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 inner 4f and the outer 6s5d shells 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 the 4f shell. 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 on graphene=Irð111Þ single-atom magnets. Density functional theory calculations of the 6s5d shell spin polarizations and of their intra-atomic exchange constants with the 4f shell yield exchange energies in agreement with the experimental values. These results prove that the description of the spin dynamics in RE considering only the 4f − 5d interaction is oversimplified. A more realistic treatment requires us to consider a multishell intra-atomic exchange in which both 6s and 5d shells are taken into account, with the 4f − 6s contribution possibly prevailing over the 4f − 5d one. 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.

Correlation between Electronic Configuration and Magnetic Stability in Dysprosium Single Atom Magnets

“…Depending on the ligand environment, lanthanide complexes and surface-adsorbed atoms and can be mostly found in either atom-like (4f n ) 4,[11][12][13][14][15] or bulk-like configuration (4f n-1 ) [1][2][3][16][17][18][19][20][21][22][23] . Not only does the 4f occupation determine the magnetic moment and stability of the atom, but it also has an impact on the population of the 6s5d valence orbitals 15,22,24,25 determining the behavior in spin transport measurements 8,[26][27][28] . Hence, understanding how to control the lanthanide atom's electronic configuration is a crucial step to develop novel approaches to design quantum magnets with tailored stability and accessibility of their magnetic states 22,29 .…”

Mapping Orbital-Resolved Magnetism in Single Lanthanide Atoms

“…The charge is mostly transferred from the lanthanide to the states localized at the MgO/Ag(100) interface (Figure e). Such an effect has been quite commonly observed to occur in ultrathin oxide layers on metal surfaces as well as on metal-supported graphene layers . Due to this charge transfer, the atom is left in a formal “Gd 1+ ” charge state.…”

“…In lanthanide elements, the spatial localization of electrons in the 4f orbitals enables strong electron correlations and gives rise to large free-atom like spin and orbital magnetic moments. When surrounded by a suitable ligand environment, lanthanide atoms can exhibit long magnetic lifetime and thereby offer an exquisite platform for information storage at the atomic scale. − In diluted samples, where magnetic interactions are negligible, one can characterize their magnetic properties using spatially averaging techniques, such as superconducting quantum interference devices ,,, and synchrotron-based X-ray spectroscopies. − The latter not only allow probing lanthanide atoms and lanthanide-based molecules localized on supporting surfaces, but also addressing the impact of the surrounding ligand. − , While individual access and manipulation of magnetic states of molecules was demonstrated using spin transport through electro-migrated molecular junctions, , the quantum states of individual surface-adsorbed lanthanide atoms can be sensed using spin-polarized scanning tunneling microscopy. − In these measurement schemes, the contribution of 4f electrons to spin transport is severely limited due to their strong localization. Nevertheless, access to their magnetic states is enabled through their magnetically coupled valence electrons, either localized on the molecular ligand ,, or on the outer orbitals of the atoms. , Therefore, probing the electronic configuration and spin-polarization of the valence orbitals not only allows one to design molecular magnets with multiorbital spin configuration and accessible magnetic states , but also to rationalize the quantum level structure of surface-adsorbed atoms with open valence shells .…”

Exchange scaling of ultrafast angular momentum transfer in 4f antiferromagnets

“…Instead, optical pulses excite the conduction electrons, which mediate the RKKY coupling between the 4 f spins. In equilibrium, RKKY acts as a Heisenberg exchange, with a coupling energy expressed as 5 j ∝ | I | 2 χ , in which χ is the non-local susceptibility of the conduction electrons and I is the on-site exchange integral between the 4 f states and the conduction electrons 6 . The participation of dispersive electronic states renders RKKY exceptionally sensitive to external factors.…”