MAGNETIC EXCITATIONS AND SUCCESSIVE PHASE TRANSITION IN RbFeBr//3-TYPE MODIFIED TRIANGULAR ANTIFERROMAGNETS.

We study magnetic excitations in RbFeBr//3-type singlet-ground-state modified triangular-antiferromagnets (SGS-MTA) in which one third magnetic ions loses the crystallographic equivalence to the others. This SGS-MTA can undergo successive phase transitions, triangular structure (TS) yields partial disorder (PD) yields paramagnetic (P) or TS yields ferrimagnetic (Fr) yields P, depending on the ratio between the two inequivalent exchange couplings. It is shown that a difference between TS yields PD and TS yields FR transitions appears in the temperature dependence of one of the magnon branches ( omega //3-mode) which originates from the single-ion excitation between the excited states. In case of TS yields PD the omega //3-mode energy softens with increasing temperature and becomes zero at the TS yields PD transition temperature and then remains vanishing in the PD phase. In case of TS yields FR the omega //3-mode energy remains finite in the FR phase. This omega //3-mode is characteristic of the SGS magnets and can be observed by inelastic neutron scattering.