Pulsed-NMR Study of Protonic Relaxation in Paramagnetic and Antiferromagnetic CoCl2· 6H2O

Protonic spin-lattice relaxation times have been measured in crystalline Co${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O in the temperature range 4.2-1.24°K, which includes the Néel transition region. Spin-echo experiments have also been carried out over this temperature range to determine the homogeneous transverse relaxation time $T_2$ and the extent of the inhomogeneous broadening. All experiments in this series were performed with the position of the external field along the intermediate anisotropy axis ($b$). In the paramagnetic region, the spin-lattice relaxation time $T_1$ and $T_2$ became increasingly temperature-dependent below 3°K, changing most rapidly in the range 2.30-2.20°K. Moriya has pointed out that such rapid fluctuations in relaxation rates are to be associated with critical fluctuations in the electronic spin system. Below the Néel temperature $T_N$, the protonic $T_1$ is found to have a very strong temperature dependence. Below 2°K, $T_1\propto T^{-7\mathrm{}}$. The observed temperature dependence of $T_1$ in the antiferromagnetic state is in accord with theoretical predictions based on a magnetoelastic coupling between magnon and phonon modes. $T_2$ exhibits much weaker temperature dependence below $T_N$.