Suchergebnisse

The development of graphene (Gr) spintronics requires the ability to engineer epitaxial Gr heterostructures with interfaces of high quality, in which the intrinsic properties of Gr are modified through proximity with a ferromagnet to allow for efficient room temperature spin manipulation or the stabilization of new magnetic textures. These heterostructures can be prepared in a controlled way by intercalation through graphene of different metals. Using photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we achieve a nanoscale control of thermally activated intercalation of a homogeneous ferromagnetic (FM) layer underneath epitaxial Gr grown onto (111)-oriented heavy metal (HM) buffers deposited, in turn, onto insulating oxide surfaces. XPS and STM demonstrate that Co atoms evaporated on top of Gr arrange in 3D clusters and, upon thermal annealing, penetrate through and diffuse below Gr in a 2D fashion. The complete intercalation of the metal occurs at specific temperatures, depending on the type of metallic buffer. The activation energy and the optimum temperature for the intercalation processes are determined. We describe a reliable method to fabricate and characterize in situ high-quality Gr-FM/HM heterostructures, enabling the realization of novel spin-orbitronic devices that exploit the extraordinary properties of Gr ; This research was supported by the Regional Government of Madrid through projects P2018/NMT-4321 (NANOMAGCOST-CM) and P2018/NMT-4511 (NMAT2D) and by the Spanish Ministry of Economy and Competitiveness (MINECO) through projects RTI2018-097895-B-C42, FIS2016-78591-C3-1-R, PGC2018-098613-B-C21, PGC2018-093291-B-I00, FIS2015-67367-C2-1-P, and PCIN-2015-111 (FLAGERA JTC2015 Graphene Flagship "SOgraph"). IFIMAC acknowledges support from the ″Maria de Maeztu″ programme for units of Excellence in R&D (MDM-2014-0377). IMDEA Nanoscience is supported by the "Severo Ochoa" programme for the Centres of Excellence in R&D, MINECO (grant number SEV-2016-0686)

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/Con/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). ; Financial support from MINECO (Grant Nos. RTI2018-097895-B-C41, RTI2018-097895-B-C42 and RTI2018-097895-B-C43 (FUN-SOC), PID2019-103910GB-I00, FIS2016-78591-C3-1-R and FIS2016-78591-C3-2-R (SKYTRON), PGC2018-098613-B-C21 (SpOrQuMat), PCI2019-111908-2 and PCI2019-111867-2 (FLAGERA 3 grant SOgraphMEM)], from Regional Government of Madrid (grant number P2018/NMT-4321 (NANOMAGCOST-CM)) and from Gobierno Vasco-UPV/EHU (grant numbers GIU18/138 and IT-1246-19). We acknowledge experiments at ALBA BL29 via proposal no. 2019023333. IMDEA-Nanociencia acknowledges support from the "Severo Ochoa" Program for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686). ; Peer reviewed