Revealing the exchange coupling of Lanthanide atoms for tailored atomic-scale spin structures.

Lanthanide atoms at surfaces with highly localized 4f electrons are promising candidates to realize atomic-scale spin qubits architectures. Their strong spin–orbit coupling and magnetic anisotropy can lead to robust magnetic moments and long spin lifetimes [1]. To optimize their quantum coherence and realize surface-based spin qubits, it is crucial to understand their electronic configurations and the interactions between open shell valence spins and substrates. While late lanthanides on MgO/Ag (100) exhibit closed outer shells, early lanthanides such as Eu and Sm possess an additional open 6s shell, introducing an extra angular momentum that couples with the 4f spin and orbital moments and strongly affects magnetic properties [2]. Here, we employ multi-momenta spin Hamiltonian approach to model the intra- and inter-atomic couplings in single lanthanide atoms and dimers on MgO, by fitting scanning tunnelling microscopy spin excitations and electron spin resonance data using Bayesian optimization. Our model reveals antiferromagnetic coupling between Sm dimers with nearly isotropic exchange, demonstrating the capability of such lanthanide dimers to serve as building blocks for tailored atomic-scale spin architectures.

[1] Donati F., et al, Science 2016, 352, 318-321

[2] Czap G., Noh K., et al, ACS Nano 2025, 19, 3705-3713