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

Abstract

This paper is concerned with a detailed investigation of the dynamic polarization of the protons in ${(\mathrm{C}\mathrm{e},\mathrm{L}\mathrm{a})}_{2}$${\mathrm{Mg}}_{3}$${(\mathrm{N}{\mathrm{O}}_{3})}_{12}$\ifmmode\cdot\else\textperiodcentered\fi{}24${\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 ${T}_{1e}\ensuremath{\propto}{T}^{\ensuremath{-}14\ifmmode\pm\else\textpm\fi{}2}$ for temperatures in the range $1.9\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}<T<2.7\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}$. In the same crystals, the proton relaxation time ${T}_{1n}$ is also measured by a transient method and found to be ${T}_{1n}\ensuremath{\propto}{T}^{\ensuremath{-}7}$ 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 $T\ensuremath{\approx}1.5\ifmmode^\circ\else\textdegree\fi{}$K and a microwave frequency ${\ensuremath{\nu}}_{e}\ensuremath{\approx}9.3$ 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 (${\ensuremath{\nu}}_{e}\ensuremath{\approx}50$ kMc/sec) it should be possible to obtain in this crystal sufficient proton polarization (\ensuremath{\sim}25%) to be useful for dynamic nuclear cooling experiments and nuclear targets.