  We present a scanning magnetic force sensor based on an individual
magnet-tipped GaAs nanowire (NW) grown by molecular beam epitaxy. Its magnetic
tip consists of a final segment of single-crystal MnAs formed by sequential
crystallization of the liquid Ga catalyst droplet. We characterize the
mechanical and magnetic properties of such NWs by measuring their flexural
mechanical response in an applied magnetic field. Comparison with numerical
simulations allows the identification of their equilibrium magnetization
configurations, which in some cases include magnetic vortices. To determine a
NW's performance as a magnetic scanning probe, we measure its response to the
field profile of a lithographically patterned current-carrying wire. The NWs'
tiny tips and their high force sensitivity make them promising for imaging weak
magnetic field patterns on the nanometer-scale, as required for mapping
mesoscopic transport and spin textures or in nanometer-scale magnetic
resonance.
