Magnetic Moment and Anisotropy of Individual Co Atoms on Graphene

Abstract

We report on the magnetic properties of single Co atoms on graphene on Pt(111). By means of scanning tunneling microscopy spin-excitation spectroscopy, we infer a magnetic anisotropy of K ¼ À8:1 meV with out-of-plane hard axis and a magnetic moment of 2:2 B . Co adsorbs on the sixfold graphene hollow site. Upon hydrogen adsorption, three differently hydrogenated species are identified. Their magnetic properties are very different from those of clean Co. Ab initio calculations support our results and reveal that the large magnetic anisotropy stems from strong ligand field effects due to the interaction between Co and graphene orbitals. DOI: 10.1103/PhysRevLett.111.236801 PACS numbers: 73.22.Pr, 32.10.Dk, 75.30.Gw, 75.70.Rf Graphene is a promising material for spintronics due to the possibility of realizing controllable spin transport [1], its perfect spin filtering [2], and spin-relaxation lengths of several micrometers at room temperature [3]. Doping graphene by magnetic impurities opens further possibilities [4][5][6]. In particular, the creation of extended magnetic phases [7], quantum critical Kondo anomalies [8,9], and the strong scattering of spin currents [10] have been predicted. Moreover, for 3d metal atoms on graphene, the calculated uniaxial magnetic anisotropies [11,12] are beyond the current record value for a surface-adsorbed atom [13]. Furthermore, hydrogen adsorption has been predicted to change the spin of the adatoms [14], underlining its potential to tailor the magnetic properties. However, the predicted anisotropies and moments are highly controversial and largely depend on how electron correlations are treated. For the prototypical system of Co atoms on graphene, spin moments between 1 and 3 B and anisotropies of different signs have been calculated [12,[15][16][17]. At present, there is only one experiment addressing the magnetic properties of transition-metal adatoms on graphene. It reports on-top adsorption, a high-spin ground state, and weak magnetic anisotropy for Co=graphene=SiCð0001Þ [18].Here we present the first local measurement of the magnetic moment and anisotropy of individual Co atoms on graphene on Pt(111). This substrate was chosen since graphene binds very weakly to it [19], thus approaching freestanding graphene. Using scanning tunneling microscopy (STM) spin-excitation spectroscopy [20], we find an exceptionally large magnetic anisotropy of K ¼ À8:1 AE 0:4 meV with out-of-plane hard axis and a magnetic moment of 2:2 AE 0:4 B . Fully relativistic density functional theory (DFT) calculations show the anisotropy to be mainly a hybridization effect. In addition to clean Co, we identify three hydrogenated species, CoH n , n 2 f1; 2; 3g, with very different magnetic behavior. The coexistence of clean and hydrogenated adatoms is expected to be a general feature of transition metal atoms on graphene that has to be considered in the interpretation of any ensemble measurement.The experiments were performed with a homebuilt STM operating at T ¼ 0:4 K and in magnetic fields up to ...

Origin of Perpendicular Magnetic Anisotropy and Large Orbital Moment in Fe Atoms on MgO

“…Previous works have only focused on Fe atoms diluted in bulk MgO since they have long been considered as a model system for studying the interplay of crystal field, spin-orbit coupling (SOC), and magnetic moment on the low energy excitation spectra of transition-metal impurities in insulators [17][18][19]. However, they have never been investigated at the surface, where the local symmetry is drastically altered with respect to the octahedral environment found inside MgO crystals and can potentially enhance the anisotropy energy, as similarly observed in previous studies of adatoms on nonmagnetic surfaces [20][21][22][23][24][25][26][27][28].…”

Theoretical probing of inelastic spin-excitations in adatoms on surfaces

“…Refs. [2][3][4][5][6][7][8][9][10]), and with which many of the predictions of Douglas L. Mills were verified. Understanding the excitation and the dynamical behavior of such nano-magnetic systems is naturally a topic of prime importance, actively studied recently both to provide insight into fundamental aspects of magnetism and as possible elements for future information technology.…”

Ab-<i>initio</i> studies of adsorbate-surface interactions

“…Most DFT investigations of Gr-supported TM clusters have been focused on the structure of TMatom or small TM n -cluster adsorption on Gr, with n = 1 − 4 or 5 atoms, and TM = Co, 37, 131 Au, 132 Pt, 12, 13, 133 Pd, 10,11,134 Pt-Au alloys, 8 and 3d TM atoms (TM = Fe, Co, Ni). 37,131,135,136 The TM-adatom adsorption site on Gr has been found to depend strongly on the TM. For example, 3d adatoms bind on the hollow sites, 37,131,135 while Pd and Pt adatoms bind on the bridge sites.…”

“…37,131,135,136 The TM-adatom adsorption site on Gr has been found to depend strongly on the TM. For example, 3d adatoms bind on the hollow sites, 37,131,135 while Pd and Pt adatoms bind on the bridge sites. 10-13 Rudenko et.…”

“…35,36 Yet another example is the large magnetic anisotropy of certain TM's, such as Co, adsorbed on Gr. 37 Even though, many theoretical studies have been conducted to describes the geometrical and eletronic properties of (TM) clusters on Gr support, they agree and disagree on many points. [8][9][10][11][12][13][14][15][16] To our knowledge, the dependence of structural and adsorption properties on cluster size has never been systematically studied from a theoretical perspective.…”