Tunneling magnetoresistance and exchange interaction in single-atom contacts

The tunneling magnetoresistance of single-atom junctions is shown to depend on the electrode separation owing to exchange forces and the resulting geometric relaxations. An analytical model is proposed to extract relaxations from the magnetoresistances measured with a scanning tunneling microscope. Exchange forces and relaxations calculated within density functional theory demonstrate the validity of the model for a range of distances between tip and sample which extends from tunneling close to the point of maximal attraction.