Large Perpendicular Magnetic Anisotropy in Nanometer-Thick Epitaxial Graphene/Co/Heavy Metal Heterostructures for Spin–Orbitronics Devices

Abstract

Nanometer-thick epitaxial Co films intercalated between graphene (Gr) and a heavy metal (HM) substrate are promising systems for the development of spin–orbitronic devices due to their large perpendicular magnetic anisotropy (PMA). A combination of theoretical modeling and experiments reveals the origin of the PMA and explains its behavior as a function of the Co thickness. High quality epitaxial Gr/Co n /HM­(111) (HM = Pt,Ir) heterostructures are grown by intercalation below graphene, which acts as a surfactant that kinetically stabilizes the pseudomorphic growth of highly perfect Co face-centered tetragonal (fct) films, with a reduced number of stacking faults as the only structural defect observable by high-resolution scanning transmission electron microscopy (STEM). Magneto-optic Kerr effect (MOKE) measurements show that such heterostructures present PMA up to large Co critical thicknesses of about 4 nm (20 ML) and 2 nm (10 ML) for Pt and Ir substrates, respectively. X-ray magnetic circular dichroism (XMCD) measurements show an inverse power law of the anisotropy of the orbital moment with Co thickness, reflecting its interfacial nature, that changes sign at about the same critical values. First principles calculations show that, regardless of the presence of graphene, ideal Co fct films on HM buffers do not sustain PMAs beyond around 6 mLs due to the in-plane contribution of the inner bulk-like Co layers. The large experimental critical thicknesses sustaining PMA can only be retrieved by the inclusion of structural defects that promote a local hcp stacking such as twin boundaries or stacking faults. Remarkably, a layer resolved analysis of the orbital momentum anisotropy reproduces its interfacial nature, and reveals that the Gr/Co interface contribution is comparable to that of the Co/Pt­(Ir).

Nature of Interfacial Dzyaloshinskii-Moriya Interactions in Graphene/Co/Pt(111) Multilayer Heterostructures

“…In the present work we show that this regime is reached at Co 3 thickness and that the DMI contributions of the stacking defects cancel out. Interestingly, this DMI behaviour contrasts with that of the magnetocrystalline anisotropy of f ct Gr/Co n /Pt(111), where a complex behaviour with Co n thickness is found that depends on the competition between the contributions from the f ct bulk (in-plane) and twin boundary defects (out-of-plane) [35].…”

“…Our interface-resolved DFT study of DMI in Gr/Co n /Pt(111) heterostructures with varying Co layer thickness shows that the regime of additivity of interfacial DMI is reached already at n = 3 atomic planes and also that the D-vectors have an almost negligible outof-plane component. As the perpendicular magnetocrystalline anisotropy prevails at much larger n values [35], a sizable DMI interaction to have spin canting and chiral exchange effects are expected to be robust. However, unlike the magnetocrystalline anisotropy itself, interfacial DMI is insensitive to the internal structure of the Co layer.…”

“…We have used Co layers of one to five atom thicknesses, pseudomorphic on a Pt substrate (lateral periodicity 2.772 Å) of five atomic planes, with the relaxed interplanar spacings found in Ref. [35]. The effect of the substrate thickness on the DMI is shown in the Supplemental Material Fig.…”

“…Motivated by recent experimental work on multilayer Gr/Co n /Pt(111) heterostructures [26,35] and by the existing density-functional theory (DFT) work on the individual interfaces of them, in this paper we show that that DMI does not adjust satisfactorily to either limit model. Our DFT calculations reveal that the Pt SOC dominates overall and that there are sizable contributions from Co SOC locally, although cancellations occur for Co at both interfaces.…”

“…We investigate next the influence on the DMI of the Co stacking, known to be a key factor to explain the magnetocrystalline anisotropy of Gr/Co/Pt heterostructures. When Co is pseudomorphically grown by intercalation in Gr/Pt(111), it results in a f ct stacking rather than hcp [26,48], with stacking defects scattered through- out, nucleating predominantly near the Pt(111) substrate steps [35]. In a perfect f ct heterostructure, DFT calculations show that the magnetocrystalline anisotropy energy (MAE) follows a bulk-like behaviour with in-plane anisotropy starting at n Co = 8 (0.09 meV per Co atom, imposed by the lateral f ct lattice strain) and the critical thickness for perpendicular to in-plane switching at n c Co = 4.…”