Spin excitations of weakly coupled magnetic atoms

Atheoretical description of the measured differential conductance through magnetic atoms on a Cu2N/Cu(100) substrate is presented [Otte et al., Phys. Rev. Lett. 103, 107203 (2009)]. In particular, we analyze the case of a weakly coupled Co/Fe dimer. The starting point of our model is an ionic Hamiltonian which describes the inelastic electron tunneling excitations and the Kondo resonances as due to atomic spin fluctuations associated with electron cotunneling processes in the tip-atom-surface system. The interaction terms of this Hamiltonian are written in the basis set of the eigenstates of the atomic part, which in the present case includes the crystalline and Zeeman fields and also, a Heisenberg exchange coupling between the two spins. The appropriate Green's functions that define the differential conductance spectra are calculated by means of the equation-of-motion method. We obtain, in this form, a very satisfactory description of the overall experimental findings related to the behavior of the spin state excitations and Kondo resonance structures in the presence of an applied magnetic field.