  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.
