Magnetic anisotropy of heteronuclear dimers in the gas phase and supported on graphene: relativistic density-functional calculations

Abstract

The structural and magnetic properties of mixed PtCo, PtFe, and IrCo dimers in the gas phase and supported on a free-standing graphene layer have been calculated using density-functional theory, both in the scalar-relativistic limit and self-consistently including spin-orbit coupling. The influence of the strong magnetic moments of the 3d atoms on the spin and orbital moments of the 5d atoms, and the influence of the strong spin-orbit coupling contributed by the 5d atom on the orbital moments of the 3d atoms have been studied in detail. The magnetic anisotropy energy is found to depend very sensitively on the nature of the eigenstates in the vicinity of the Fermi level, as determined by band filling, exchange splitting and spin-orbit coupling. The large magnetic anisotropy energy of free PtCo and IrCo dimers relative to the easy direction parallel to the dimer axis is coupled to a strong anisotropy of the orbital magnetic moments of the Co atom for both dimers, and also on the Ir atom in IrCo. In contrast the PtFe dimer shows a weak perpendicular anisotropy and only small spin and orbital anisotropies of opposite sign on the two atoms. For dimers supported on graphene, the strong binding within the dimer and the stronger interaction of the 3d atom with the substrate stabilizes an upright geometry. Spin and orbital moments on the 3d atom are strongly quenched, but due to the weaker binding within the dimer the properties of the 5d atom are more free-atom-like with increased spin and orbital moments. The changes in the magnetic moment are reflected in the structure of the electronic eigenstates near the Fermi level, for all three dimers the easy magnetic direction is now parallel to the dimer axis and perpendicular to the graphene layer. The already very large magnetic anisotropy energy (MAE) of IrCo is further enhanced by the interaction with the support, the MAE of PtFe changes sign, and that of the PtCo dimer is reduced. These changes are discussed in relation to the relativistic electronic structure of free and supported dimers and it is demonstrated that the existence of a partially occupied quasi-degenerate state at the Fermi level favors the formation of a large magnetic anisotropy.

Cu(1 1 1) supported graphene as a substrate for magnetic dimers with a large magnetic anisotropy: relativistic density-functional calculations

“…IrCo , resulting from a modest spin and a strong orbital anisotropy [8,10]. The MAE of 11.7 meV/PtCo dimer is reduced by 32% compared to the free PtCo, while that for IrCo it is even enhanced from about 35% from 69 meV for the free dimer to 93 meV for the dimer supported on the freestanding graphene [10].…”

“…The interaction with graphene enhances the MAE of IrCo, but reduces that of PtCo. The mechanism determining the MAE is the same for free and graphene-supported IrCo, whereas for PtCo the broadening of the eigenstates due to the interaction with the substrate reduces the influence of the SOC-induced shift of the eigenstates on the MAE [10]. The situation is changed for graphene deposited on a matching metallic surface: for homoatomic dimers the stronger interaction of the adatoms with the substrate stabilizes a flat adsorption configuration, while an upright dimer is only metastable.…”

“…The local spin moments are of the same order of magnitude as in the homoatomic Pt 2 and Co 2 dimers, but the orbital moment is strongly reduced on Pt and increased on the Co atom. Details of the structural, energetic and magnetic properties of heteroatomic dimers in the gas phase and on a freestanding graphene layer may be found in [10].…”

“…For the supported dimers the essential point is the stable adsorption geometry-a large MAE can be achieved only for a dimer perpendicular to the substrate. On a freestanding graphene layer, an upright adsorption geometry is stable for both homo-and heteroatomic dimers, because the interaction between the metallic atoms is stronger than their binding to the support [6,10]. However, while graphene-supported Pt 2 remains magnetic, with spin and orbital moments only reduced compared to the free dimer, an upright Ir 2 dimer on graphene is non-magnetic.…”

Giant magnetic anisotropy of heavyp-elements on high-symmetry substrates: a new paradigm for supported nanostructures

“…The outermost atom of these vertical dimer could generate a giant MAE as large as 100 meV and might be increased to 150 meV with the tuning of external electrical fields. These discoveries lead to a new research field of building magnetic recording devices at the atomic scale [43][44][45][46][47][48][49][50][51][52][53].…”