Magnetic excitations and successive phase transition in RbFeBr₃-type modified triangular antiferromagnets

We study magnetic excitations in RbFeBr₃-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 (ω₃-mode) which originates from the single-ion excitation between the excited states. In case of TS → PD the ω₃-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 ω₃-mode energy remains finite in the FR phase. This ω₃-mode is characteristic to the SGS magnets and can be observable by inelastic neturon scattering.