spin-wave approximation.…”

“…Motivated by the recent experiments on the Br doped DTN (so called DTNX) 18,22 we discuss theoretically the spin-wave spectrum of antiferromagnet with large singleion easy-plane anisotropy in the magnetic field induced ordered phase. In particular, we obtain analytical expression for energy of the optical magnon ∆(h) at the center of the Brillouin zone which can be measured in ESR experiment.…”

“…This naturally leads to the disappearance of MI phase at higher Br concentrations 20 . In contrast, electron spin resonance (ESR) experiments 21,22 show that the ESR spectrum in magnetically-ordered phase stays almost intact upon doping with 7% of Br at T = 0.5 K. This observation is quite counter-intuitive, because larger system couplings should lead to larger optical magnon energies at k = 0 point in the linear spin-wave approximation.…”

“…Moreover, we observe that the ESR spectrum calculated to the first order in 1/S expansion describes well the experimental data of Refs. 21,22 . Next, taking into account the quantum corrections gives rise to nontrivial spectrum dependence on exchange couplings and single-ion anisotropy constant which allows to solve the described above problem in different experiments data interpretation.…”