Influence of 4f filling on electronic and magnetic properties of rare earth-Au surface compounds

Abstract

One-atom-thick rare-earth/noble metal (RE-NM) compounds are attractive materials to investigate two-dimensional magnetism, since they are easy to synthesize into a common RE-NM₂ structure with high crystal perfection. Here we perform a comparative study of the GdAu₂, HoAu₂, and YbAu₂ monolayer compounds grown on Au(111). We find the same atomic lattice quality and moiré superlattice periodicity in the three cases, but different electronic properties and magnetism. The YbAu₂ monolayer reveals the characteristic electronic signatures of a mixed-valence configuration in the Yb atom. In contrast, GdAu₂ and HoAu₂ show the trivalent character of the rare-earth and ferromagnetic transitions below 22 K. Yet, the GdAu₂ monolayer has an in-plane magnetic easy-axis, versus the out-of-plane one in HoAu₂. The electronic bands of the two trivalent compounds are very similar, while the divalent YbAu₂ monolayer exhibits different band features. In the latter, a strong 4f–5d hybridization is manifested in neatly resolved avoided crossings near the Fermi level. First principles theory points to a residual presence of empty 4f states, explaining the fluctuating valence of Yb in the YbAu₂ monolayer.