RAPID COMMUNICATIONS PHYSICAL REVIEW B 87, 041403(R) (2013) Atomic-scale magnetism of cobalt-intercalated graphene R´ egis Decker, 1 Jens Brede, 1,* Nicolae Atodiresei, 2,† Vasile Caciuc, 2 Stefan Bl¨ ugel, 2 and Roland Wiesendanger 1 1 Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany 2 Peter Gr¨ unberg Institut and Institute for Advanced Simulation, Forschungzentrum J¨ ulich, D-52425 J¨ ulich, Germany (Received 4 July 2012; published 7 January 2013) Using spin-polarized scanning tunneling microscopy and density functional theory, we have studied the structural and magnetic properties of cobalt-intercalated graphene on Ir(111). The cobalt forms monolayer islands being pseudomorphic with the Ir(111) beneath the graphene. The strong bonding between graphene and cobalt leads to a high corrugation within the Moir´ e pattern which arises due to the lattice mismatch between the graphene and the Co on Ir(111). The intercalation regions exhibit an out-of-plane easy axis with an extremely high switching field, which surpasses the significant values reported for uncovered cobalt islands on Ir(111). Within the Moir´ e unit cell of the intercalation regions, we observe a site-dependent variation of the local effective spin polarization. State-of-the-art first-principles calculations show that the origin of this variation is a site-dependent magnetization of the graphene: At top sites the graphene is coupled ferromagnetically to the cobalt underneath, while it is antiferromagnetically coupled at fcc and hcp sites. DOI: 10.1103/PhysRevB.87.041403 PACS number(s): 73.22.Pr, 68.37.Ef, 75.75.−c The electronic properties of graphene 1 depend critically on its environment and in particular on its substrate. 2–4 Therefore, it is important to be able to precisely design and control the physical properties of the graphene-substrate interfaces. Inspired by the extensively studied graphite in- tercalation compounds, 5–8 an emerging method to engineer technologically relevant hybrid organic-metal interfaces is to intercalate specific elements at the interface. 9,10 So far, most efforts in that direction have been employed to use intercalants to reduce the graphene-substrate interaction in order to keep the unprecedented exotic properties of bare graphene intact, 11–13 open a gap at the Dirac point, 14 or induce superconductivity. 15,16 However, this approach has not yet been exploited to create an interstitial ferromagnetic layer and study its effects on the properties of graphene. Being made of light atoms, the spin-orbit coupling in graphene is known to be very weak and the intrinsic magnetic properties of graphene are difficult to observe experimentally. Therefore, most studies concerning the magnetic properties of graphene have been focusing on its edges, defects, and impurities. 17 Experimentally, however, many features pre- dicted by theory were not observed yet and the magnetic properties of graphene in general remain an almost unexplored and challenging research topic. 18,19 Since graphene is only one layer thick and is known to be an inert material, graphene- based ferromagnetic heterostructures are an ideal candidate for a new class of tunnel magnetoresistance (TMR) 20 or giant magnetoresistance (GMR) 21,22 devices. A detailed state-of- the-art experimental and theoretical description at the atomic scale of the topography as well as of the complex phenomena occurring at the hybrid graphene-ferromagnetic surface and interface represents the first necessary steps towards the development of graphene-based spintronics. In this combined experimental and theoretical study, we gain a unique and detailed insight into the physical properties of the cobalt-intercalated graphene/Ir(111) system. Using spin- polarized scanning tunneling microscopy (SP-STM), 23 we resolve simultaneously the morphology and the local magnetic properties of graphene on top of monolayer cobalt islands which were intercalated between the graphene and the Ir(111) substrate. State-of-the-art first-principles calculations were performed to gain a thorough insight into the nature of the local bonding and the magnetic interactions present at the hybrid graphene-ferromagnet interface that lead to the experimentally observed topography and local spin polarization in this system. SP-STM measurements were performed under UHV condi- tions (P base < 1.10 −10 mbar). All tip and sample preparations were done in vacuo. The Ir(111) substrate was prepared by repeated cycles of sputtering and annealing in O 2 atmosphere (P O 2 = 10 −7 mbar) followed by a flash at ∼1000 ◦ C. After the cobalt intercalation process (see below), samples were transferred in situ into a home-built SP-STM operated at ∼6 K. Fe-coated (∼50 ML) tungsten tips were used to observe the magnetic structure of the sample. All data were acquired in the constant-current mode. Spin-polarized differential tun- neling conductance maps were recorded using the lock-in technique, by detecting the ac tunneling current induced by a sinusoidal voltage added to the dc sample bias V b , with the magnetic orientation of the tip aligned by applying an external magnetic field |B |  1 T. Calculations were performed via DFT in the generalized gradient approximation + Hubbard term (GGA + U ), including van der Waals interactions. 24,25 As observed experimentally, 26 the spin polarization above the Co/Ir(111) surface and the out-of-plane magnetization direction is reproduced when using a U = 4 eV in our calculations. Thus, this value has been chosen also for the graphene/Co/Ir(111) system. The graphene/Co/Ir(111) system was modeled by a (10 × 10) graphene unit cell (200 C atoms) on a (9 × 9) Co/Ir(111) unit cell (one layer of Co and three layers of Ir, i.e., 81 Co atoms and 243 Ir atoms). The Ir(111) surface was partially covered by micron-sized graphene patches obtained by a chemical vapor deposition as described in Ref. 27. On the one hand, graphene on transition metal systems such as Ru(0001) 28,29 is dominated by strong chemical interaction between the graphene and the surface. On the other hand, the bonding of graphene on Ir(111) is primarily of van der Waals type, with a chemical modulation. 30 A Moir´ e pattern arises in the STM topography due to the lattice 041403-1 1098-0121/2013/87(4)/041403(5) ©2013 American Physical Society