Zero-Dimensional Magnetic Structures with Correlated Electrons

*Harald Brune
Institute of Physics, EPFL, CH-1015 Lausanne, Switzerland

Zero-dimensional magnetic structures are single magnetic atoms. In order for them to have non-trivial magnetic properties that can be addressed and potentially used, they are adsorbed onto single crystal surfaces. The magnetic eigenstates of their electrons emerge from the crystal field of the adsorption site and from spin-orbit coupling, thus from electron correlations. The stability of the ground state depends on the excited states that can be reached by electron or phonon scattering. Also the nuclear spin states play an important role in the magnetic stability.

Single Ho atoms on MgO(100)/Ag(100) are the smallest and most stable magnets known today. We show recent results on their thermal and magnetic field stability. They yield an onset temperature for magnetization switching of 45 K and an estimated coercitive fields of 60 T. We show that Landau-Zener tunneling through avoided level crossings involving the nuclear spin states can be used to prepare the magnetic states by magnetic field sweeps that are by orders of magnitude smaller than the coercitive field.

Many rare-earth atoms adsorbed on graphene exhibit intense inelastic conductance steps in scanning tunneling spectroscopy. Their origin is an intra-atomic spin-excitation between the 4f and 6s spin-moments. For a given valency, the exchange energy between these moments is largely independent of the 4f element [H. S. Li et al., J. Phys.: Condens. Matter 3, 7277 (1991)] and therefore we observe a linear behavior of excitation energy with filling of 4f levels. Note that this excitation takes place on the 6s, and possibly also 5d, states and not on the 4f levels that are to localized to interact with the tunnel electrons. Due to charge transfer to graphene, the 6s spin moment is high. This leads to giant spin-polarization observed for Dy atoms on graphene grown on Ir(111). The first 4f adatom for which STM spin-contrast was reported is Ho/MgO, in that case the spin-contrast was only 2–4 pm in apparent height. Dy atoms on graphene exhibit up to 60 pm apparent height contrast between the up and down states. We attribute this to Dy transferring almost an entire electron to graphene [M. Pivetta et al., Phys. Rev. B 98, 115417 (2018)], leading to a half-filled 6s shell. Its spin-polarization is therefore close to 100 %, and the projection of the spin onto the out-of-plane axis is stable due to the stability of the Dy 4f states and the exchange coupling to them.