Softening Effect of Magnetic Excitation on Nuclear Spin-Lattice Relaxation of 133Cs in Singlet-Ground-State Magnet CsFeCl 3

The temperature dependence of the spin-lattice relaxation time T 1 of 133 Cs in CsFeCl 3 was measured from 1.5 to 20 K with external fields up to 7 T applied along the c -axis, and the analysis was performed on the basis of the dynamical correlated-effective-field approximation theory. T 1 -1 below 2 T, which decreases markedly with decreasing temperature, was well interpreted in terms of the longitudinal spin fluctuation characterized by a large energy gap. For higher fields however, T 1 -1 exhibited an almost temperature-independent behavior accompanied by an significant increase below 5 K. The latter feature was ascribed to the transverse spin fluctuation associated with the softening of the lower branch of the excited doublet around the K -point.