Effect of Magnetic Soft Mode on ⁸⁷Rb Nuclear Spin-Lattice Relaxation in Singlet-Ground-State System RbFeCl₃

Abstract

The nuclear spin-lattice relaxation time T₁ of ⁸⁷Rb in the singlet-ground-state easy-plane antiferromagnet RbFeCl₃ has been measured at the temperatures above and below T_N≈2.5 K with the external field applied both along and perpendicularly to the hard c-axis. From the analysis based on the dynamical correlated-effective-field approximation, it is found that the trend of the divergence of T₁⁻¹ in the paramagnetic phase around T_N is closely related to the remarkable softening of the magnetic excitation for the K-point mode (q=q₀) via the in-plane component of the static susceptibility χ(q₀).