Magnetic coupling in constructed arrays of lanthanide atoms

Lanthanide atoms show large magnetic moments, long magnetic relaxation times, and a rich variety of spin excitations because of their protected 4f electrons. We recently measured spin excitations in individual lanthanide atoms in a low-temperature STM by using inelastic electron tunneling spectroscopy (IETS) and electron spin resonance (ESR). The 4f electrons are confined near the core so that their spins are normally isolated from scattering by the environment. In contrast, we found that Sm and Eu atoms adsorbed on a thin insulating film of MgO form a +1 charge state having an open-valence-shell configuration 6s¹ that allows direct electrical access by tunneling electrons for IETS and ESR [1]. We further show that the STM tip provides controlled sliding of these lanthanide adatoms along the MgO film to construct atomically-precise magnetically-coupled chains and arrays. The magnetic interaction between atomic spins varies from weak dipole coupling detected by ESR for distances greater than 1 nm, to exchange couplings of ~1 meV for spacings of a few atomic diameters. For chains of close-packed atoms, the 6s electrons delocalize and display the standing wave patterns of a particle in a 1D box. For chains having odd electron occupation, the unpaired delocalized electron shows Zeeman splitting detected by IETS and by ESR. The combination of strong exchange coupling, delocalized valence electrons, and large unpaired 4f spins provides a new testbed for quantum magnetism of low-dimensional nanostructures.



[1] Czap et al. arXiv:2408.11335 (2024).