Abstract

We present an approach to infer the surface density of magnetic moments I-s 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 I-s 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 I-s induced by local irradiation with He+ ions in an ultrathin ferromagnetic wire. This method offers a route to study variations of magnetic properties at the nanoscale.