Relaxation and exchange in a nuclear cooling agent: NMR and EPR in single crystal PrNi5

NMR of Pr and ESR of Gd in single crystals of PrNi5 are observed to exhibit a dramatic increase of the resonance linewidth with temperature in the range 1.2 K =< T =< 10 K. The observations are interpreted, within the framework of a crystal field spin Hamiltonian model calculation, in terms of low frequency fluctuations of the Pr ions induced by Pr-Pr exchange coupling. A fitting procedure of the theory to the experimental data was used to extract the various Pr-Pr and Pr-Gd exchange parameters. The former yield independent information concerning the lowest temperature that can be achieved in nuclear cooling experiments. (author)