  Electron paramagnetic resonance (EPR) performed with a scanning tunneling
microscope (STM) allows for probing the spin excitation of single atomic
species with MHz energy resolution. One of the basic applications of
conventional EPR is the precise determination of magnetic moments. However, in
an STM, the local magnetic fields of the spin-polarized tip can introduce
systematic errors in the measurement of the magnetic moments by EPR. We propose
to solve this issue by finding tip-sample distances at which the EPR resonance
shift caused by the magnetic field of the tip is minimized. To this end, we
measure the dependence of the resonance field on the tip-sample distance at
different radiofrequencies and identify specific distances for which the true
magnetic moment is found. Additionally, we show that the tip's influence can be
averaged out by using magnetically bistable tips, which provide a complementary
method to accurately measure the magnetic moment of surface atoms using
EPR-STM.
