Magnetic properties of rare earth single atoms on metal substrates

The interaction of individual rare earth (RE) atoms with single-crystal surfaces leads to magnetic ground and excited states that determine their magnetic properties, e.g., magnetic relaxation time, total magnetic moment, zero-field splitting, and magnetic anisotropy energy. We present a systematic study of several RE elements (Dy, Ho, Er, and Tm) on different metal surfaces (Pt(111), Cu(111), Ag(100), and Ag(111)). Using x-ray absorption spectroscopy and magnetic circular dichroism we reveal two $4f$ configurations, i.e., $4f^{n}$ and $4f^{n-1}$, where $n$ corresponds to the free atom occupation. We identify two factors governing the valency of these adatoms: (a) the ionization potential of the $4f$ elements and (b) the substrate density of states at the Fermi level. Magnetization loops at $2.5$ K reveal that all RE adatoms are paramagnetic, i.e., their magnetic relaxation is faster than about $10$ s. Comparison of our experimental spectra with multiplet calculations identify the role of the crystal field in determining the magnetic quantum levels of RE adatoms.