Neutron scattering study of magnetic excitations in pseudo-one-dimensional singlet ground state ferromagnets CsFeCl3 and RbFeCl3

The magnetic excitation spectra in pseudo-one-dimensional planar ferromagnets CsFeCl3 and RbFeCl3 were observed using inelastic neutron scattering technique. The observed excitation spectrum of CsFeCl3 has a gap at the magnetic zone center, while that of RbFeCl3 has no gap with a feature of XY system. This prominent difference can be understood from the fact that they are singlet-ground-state systems. As RbFeCl3 has a 3D LRO due to an exchange induced moment, CsFeCl3 shows no sign of the LRO within the temperature region we measured. Moreover, from a viewpoint of the temperature dependence, it is noticeable that the excitation spectrum of RbFeCl3 shows the feature of XY system even in the paramagnetic phase. The one-dimensionality is evaluated to be about 0.05 for CsFeCl3 and 0.1 for RbFeCl3 showing that they have unexpectedly poor one-dimensionality. (author)