Spin-contrast in non-contact SFM on oxide surfaces: theoretical modelling of NiO(0 0 1) surface

Low-temperature non-contact scanning force microscopy (NC-SFM) is now being used to try and measure the exchange force with metal tips on magnetic oxide surfaces. This demanding test of NC-SFM has yet to prove successful and therefore it is important to use a theoretical model to predict the magnitude and range of the exchange force, and the best way to measure it. In this study, a simple model of a spin-polarized tip has been used to measure the difference in interaction over opposite spin Ni ions in an ab initio simulation of the NiO(0 0 1) surface. The contribution of the exchange force and the role of ion instabilities in the tip–surface interaction are calculated, and used as an example to predict whether spin-contrast over ions should be observable in NC-SFM experiments and how best to achieve this.