Measurement of Magnetic Anisotropy for Individual Atomic Spins on Surfaces

We measure the effects of magnetic anisotropy on individual magnetic atoms on a thin-insulating surface. Using the inelastic electron tunneling spectroscopy capabilities of a scanning tunneling microscope, we probe the spin excitation spectra of Mn and Fe atoms adsorbed on a single copper nitride layer. Magnetic anisotropy is directly manifested as finite-energy spin excitations that exist even in the absence of a magnetic field. The effects of anisotropy are found to be relatively weak for Mn atoms but are substantially larger for Fe atoms, in which spin-orbit coupling is prominent. When a magnetic field is applied to the Fe atoms, the spin excitations shift in a manner that is strongly dependent on the direction of the applied field. These shifts in energy can be understood both qualitatively and quantitatively with a Hamiltonian containing in-plane and out-of-plane magnetic anisotropies.