Inelastic Electron Tunneling Spectroscopy of a Mn Dimer

F. Delgado and J. Fernández-Rossier INL-International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga, 4715-310 Braga, Portugal and Departamento de Física Aplicada, Universidad de Alicante, San Vicente del Raspeig, 03690, Spain A scanning tunneling microscope can probe the inelastic spin excitations of single magnetic atoms in a surface via spinip assisted tunneling. A particular and intriguing case is the Mn dimer case. We show here that the existing theories for inelastic transport spectroscopy do not explain the observed spin transitions when both atoms are equally coupled to the scanning tunneling microscope tip and the substrate, the most likely experimental situation. The hyper ne coupling to the nuclear spins is shown to lead to a nite excitation amplitude, but the physical mechanism leading to the large inelastic signal observed is still unknown. We discuss some other alternatives that break the symmetry of the system and allow for larger excitation probabilities.