  The magnetic moment of rare earth elements originates from electrons in the
partially filled 4f orbitals. Accessing this moment electrically by scanning
tunneling spectroscopy is hampered by shielding of outer-lying orbitals. Here
we show that we can detect the magnetic moment of an individual Ce atom
adsorbed on a Cu2N ultrathin film on Cu(100) by using a sensor tip that has its
apex functionalized with a Kondo screened spin system. We calibrate the sensor
tip by deliberately coupling it to a well characterized Fe atom. Subsequently,
we use the splitting of the tip's Kondo resonance when approaching a
spectroscopically dark Ce atom to sense its magnetic moment.
