Dynamic Polarization of Nuclei by Electron-Nuclear Dipolar Coupling in Crystals

This paper is concerned with a detailed investigation of the dynamic polarization of the protons in ${\mathrm{}(\mathrm{C}\mathrm{e},\mathrm{L}\mathrm{a})\mathrm{}}_2$ ${\mathrm{Mg}}_3$ ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_{12}$·24${\mathrm{H}}_2$O which occurs when one saturates the "forbidden" microwave transitions that simultaneously flip a proton spin and a ${\mathrm{Ce}}^{3+}$ electron spin. The rate equations for the electron and nuclear polarization are solved for (a) a simple ideal model, (b) a model for the case where the forbidden lines are not resolved, and (c) a model taking into account nuclear-spin temperature diffusion. An apparatus for simultaneous observation of proton magnetic resonance and ${\mathrm{Ce}}^{3+}$ paramagnetic resonance at liquid helium temperatures is described. The ${\mathrm{Ce}}^{3+}$ spin-lattice relaxation time $T_{1e}$ is directly measured by a transient method, and it is found that $T_{1e}\propto T^{-14\mathrm{}\pm 2}$ for temperatures in the range $1.9°\mathrm{K}<T<\mathrm{}2.7\mathrm{}°\mathrm{K}$. In the same crystals, the proton relaxation time $T_{1n}$ is also measured by a transient method and found to be $T_{1n}\propto T^{-7\mathrm{}}$ and dependent on the concentration of ${\mathrm{Ce}}^{3+}$ ions. The relative magnitudes of $T_{1n}$ and $T_{1e}$ are best explained by a model intermediate between (a) and (c). At $T\approx 1.5°$K and a microwave frequency ${\nu }_e\approx 9.3$ kMc/sec, the proton polarization is observed for a number of different concentrations of ${\mathrm{Ce}}^{3+}$. The magnitude of the polarization, its dependence on magnetic field and microwave power, and the transient behavior are studied and qualitatively explained. In a crystal containing 1% Ce, the proton polarization is observed to become greater than the thermal equilibrium value by the factor 150, which is about one-quarter of the theoretical ideal. At higher microwave frequencies (${\nu }_e\approx 50$ kMc/sec) it should be possible to obtain in this crystal sufficient proton polarization (∼25%) to be useful for dynamic nuclear cooling experiments and nuclear targets.