Abstract
We present the first experimental determination of the intra-atomic exchange energy between the inner and the outer 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 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 single-atom magnets. Density functional theory calculations of the shell spin polarizations and of their intra-atomic exchange constants with the 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 interaction is oversimplified. A more realistic treatment requires us to consider a multishell intra-atomic exchange in which both and shells are taken into account, with the contribution possibly prevailing over the 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.
- Received 29 January 2020
- Revised 5 June 2020
- Accepted 23 July 2020
DOI:https://doi.org/10.1103/PhysRevX.10.031054
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular 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-localized orbitals and the spatially extended and orbitals. 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 the spin moment, as expected for the exchange interaction. Our results also demonstrate that the and shells 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 the magnetic moment, is expected to play a key role in these dynamics.
