Abstract
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. We determine that the six-fold symmetry hollow site is the preferred adsorption site and we investigate electronic spectrum for different adatom oxidation states including Tb3+, Tb2+,Tb1+, and Tb0. For all charge states, the Tb 4f8configuration is retained with other adatom valence electrons being distributed over 5dxy, 5dx2+y2, and 6s/5d0single-electron orbitals. We find strong intra-site adatom exchange coupling that ensures that the 5d6sspins are parallel to the 4fspin. For Tb3+, the energy levels can be described by theJ= 6 multiplet split by the graphene crystal field. For other oxidation states, the interaction of 4felectrons with spin and orbital degrees of freedom of 6s5delectrons in the presence of spin-orbit coupling results in the low-energy spectrum composed closely lying effective multiplets that are split by the graphene crystal field. 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 the 6s5dorbitals can dramatically affect the magnetic anisotropy and magnetic moment stability of rare earth adatoms.
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