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