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)→partial disorder (PD)→paramagnetic (P) or TS→ferrimagnetic (FR)→P, depending on the ratio between the two inequivalent exchange couplings. It is shown that a distinct difference between TS→PD and TS→FR transitions appears in the temperature dependence of one of the magnon branches (ω ₃-mode) which originates from the single-ion excitation between the excited states. In case of TS→PD the ω ₃-mode energy softens with increasing temperature and becomes zero at the TS→PD transition temperature, and then remains vanishing in the PD phase. On the other hand, in case of TS→FR the ω ₃-mode energy remains finite in the FR phase. This ω ₃-mode is characteristic to the SGS magnets and can be observable by inelastic neturon scattering.