The spatial resolution of imaging magnetometers has benefitted from scanning probe techniques. The requirement that the sample perturbs the scanning probe through a magnetic field external to its volume limits magnetometry to samples with static moments. We propose a magnetometer in which the perturbation is reversed; the probe’s magnetic field generates a response of the sample, which acts back on the probe and changes its energy. For an NV$^-$~spin center in diamond this perturbation changes the fine structure splitting of the spin ground state. Sensitive measurement techniques using coherent detection schemes then permit detection of the magnetic response of paramagnetic and diamagnetic materials, including especially superconductors. This technique can measure the thickness of magnetically dead layers with better than 0.01 nm accuracy.
