Spin-lattice relaxation of dipolar order in paramagnetic-impurity-doped CaF2

Pulse NMR techniques have been used to study the spin-lattice relaxation of dipolar order in single crystals of Ca${\mathrm{F}}_2$ doped with either ${\mathrm{Mn}}^{2+}$ of ${\mathrm{Ce}}^{3+}$ paramagnetic ions. The spin-lattice relaxation time of dipolar order, $T_1^d$, has been obtained as a function of impurity concentration, temperature, and field orientation. An initial nonexponential dependence in dipolar relaxation was observed. The method of adiabatic demagnetization in the rotating frame, for creating dipolar order, was compared with the pulse method proposed by Jeneer and Broekaert, and differences in the resulting relaxation have been noted. Finally, $T_1^d$ has been compared with the rotating-frame relaxation time $T_1^r$, and no correlation between the two was found.