4f occupancy and magnetism of rare-earth atoms adsorbed on metal substrates

Abstract

We report x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements as well as multiplet calculations for Dy, Ho, Er, and Tm atoms adsorbed on Pt(111), Cu(111), Ag(100), and Ag(111). In the gas phase, all four elements are divalent and we label their 4f occupancy as 4f n . Upon surface adsorption, and depending on the substrate, the atoms either remain in that state or become trivalent with 4f n−1 configuration. The trivalent state is realized when the sum of the atomic correction energies (4f →5d promotion energy E f d + intershell coupling energy δE c ) is low and the surface binding energy is large. The latter correlates with a high substrate density of states at the Fermi level. The magnetocrystalline anisotropy of trivalent RE atoms is larger than the one of divalent RE atoms. We ascribe this to the significantly smaller covalent radius of the trivalent state compared to the divalent one for a given RE element. For a given valency of the RE atom, the anisotropy is determined by the overlap between the spd states of the RE and the d states of the surface. For all investigated systems, the magnetization curves recorded at 2.5 K show absence of hysteresis indicating that magnetic relaxation is faster than about 10 s.

Magnetic properties of single rare-earth atoms on graphene/Ir(111)

“…1(a) and 1(b)]. This peak indicates the presence of Dy 4f 9 occupancy [39] and can be associated with the formation of Dy dimers. To verify this assignment, and to further investigate the influence of dimers and bigger clusters on the magnetic properties of the ensemble, we performed combined coverage dependent XAS and STM measurements.…”

“…The increasing intensity of peak B and of its tail also result in the shift of peak A toward higher energies. From this coverage dependence we attribute the peak B to the formation of Dy dimers and bigger clusters, and the corresponding change in the valency to the increased coordination of the Dy atoms [39].…”

“…The surface binding energy strongly depends on the atom's environment as it is ruled by the hybridization between its outer spd orbitals with the valence electrons of the substrate. The degree of hybridization, and hence the binding energy, increases with the increasing substrate density of states (DOS) at E F [39].…”

Direct capture and electrostatic repulsion in the self-assembly of rare-earth atom superlattices on graphene

“…Former examples of self-assembled superlattices of 3d transition metals [14][15][16][17][18] or rare earths [19][20][21][22] used long-range adsorbate-adsorbate interactions mediated by surface-state Friedel oscillations on close-packed metal surfaces. However, for the single-atom-magnet systems mentioned above, direct adsorption onto metal substrates is unsuited since long-lived magnetic quantum states require protection against electron and phonon scattering [12,22,23], explaining why thin MgO or graphene separation layers are needed. Cs atom superlattices were self-assembled on graphene/SiC(0001) [24] and on graphene on Ir(111) [25], thanks to the combined effect of the graphene moiré pattern and repulsive adatom interactions.…”

“…These fascinating properties are unique to the respective atom/substrate combination. Regarding magnetism, Ho is a single-atom magnet when adsorbed on MgO [2], but neither on graphene [10] nor on several metal substrates [11,12]; conversely, Dy shows long spin lifetime when adsorbed on freestandinglike graphene on metals, as for graphene on Ir(111), while it is paramagnetic when placed directly on Ir(111) or other metals, on graphene on Ru(0001), and onto hexagonal boron nitride, h-BN/Rh(111) [3,12].…”