  We demonstrate quantitative magnetic field mapping with nanoscale resolution,
by applying a lock-in technique on the electron spin resonance frequency of a
single nitrogen-vacancy defect placed at the apex of an atomic force microscope
tip. In addition, we report an all-optical magnetic imaging technique which is
sensitive to large off-axis magnetic fields, thus extending the operation range
of diamond-based magnetometry. Both techniques are illustrated by using a
magnetic hard disk as a test sample. Owing to the non-perturbing and
quantitative nature of the magnetic probe, this work should open up numerous
perspectives in nanomagnetism and spintronics.
