Crossing and Anticrossing of Exchange-Coupled Molecular Spin Excitation Energy Levels

The superconducting Pb tip of a scanning tunneling microscope (STM) is functionalized with a single nickelocene (Nc) molecule and approached to individual Nc molecules embedded in a molecular island adsorbed on the (111) surface of the conventional superconductor Pb. Excitations of the tip and surface molecular spins that are coupled across the vacuum barrier via the magnetic exchange interaction are explored by inelastic electron tunneling spectroscopy (IETS) from the far tunneling to the contact range of intermolecular distances. Depending on the tilt angle of Nc at the microscope probe, single spin and double spin flip energy levels cross for the straight Nc-terminated tip and exhibit avoided crossing for the tilted configuration. The crossing-to-anticrossing transition of spin excitation energy levels can be reproduced by simulations based on a spin Hamiltonian where the magnetic anisotropy tensor of the Nc tip is rigidly rotated.