Phase diagram and magnons in quasi-one-dimensional dipolar antiferromagnets

We investigate antiferromagnetic spin chains, which are coupled by a weak antiferromagnetic exchange interaction. The spins are located on a hexagonal lattice, i.e., frustration is present when three-dimensional order sets in. Typical realizations of such systems are the halides ABX3. In this work we particularly study the role of the long-range dipolar interaction within the framework of a Heisenberg model with nearest-neighbor exchange and additional dipolar interaction. We perform a classical ground-state analysis and show that the spin configuration is sensitively dependent on κ′, the ratio of the dipolar interaction to the interchain interaction, as a consequence of their competing character. Several commensurate and incommensurate phases arise in the different regions of the parameter space. The ground-state investigations are supplemented by a stability analysis by means of a linear spin-wave calculation. From the magnon spectra we can show that all commensurate phases are stable against fluctuations. In comparison with experiments (CsMnBr3, RbMnBr3) we obtain good agreement for the energy gaps. From this we conclude that the dipolar interaction is the most important source of anisotropy in these Mn compounds.