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We investigate the dynamically polarized nuclear-spin system inFe/\emph{n}-GaAs heterostructures using the response of the electron-spinsystem to nuclear magnetic resonance (NMR) in lateral spin-valve devices. Thehyperfine interaction is known to act more strongly on donor-bound electronstates than on those in the conduction band. We provide a quantitative model ofthe temperature dependence of the occupation of donor sites. With this model wecalculate the ratios of the hyperfine and quadrupolar nuclear relaxation ratesof each isotope. For all temperatures measured, quadrupolar relaxation limitsthe spatial extent of nuclear spin-polarization to within a Bohr radius of thedonor sites and is directly responsible for the isotope dependence of themeasured NMR signal amplitude. The hyperfine interaction is also responsiblefor the $2\text{ kHz}$ Knight shift of the nuclear resonance frequency that ismeasured as a function of the electron spin accumulation. The Knight shift isshown to provide a measurement of the electron spin-polarization that agreesqualitatively with standard spin transport measurements.

Knight shift and nuclear spin relaxation in Fe/n-GaAs heterostructures