AVS2011 Session GR+MI-TuA: Graphene: Magnetic Properties and Spin-Dependent Phenomena

Hybrid systems consisting of transition metal (TM) atoms in contact with graphene are expected to show outstanding magnetic effects, from Kondo screening to long range ferromagnetism due to the large Fermi wavelength in graphene. First recent experimental evidence supports this scenario, however, little is known about the nature of the chemical interaction between TM atoms and graphene, which is the necessary starting point for any advanced application. Here we present recent X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) experiments probing the electronic configuration and magnetism of Fe, Co and Ni impurities on graphite and various graphenes. We find a rich physical scenario with marked differences between graphite and silicon oxide supported graphene on one hand and few layer epitaxial graphene on the C-face of silicon carbide on the other hand.

Under the external magnetic field, carriers of graphene are quantized to show an unusual Landau level (LL) energy spectrum due to mass-less Dirac fermions (DFs).¹ The LL energies are not equally spaced and include a characteristic zero-energy state (the n = 0 LL) contrary to the case of normal metals or two-dimensional electron gases. As a result, anomalous quantum Hall effect of graphene has been observed.^{2, 3} The quantization of the graphene carrier also occurs without external magnetic field if the appropriate strain is induced.^4,5Here, we report spontaneous LLs formation of mass-less DFs on potassium intercalated graphite (K-Graphite) and nitrogen-doped graphite (N-Graphite) under zero external magnetic field with the use of scanning tunneling spectroscopy (STS). On the basis of the calculation with the density functional theory, the top-most graphene layer is found to be decoupled with the graphite due to the partial intercalation of potassium atom or nitrogen-doping on graphite. Partially decoupled graphene layer has a sufficient strain to generate the pseudo-magnetic field with about 280 T and 60 T for K-Graphite and N-Graphite, respectively, leading to the LLs formation on the top-most graphene layer on graphite.

1. A. H. Castro Neto et al., Rev. Mod. Phys. 81 (2009) 109.

3. K. S. Novoselov et al., Science. 306 (2004) 666.

4. F. Guinea et al., Nat Phys. 6 (2010) 30.

5. N. Levy et al., Science. 329 (2010) 544.

Graphene may be an ideal material for spin field effect transistors because of its high charge carrier mobility and long spin relaxation times due to small spin-orbit coupling.¹ However, efficient spin injection into graphene requires overcoming conductivity mismatch through the use of tunnel barriers and/or spin filters.² It is possible that organic films can serve as tunnel barriers/spin filters with highly tailorable properties. In particular, metal phthalocyanines have recently been shown to exhibit spin dependent interfacial coupling on magnetic electrodes.³ A study of the coupling and morphology of such molecules on graphene is a crucial first step to understand potential spin enhanced interfaces.

We deposit monolayer iron phthalocyanine (FePc) on both single layer and bilayer epitaxial graphene on the Si-terminated polar face of SiC, named SiC(0001). Scanning tunneling microscopy reveals an adsorbed molecular lattice periodicity of 1.8 nm, close to that of the graphene/SiC buffer layer corrugation periodicity. This lattice spacing is larger than that of FePc adsorbed on a graphite surface that shows a smaller spacing of ~1.4 nm. This implies a stronger interaction of the FePc with epitaxial graphene than expected and is possibly due to the modification of graphene by the SiC substrate. Tunneling spectroscopy has been used to study the occupied and unoccupied electronic states of the absorbed monolayer FePc. Broad unoccupied states indicate significant electronic coupling between the molecules and the graphene and suggest a promising future for molecular strategies for spin injection.

*Supported by the NSF Center for Chemical Innovation: Center for Molecular Spintronics under CHE-0943975.

1. Y. G. Semenov, K. W. Kim and J. M. Zavada, Appl. Phys. Lett. 91 (15), 3 (2007).

2. W. Han, K. Pi, K. M. McCreary, Y. Li, J. J. I. Wong, A. G. Swartz and R. K. Kawakami, Phys. Rev. Lett. 105 (16), 4 (2010).

3. C. Iacovita, M. V. Rastei, B. W. Heinrich, T. Brumme, J. Kortus, L. Limot and J. P. Bucher, Physical Review Letters 101 (11), 116602-116604 (2008).

We have performed scanning tunneling spectroscopy measurements on a gated bilayer graphene device. In graphene bilayer, a potential asymmetry between the layers induces an energy gap in the electron spectrum. The formation of the energy gap is investigated as a function of carrier density and magnetic field. We found that in zero magnetic field, the reliable determination of the gap can be complicated because of disorder scattering. However, in the quantum Hall regime, the energy gap can be quantitatively determined by measuring the layer-polarized low index Landau levels.

Metal/graphene interfaces play an important role in both surface science studies of the epitaxial growth of graphene on metallic substrates, as well as in metal/graphene contacts in graphene nanoelectronic devices. We present results of first-principles density functional theory (DFT) investigations of structural, electronic, and magnetic properties for graphene/Ni(111) and graphene/Cu(111) interfaces relevant to experimental studies of graphene growth on metallic substrates. The favored interface geometries and binding sites for different interface configurations were identified. Additional adlayers of Ni and Cu were either adsorbed on top of the graphene/metal interface, or placed between the graphene and substrate to model processes of metal intercalation. It was also found that the interaction between graphene/Ni(111) and the top Cu adlayer is much weaker compared to that for a Ni adlayer. The atomic, electronic, and magnetic properties of these interfaces, including induced magnetic moments in graphene/Ni(111), Ni/graphene/Ni(111) systems, are also discussed.

We investigate the magnetic field and temperature-dependent transport properties of CVD-grown graphene subjected to different strains. The graphene is transferred to kapton substrates to which a blending force can be applied. In zero magnetic field, the prefactor to the logarithmic-in-temperature conductivity correction decreases by an approximate factor of 3 for strains as high as 0.6 %. There is also a concomitant decrease in diffusivity by a factor of 6. At 5 K we observe negative magnetoresistance for fields up to 0.5 Tesla followed by positive magnetoresistance at higher fields. We attribute the low field negative magnetoresistance to weak-localization and find that it is well described by theory. The strains resulting from the applied blending force inhibit the intervalley scattering more than an order of magnitude and decrease the phase coherence length, thereby leading to a suppression of weak-localization.

In the framework of real-time real-space time-dependent density functional theory (TDDFT) we have studied coupled electron-ion dynamics in small fragments of graphene, graphane, and fluorinated graphene subjected to short (a few femtoseconds) intense laser pulses or irradiated by energetic ions. The goal of this study is to investigate the possibility of defect creation in graphene and desorption of hydrogen/fluorine from graphene surface. We will present the results of our simulations, discuss the mechanisms that take place, and identify the parameters of the laser or energetic ions necessary for those processes to occur.