  We applied the recently developed technique of force-detected nuclear
magnetic resonance (NMR) to observe 71Ga, 69Ga, and 75As in GaAs. The nuclear
spin-lattice relaxation time is 21$\pm$5 min for 69Ga at $\sim 5$ K and 4.6
Tesla. We have exploited this long relaxation time to first create and then
observe spatially varying nuclear spin polarization within the sample,
demonstrating a new form of contrast for magnetic resonance force microscopy
(MRFM). Such nuclear spin contrast could be used to indirectly image electron
spin polarization in GaAs-based spintronic devices.
