Low-energy magnetic states of Tb adatom on graphene

Electronic structure and magnetic interactions of a Tb adatom on graphene are investigated from first principles using combination of density functional theory and multiconfigurational quantum chemistry techniques including spin-orbit coupling (SOC) . We determine that the six-fold symmetry hollow site is the preferred adsorption site and investigate electronic spectrum for different adatom oxidation states including Tb³⁺, Tb²⁺, Tb¹⁺, and Tb⁰. For all charge states, the Tb4f8configuration is retained with other adatom valence electrons being distributed over5dxy,5dx2+y2, and6s/5d0single-electron orbitals. We find strong intra-site adatom exchange coupling that ensures that the5d6sspins are parallel to the<b>4<i>f</i></b>spin. For Tb³⁺, the energy levels can be described by the<i>J</i> = 6 multiplet split by the graphene crystal field (CF). For other oxidation states, the interaction of<b>4<i>f</i></b>electrons with spin and orbital degrees of freedom of6s5delectrons in the presence of SOC results in the low-energy spectrum composed closely lying effective multiplets that are split by the graphene CF. Stable magnetic moment is predicted for Tb³⁺and Tb²⁺adatoms due to uniaxial magnetic anisotropy and effective anisotropy barrier around 440 cm^-1controlled by the temperature assisted quantum tunneling of magnetization through the third excited doublet. On the other hand, in-plane magnetic anisotropy is found for Tb¹⁺and Tb⁰adatoms. Our results indicate that the occupation of the6s5dorbitals can dramatically affect the magnetic anisotropy and magnetic moment stability of rare earth adatoms.