Effects of Magnetic Field on Magnetic Excitations in Singlet-Ground-State Hexagonal Magnet RbFeCl₃

Abstract

Magnetic excitations in the field-induced ferromagnetic states of hexagonal singlet-ground-state magnet RbFeCl₃ are studied on the basis of the dynamical correlated-effective-field theory. It is shown that transition to a fan state (H⊥c) or to a cone state (H⁄⁄c) with decreasing field and/or temperature is driven by softening of a magnon mode at the K point in the Brillouin zone. The obtained softening behavior of the K magnon mode can explain qualitatively the temperature and field dependences of the NMR relaxation time T₁ of Rb observed by Goto et al. It is also shown that the magnon mode originating from the single-ion excitation between the single-ion excited states for H⊥c can be observable by inelastic neutron scattering measurements for moderately high temperatures and strong fields.