Measuring the Intra-Atomic Exchange Energy in Rare-Earth Adatoms

Abstract

We present the first experimental determination of the intra-atomic exchange energy between the inner4fand the outer6s5dshells in rare-earth elements. Inelastic electron tunneling spectroscopy on individual rare-earth atoms adsorbed on metal-supported graphene reveals an element-dependent excitation, with energy between 30 and 170 meV, linearly increasing with the spin angular momentum of the4fshell. This observation is possible owing to the strong spin polarization of the outer shells, characteristic of rare-earth adatoms on graphene. This polarization gives rise to a giant magnetoresistance of up to 75% observed for Dy ongraphene/Ir(111)single-atom magnets. Density functional theory calculations of the6s5dshell spin polarizations and of their intra-atomic exchange constants with the4fshell yield exchange energies in agreement with the experimental values. These results prove that the description of the spin dynamics in RE considering only the4f−5dinteraction is oversimplified. A more realistic treatment requires us to consider a multishell intra-atomic exchange in which both6sand5dshells are taken into account, with the4f−6scontribution possibly prevailing over the4f−5done. Our findings are important for the general understanding of magnetism in rare earths, whether they are in bulk compounds or as surface adsorbed atoms and clusters. The results presented here also push for a revision of the description of the spin dynamics in rare-earth-based systems.

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Plain Language Summary

Magnetic order in solids arises from parallel or antiparallel alignment of the spin magnetic moments among adjacent atoms. While this interaction is interatomic, there is also a similar interaction among electrons in different orbitals within the same atom. This exchange energy is important in the strongest known magnets, such as SmCo and NdFeB alloys, but researchers have not been able to measure it until now. Here, we present the first measurements of intra-atomic exchange energies in rare-earth atoms.

In rare-earth atoms, intra-atomic exchange energy acts between the well-localized4forbitals and the spatially extended5dand6sorbitals. In our experiments, we measure this energy by probing individual rare-earth atoms adsorbed onto graphene with a scanning tunneling microscope. The observations reveal prominent inelastic conductance steps at energies that vary linearly with the4fspin moment, as expected for the exchange interaction. Our results also demonstrate that the5dand6sshells are spin polarized, which allows us to read the magnetization of the individual atoms by means of tunnel magnetoresistance.

We shed new light on rare-earth-based magnetic materials as well as surface-supported nanomagnets down to the single-atom size limit. Recent experiments have shown that some atom-surface combinations and single-ion molecules have long spin lifetimes, thus they can be considered as prototypes for single-atom bits. The mechanisms responsible for the magnetization dynamics in these systems are still debated. In this respect, our findings are timely, since the intra-atomic exchange interaction, coupling the spin-polarized outer shells to the4fmagnetic moment, is expected to play a key role in these dynamics.