Atomic-scale spin-polarization maps using functionalized superconducting probes

A scanning tunneling microscope (STM) with a magnetic tip that has a sufficiently strong spin-polarization can be used to map the sample's spin structure down to the atomic scale but usually lacks the possibility to absolutely determine the value of the sample's spin-polarization. Magnetic impurities in superconducting materials give rise to pairs of perfectly, i.e. 100% spin-polarized sub-gap resonances. In this work, we functionalize the apex of a superconducting Nb STM-tip with such impurity states by attaching Fe atoms to probe the spin-polarization of atom-manipulated Mn nanomagnets on a Nb(110) surface. By comparison with spin-polarized STM measurements of the same nanomagnets using Cr bulk tips we demonstrate an extraordinary spin-sensitivity and the possibility to measure the sample's spin-polarization values quantitatively with our new functionalized probes.