Probing and Visualizing Spin-Exchange Interactions Between Two Single Magnetic Molecules Using Scanning Tunneling Microscopy and Spectroscopy*

Over the last decade single-molecule functionalized scanning probe tips have been shown to provide novel sensing capabilities at the nanoscale, such as visualization of chemical structures and bonds as well as electrostatic fields. However, the use of magnetic molecules to sense spin densities and quantify spin-spin interactions has yet to be demonstrated. Here we show that a single S = 1 nickelocene (NiCp2) molecule attached to a scanning tunneling microscope tip can be used to probe spin-spin interactions with other magnetic entities on the surface using inelastic electron tunneling spectroscopy. When the NiCp2-tip is positioned above another NiCp2 molecule, antiferromagnetic superexchange interactions act across the vacuum gap which are sensitive to sub-Ångström scale changes in the intermolecular separation. Using this sensitivity, we demonstrate the ability to image contours of spin-spin interaction strength and provide a means to visualize spin density with a molecular probe tip. Our results extend the capabilities of molecule-functionalized tips to the detection of continuously-tunable magnetic interactions with other objects in the local environment.