Spin-lattice relaxation processes in pure and gadolinium-doped SrF2 at low and high temperatures

Abstract

The nuclear spin-lattice relaxation time, T1, of F19 nuclei in single crystals of SrF2, pure and doped with 0.2 mole % trivalent gadolinium, was measured over a temperature range from 300°K to above 1000°K. T1 measurements were also made in two SrF2 crystals doped with 0.01 and 0.05 mole % trivalent gadolinium from 4.2°K to approximately 900°K. The electronic spin-lattice relaxation time, tau_e of gadolinium in the 0.01% sample was measured from 12°K to 80°K. NMR measurements above 300°K were made using the pulsed technique at 26.8 MHz; below 300°K they were made at 14 MHz. (3500 G), EPR measurements were made at 9.5 GHz. (3400 G) using the saturation technique. The magnetic field was along the [111] crystallographic direction. The high temperature results indicate that the nuclear relaxation is due to translational diffusion of F19 in the presence of paramagnetic impurities. Activation energies obtained from the measurements yield a value of 2.18eV for the formation energy of an anti-Frenkel defect which agrees well with values obtained by ionic conductivity measurements. The jump frequency obtained from pure sample measurements is v_f(pure) = 3.17 X 10^17exp(+2.08eV/kT) and is believed to represent the jump frequency of a fluorine vacancy. For the doped samples, v_f(doped) ~= 4 x 10^13exp(+1.0eV/kT) and is believed to represent the jump frequency of fluorine ion interstitials. The low temperature measurements indicate that the nuclear relaxation mechanism is due to the paramagnetic impurities. The analysis of the data indicates that the transition from diffusion limited to rapid diffusion relaxation occurs around 25°K. Values of tau_e calculated from the experimental values of T1 agree reasonably well with the experimental values. The spin-spin diffusion constant and the barrier radius are found to be 4.3 x 10^-11cm^2/sec and 1.46Å respectively. The temperature dependence of tau_e of Gd3+ in SrF2 from 25°K to 80°K is T^-2.14 which does not agree with the reported T^-5 temperature dependence of tau_e of Gd3+ in CaF2.