Dynamic Polarization and Relaxation of 75As Nuclei in Silicon at High   Magnetic Field and Low Temperature
  We present the results of experiments on dynamic nuclear polarization and relaxation of 75As in silicon crystals. Experiments are performed in strong magnetic fields of 4.6 T and temperatures below 1 K. At these conditions donor electron spins are fully polarized, and the allowed and forbidden ESR transitions are well resolved. We demonstrate effective nuclear polarization of 75As nuclei via the Overhauser effect on the time scale of several hundred seconds. Excitation of the forbidden transitions leads to a polarization through the solid effect. The relaxation rate of donor nuclei has strong temperature dependence characteristic of Orbach process. 
J. J\"arvinen, J. Ahokas, S. Sheludyakov, O. Vainio, L. Lehtonen, S.   Vasiliev, D. Zvezdov, L. Vlasenko
cond-mat.mtrl-sci