We present a new approach to infer the surface density of magnetic moments
Is in ultrathin ferromagnetic films with perpendicular anisotropy. It relies
on quantitative stray field measurements with an atomic-size magnetometer based
on the nitrogen-vacancy center in diamond. The method is applied to
microstructures patterned in a 1-nm-thick film of CoFeB. We report measurements
of Is with a few percent uncertainty and a spatial resolution in the range
of (100 nm)2, an improvement by several orders of magnitude over existing
methods. As an example of application, we measure the modifications of Is
induced by local irradiation with He+ ions in an ultrathin ferromagnetic
wire. This method offers a new route to study variations of magnetic properties
at the nanoscale.Comment: 9 pages and 7 figures including main text and Supplemental
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