Probing surface and bulk ground states of lanthanides: 4f moment orientation through 4d x-ray absorption spectroscopy

Knowledge of the ground state of lanthanides (Ln) in Ln-based materials is fundamental for understanding their properties, particularly those related to magnetism. In crystalline materials, the magnitude and orientation of $4f$ magnetic moments are often strongly influenced by the crystal electric field (CEF), making them challenging to predict ab initio. This complexity is intensified in low-dimensional systems and nanostructures, where the CEF may vary significantly near surfaces and at interfaces. The related variations of the $4f$ ground state should be probed by methods with adjustable surface sensitivity. Here, we apply x-ray absorption spectroscopy (XAS) at the Ln $4d$ edge, along with modeling of XAS spectra, to probe the $4f$ ground state of Ln atoms both at the surface and in the bulk of ${\mathrm{LnRh}}_{2}{\mathrm{Si}}_{2}$ layered crystals. Our results explicitly reveal significant changes of the $4f$ ground state, which may lead to the reorientation of $4f$ moments at the surface compared to the bulk, driven by alterations of the CEF. We also provide a comprehensive database of calculated Ln $4d$ XAS spectra in numeric format for all lanthanides. Our findings will facilitate studies of Ln-based materials and molecular complexes by enabling comprehensive analysis of XAS data, including those obtained through magnetic linear and circular dichroism techniques, as well as related spectromicroscopy studies.