Spin-lattice relaxation below 1 K: A new mechanism for unexpected nuclear spin relaxation

We propose a physical mechanism for the unexpectedly rapid relaxation of the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei in ${\mathrm{CaF}}_2$ at millikelvin temperatures. The existence of antiferromagnetically coupled impurity dimers possessing two nearly degenerate electronic levels is postulated. Thermal relaxation between these two levels provides the oscillating fields needed to relax the nuclei. We find that this mechanism couples in a nonlinear fashion the magnetic field and impurity concentration dependence of the relaxation rate. Order-of-magnitude estimates of the relaxation rates are given which are in reasonable agreement with experimental values and predict an approximate linear dependence on impurity concentration.