Spin-lattice relaxation of F19 in BaF2 crystals doped with Europium in the laboratory and rotating frames

Abstract

The nuclear spin-lattice relaxation time in the laboratory frame, T1 , and in the rotating frame, T1r , of F19 in BaF2 crystals doped with europium were measured with the magnetic field, H_0, along the [111] crystallographic axis. T1 measurements were made on five samples with concentration of the dopant ranging from 1.9 x 10^20 ions/cm3 to 1.6 x 10^18 ions/cm3 over a temperature range of 4.2°K to 300°K. T1r measurements were made on the sample with 1.9 x 1020 ions/cm3 of europium in three different holding fields of 6.24 gauss, 14.68 gauss and 20.81 gauss over a temperature range of 45 K to 300 K. The minimum in the T1r vs temperature data is found to be at 60ºK, 70ºK and 80ºK corresponding to the three holding fields and yields electronic spin-lattice relaxation time, tau_e. Over this temperature range, tau_e is found to exhibit a temperature dependence of T^-5.2 which compares well with the T^-5 dependence of tau_e obtained from direct EPR measurements by Horak and Nolle and by Huang. The value of tau_e is also computed from the nuclear T1 data for the three samples containing 1.9 x 10^20 ions/ cm3 , 8.29 x 10^19 ions/cm3 and 1.6 x 10^19 ions/cm3 of europium respectively in the temperature range of 30ºK to 300ºK. The average temperature dependence is again found to be T^-5.4. The nuclear spin-spin diffusion constant D is calculated from the data at 60ºK, 70ºK and 80ºK and is found to be temperature independent. The value of D is found to be 9.5 x 10^-15 cm2/sec as compared to the theoretical value of D = 2.64 x 10^-13 cm2/sec obtained from the order of magnitude expression 50 a^2/T2. From the T1r data, it is found that T1r is independent of H1 in the region where w, tau_e<< 1. The H1 dependence of T1r is also investigated at 55ºK and 45ºK. At 55ºK, T1r exhibits a T1r alpha H1^1.04 field dependence which agrees well with the theoretical T1r alpha H1^1.0 dependence for the diffusion vanishing case. The field dependence decreases to T1r alpha H1^0.7 at 45ºK indicating that the relaxation process gradually approaches the diffusion limited case.