Nuclear magnetic resonance studies of pure and samarium-doped CaF2 crystals

Abstract

The spin-lattice relaxation time and transverse relaxation time of fluorine nuclei in single crystals of calcium fluoride, pure and doped with different amounts of samarium, were investigated over a temperature range from 77ºK to approximately 1300ºK. These crystals were investigated with the magnetic field along the [100] and [111] directions; however, most of the conclusions were based on the data taken with the magnetic field along the [111] direction. The pulse technique was used in this investigation. At room temperature the zero crossing of the NMR induction signal was investigated over a 200 degree rotation of the crystal axis. It was found that the zero crossing is quite distinct and invariant to the degree the sample was magnetized. T his investigation bas shown that after the crystals had been annealed, the spin-lattice relaxation time was inversely proportional, within two per cent, to the number of samarium impurities; before annealing there was a 73 percent deviation from this proportionality. This discrepancy was attributed to a nonuniform distribution of impurities. The high temperature results have indicated quite conclusively that the spin-lattice relaxation at high temperatures was primarily due to translational diffusion mechanisms. The entire predicted curve was obtained. From these date the jump energies, the energy required tor formation of vacancies and the atomic vibration frequencies were obtained. For the pure sample E_j = 0.36 +- 0.06 ev, E_f = 3.88 +- 0.90 ev and v_0 = 0.70 x 10^10 sec^-1. For the sample doped with samarium E_j = 0.53 +- 0.05 ev, E_f = 3.72 +- 1.00 ev and v_0 = 0.423 x 10^11 sec^-1. The difference in jump energies between the pure sample and the doped sample was attributed to different diffusion mechanisms. In the pure sample the diffusion was probably due to vacancies of lattice sites, and in the doped sample interstitial fluorine ions were probably diffusing. The temperature at which the plot of T_1 vs 1/kT was a minimum was the same tor the pure and doped crystals. The low temperature investigation of the sample doped with samarium indicated that the spin-lattice relaxation times of the fluorine nuclei were directly proportional to the absolute temperature. This indicated that the central Larmor frequency w_c of the fluorine times the samarium electronic correlation time t_e was, in this temperature range, much smaller than one. From these date it was estimated that the samarium electronic relaxation rate was at room temperature approximately 0.5 x 10^-14 sec, end was proportional to T^-4.