Dynamical critical slowing down in CsNiF3

Dynamical critical slowing down in CsNiF₃ is studied using the a.c. susceptibility measurements at 9.5 GHz in zero external static magnetic field. The dynamical critical exponent for the relaxation time of the in-plane spin fluctuations is obtained for the temperature interval 6 K < T < 20 K. For this temperature interval where one-dimensional spin fluctuations are dominant, very good qualitative and quantitative agreement with the spinwave calculation of the dynamical response is obtained at long wavelength and low frequency. The dynamical critical exponent for the relaxation time is measured to be 0.96 ± 0.6. At higher temperature, a gradual crossover to an isotropic Heisenberg chain behaviour is observed. For temperatures close to the 3-d antiferromagnetic ordering temperature T_N, a crossover to 3-d fluctuation regime gives rise to a speeding-up of the spin relaxation rate.