Critical Speeding-Up of Spin-Relaxation in CdCr2Se4

The critical behavior of magnetic resonance linewidths has been measured at fixed frequencies between 0.7 and 9.6 GHz. At sufficiently low frequencies, the derived Onsager coefficient of the spin-relaxation rate "speeds up" with ${\chi }^{\frac{7}{4}}$ in the exchange critical region ($\chi \ll 1$) and flattens around $\chi =1\mathrm{}$ when reaching the crossover to the dipolar region. These results turn out to be quantitative confirmations of current mode-coupling calculations.