Dynamic nuclear polarization of diamond. I. Solid state and thermal mixing effects

The dynamic nuclear polarization of 13C nuclei in a suite of seven natural type Ia and Ib diamonds, using continuous wave S- and X-band microwave radiation, is described. The 13C signal enhancement and polarization time have been measured for one of the type Ib diamonds as a function of magnetic field in the vicinity of the resonance field. The total paramagnetic impurity concentration (P1 and other centers) in this diamond is 2×1018 cm−3 (23 atomic parts per million), while the concentration of P1 centers is 9.3 ppm. Since the central electron spin resonance (ESR) linewidth HL is comparable with H0γC/γe, flip–flip and flip–flop forbidden transitions take place simultaneously. Consequently thermal mixing plays an important role in the 13C signal enhancement. However, the 13C spin-lattice relaxation rate is determined to a large extent by the solid state effect (forbidden transitions). The 13C polarization rates have been measured for the suite of diamonds by executing dynamic nuclear polarization (DNP) experiments on both hyperfine and central ESR lines. It is shown that the polarization rate is proportional to the paramagnetic impurity concentration of the sample, in agreement with the existing theory. It has been found that in type Ib diamonds with relatively low nitrogen impurity concentrations the dynamic nuclear polarization of a single hyperfine line yields an equilibrium 13C polarization that is one-quarter of that obtained in the case of dynamic nuclear polarization of the central line. In samples containing P1 and P2 centers (type Ia), or in type Ib samples with relatively high concentrations of P1 centers, the same equilibrium 13C polarization is obtained for the DNP of hyperfine and central transitions. This phenomenon is explained in terms of a model in which thermal contact is established between the electron Zeeman reservoir and the nuclear spin reservoirs via the spin–spin interaction reservoir if HL⩾H0γC/γe.