Dynamic nuclear polarization in samarium-doped lanthanum magnesium nitrate

We report theoretical and experimental investigations of the dynamic nuclear polarization (DNP) and nuclear spin-lattice relaxation of protons in single crystals of lanthanum magnesium nitrate (LMN) weakly doped (≅0.1-1.1-mole%) with samarium ions (LMN:Sm) at 9.1 GHz and liquid-helium temperatures. Theoretical expressions for DNP and nuclear relaxation are given for a homogeneously broadened ESR line in the high-temperature limit. DNP effects due to the electron dipole-dipole reservoir (EDDR) are predicted for the case of well-resolved satellites. Our DNP experiments on LMN:Sm are in general agreement with the conventional theory, but no evidence for direct EDDR effects on DNP with well-resolved satellites is seen, despite suggestive nuclear-relaxation data. Various explanations for this disagreement are proposed. We suggest that these EDDR effects may be seen in crystals of ≅3-5-mole% LMN:Sm or in equivalent systems.