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 CsFeCl 3 and RbFeCl 3 were observed using inelastic neutron scattering technique. The observed excitation spectruun of CsFeCl 3 has a gap at the magnetic zone center, while that of RbFeCl 3 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 RbFeCl 3 has a 3D LRO due to an exchange induced moment, CsFeCl 3 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 RbFeCl 3 shows the feature of XY system even in the paramagnetic phase. The one-dimensionality is evaluated to be about 0.05 for CsFeCl 3 and 0.1 for RbFeCl 3 showing that they have unexpectedly poor one-dimensionality.