Complex Magnetic Exchange Coupling between Co Nanostructures and Ni(111) across Epitaxial Graphene

Abstract

We report on the magnetic coupling between isolated Co atoms as well as small Co islands and Ni(111) mediated by an epitaxial graphene layer. X-ray magnetic circular dichroism and scanning tunneling microscopy combined with density functional theory calculations reveal that Co atoms occupy two distinct adsorption sites, with different magnetic coupling to the underlying Ni(111) surface. We further report a transition from an antiferromagnetic to a ferromagnetic coupling with increasing Co cluster size. Our results highlight the extreme sensitivity of the exchange interaction mediated by graphene to the adsorption site and to the in-plane coordination of the magnetic atoms.

Graphene-based synthetic antiferromagnets and ferrimagnets

“…2 ). This confirms a direct role of the graphene 2D-layer in sustaining AF superexchange-coupling between the two magnetic films, in line with earlier propositions by Hermanns et al 22 , Barla et al 23 , or Yang et al 21 among others. Indeed, for the Co–Fe interlayer distances obtained, the coupling becomes FM in absence of graphene, and reduces by an order of magnitude.…”

“…In-plane AFC through a graphene layer was early observed between a Ni thin-film and Co-porphyrin molecules at temperatures below ~200 K by Hermanns et al 22 , as well as in small-area MTJ devices with a graphene tunnel barrier and ferromagnetic metal electrodes by Li et al 16 . More recently, graphene-mediated AFC between magnetic bulk single crystals and magnetic adatoms was studied, and it was shown to weaken or evolve onto a complex coupling when adatoms assemble to form small clusters 23 . Under these conditions, up-scaling for realistic applications is challenging.…”

Large effect of metal substrate on magnetic anisotropy of Co on hexagonal boron nitride

“…Both calculations represent the data well and reveal a ground state configuration with mainly 3d 8 character on both surfaces. This predisposition to acquire an extra electron in the 3d shell is frequently observed for Co atoms adsorbed on surfaces with low electron density at the Fermi level such as alkali metals [36], graphene [10,11,37] or MgO [1].…”

Magnetic decoupling of ferromagnetic metals through a graphene spacer

“…By recent X-ray magnetic circular dichroism (XMCD) measurements [7] on FM single atoms or islands deposited on G/Ni at T<5K, we observed a complex behavior of G in transmitting the exchange interactions through the perpendicular direction. Combined experimental and theoretical results have shown that the sign of the magnetic coupling depends on the adsorption sites for isolated atoms and varies in a complex way for dimers and larger aggregates.…”