  Scanning-probe and wide-field magnetic microscopes based on Nitrogen-Vacancy
(NV) centers in diamond have enabled remarkable advances in the study of
biology and materials, but each method has drawbacks. Here, we implement an
alternative method for nanoscale magnetic microscopy based on optical control
of the charge state of NV centers in a dense layer near the diamond surface. By
combining a donut-beam super-resolution technique with optically detected
magnetic resonance spectroscopy, we imaged the magnetic fields produced by
single 30-nm iron-oxide nanoparticles. The magnetic microscope has a lateral
spatial resolution of ~100 nm, and it resolves the individual magnetic dipole
features from clusters of nanoparticles with interparticle spacings down to
~190 nm. The magnetic feature amplitudes are more than an order of magnitude
larger than those obtained by confocal magnetic microscopy due to the smaller
characteristic NV-nanoparticle distance within nearby sensing voxels. We
analyze the magnetic point-spread function and sensitivity as a function of the
microscope's spatial resolution and identify sources of background fluorescence
that limit the present performance, including diamond second-order Raman
emission and imperfect NV charge-state control. Our method, which uses less
than 10 mW laser power and can be parallelized by patterned illumination,
introduces a new format for nanoscale magnetic imaging.
