Towards an understanding of the atomic scale magnetic contrast formation in NC-AFM: a tip material dependent MExFM study on NiO(001).

Recently, magnetic exchange force microscopy (MExFM) was established as a new force microscopy based technique capable of imaging arrangements of magnetic moments with atomic resolution on NiO(001). However, before this final achievement many unsuccessful experiments were performed on this particular sample system, and it is still not entirely clear which factors are important to attain an atomic scale magnetic contrast. Varying the tip's magnetic properties, we investigate the contrast formation on NiO(001). Fe-, Ni- and Gd-coated tips yielded a chemical contrast between Ni and O atoms, but a magnetic signal between Ni atoms with magnetic moments pointing in the opposite direction could only be observed with out-of-plane sensitive Fe-coated tips. Our observations suggest that three factors are crucial to obtain a sufficiently large magnetic signal: (i) a large overlap between the spin-carrying electronic states, (ii) a collinear orientation between the magnetic moments and (iii) a large magnetic moment.