Spin dynamics in ordered phases of anisotropic triangular-lattice antiferromagnet Cs2CoBr4

TL;DR

This study investigates the complex spin dynamics of Cs2CoBr4 using ESR and theoretical models, revealing multiple magnetic excitations influenced by frustration, anisotropy, and dimensionality, with implications for understanding low-dimensional quantum magnets.

Contribution

The paper provides a combined experimental and theoretical analysis of spin excitations in Cs2CoBr4, highlighting the coexistence of 2D and 1D magnetic behaviors in an ordered phase.

Findings

Up to seven ESR branches observed in different magnetic phases.

Low-energy excitations well described by quasiparticle spectra.

High-energy modes interpreted as bound states of domain walls.

Abstract

We study spin dynamics of ordered phases of Cs2CoBr4 in a magnetic field using electron spin resonance (ESR) technique and theoretical analysis. This material hosts weakly interacting distorted-triangular-lattice planes of spin-3/2 Co(2+) ions which can be viewed as spin chains coupled by frustrating interactions. Strong single-ion anisotropy allows to describe the low-energy spin dynamics of this system by an effective strongly anisotropic pseudospin-1/2 model. Our ESR data show up to seven branches of magnetic resonance in four magnetic phases arising due to subtle interplay of frustration, low dimensionality and strong anisotropy. In particular, in the low-field collinear stripe phase, the field evolution of modes lying below 200 GHz is described reasonably good by spectra of spin-1 and spin-0 quasiparticles which we obtain using the bond-operator technique. These well-defined excitations can be treated as conventional magnons and bound states of two magnons, respectively. In contrast, numerous excitations lying above 200 GHz are not captured by our theory due to pronounced one-dimensional correlations inside spin chains which govern the spin dynamics at high enough energies. As it was shown before, these modes can be most naturally interpreted as bound states of domain walls in individual chains and their sequence resembles the so-called "Zeeman ladder" in anisotropic Ising-like spin chains. Thus, Cs2CoBr4 is a system showing spin-dynamics in ordered state characteristic of both two-dimensional and one-dimensional magnets.