  Despite decades of advances in magnetic imaging, obtaining direct,
quantitative information with nanometer scale spatial resolution remains an
outstanding challenge. Recently, a new technique has emerged that employs a
single nitrogen-vacancy (NV) defect in diamond as an atomic-size magnetometer.
Although NV magnetometry promises significant advances in magnetic imaging, the
effectiveness of the technique, when applied to realistic magnetic
nanostructures, remains to be demonstrated. Here we use a scanning NV
magnetometer to image a magnetic vortex, which is one of the most iconic object
of nanomagnetism, owing to the small size (~10 nm) of the vortex core. We
report three-dimensional, vectorial, and quantitative measurements of the stray
magnetic field emitted by a vortex in a ferromagnetic square dot, including the
detection of the vortex core. We find excellent agreement with micromagnetic
simulations, both for regular vortex structures and for higher-order
magnetization states. These experiments establish scanning NV magnetometry as a
practical and unique tool for fundamental studies in nanomagnetism.
