Spin-lattice relaxation processes in pure and europium-doped BaF2 at low- and high-temperatures

Abstract

Measurements of the spin-lattice relaxation time, T1, of F19 nuclei in a nominally pure BaF2 crystal and a BaF2 crystal containing 4.9x10^19 ions/cm3 of primarily Eu3+ as a function of temperature and crystal orientation have been made. The T1 measurements were made at 29 Mc/sec using the NMR pulse technique. The T1 measurements in the pure sample were made over a temperature range 300 to 1150°K with H0 along the [111] and [100] directions. The temperature dependence of T1 in the doped sample was studied over a temperature range of 4.2 to 1000°K with H0 along the [111] direction and additional measurements were made below 77°K at two other orientations. The europium spin-lattice relaxation time was measured at four temperatures between 4.2 and 9 0°K in order to aid in the analysis of the NMR data. The data indicates that the diffusion of the fluorine ions provides the predominant relaxation mechanism above 700°K in the pure crystal and above 500°K in the doped crystal. The minima in the T1 vs. T data appear at 990 ° K tor the pure sample and 880°K for the doped sample. The jump frequencies for the fluorine ions are Vf(pure) = 1.3x10^15exp(-1.35 w/kT) and Vf(doped) = 6.5x10^13exp (-0.62w/kT) . It is believed that Vf(pure) represents the Jump frequency of fluorine vacancies. whereas Vf(doped) represents the jump frequency of fluorine ion interstitials. The T1 measurements below 500°K in the pure sample indicate that the relaxation mechanism is due to the impurities which may be present even in a "pure" crystal. The T1 data in the doped sample indicate that the impurities are providing the relaxation mechanism below 400°K. The orientation data taken below 77°K indicate that the transition from diffusion limited relaxation to rapid diffusion relaxation occurs between 50 and 70°K. The values of T1 predicted by the theory of impurity induced relaxation have been compared with the measured values of T1 at those temperatures where Te has been measured. The calculated values of T1 between 60 and 90°K are about a factor of three smaller than the observed values, whereas the value of T1 calculated at 4.2°K 1s about 300 times larger than the observed value. Since the theories are meant to be order of magnitude only, the calculated values of T1 between 60 and 90°K agree very well with the observed value. The difference in the calculated and measured T1 at 4.2° K cannot be entirely attributed to the inadequacy of the theory. The temperature dependence which is observed tor T1 is T^-0.7 between 10 and 50°K, T^-2.4 between 60 and 125°K, and T^-1.0 between 175 and 360°K.