Session X16: Focus Session: Spins in Carbon-Based Materials -- Graphene, CNT, and C60

Enhanced spin injection and spin lifetime in Graphene

Graphene is an attractive material for spintronics due to the low intrinsic spin-orbit and hyperfine coupling, which should lead to excellent spin transport properties. Earlier studies on spin injection and transport in graphene present two major challenges: low spin injection efficiency and short spin lifetimes compared to the theoretical predictions. In our work, we utilized TiO2 Seeded MgO barriers and achieved tunneling spin injection into single layer. As a result, large nonlocal magnetoresistances were observed at room temperature, with high spin injection efficiency up to 30{\%}. Surprisingly, enhanced spin lifetimes of graphene are obtained, which is due to reducing the contract-induced spin relaxation by inserting tunnel barrier between graphene and Co electrodes.   

Electrical Detection of Spin Transport in Epitaxial Graphene Grown on the Si-face of Hexagonal SiC(0001)

Graphene has great potential for use as a spin transport channel due to its low spin orbit coupling and high mobility. Spin diffusion lengths in the microns have been demonstrated on exfoliated graphene at room temperature\footnote{Wei Han, et al. PRL 105, 167202 (2010). }. We will present our measurements of spin relaxation in both exfoliated graphene and epitaxially grown graphene on SiC from IBM using non-local Hanle measurements as a function of temperature. The diffusion lengths on epitaxial graphene were comparable to those found in exfoliated flakes. The initial results show the diffusion length is limited by contact induced relaxation that occurs at the metal/graphene interface in agreement with results from exfoliated flakes.   

Room Temperature Spin Transport in C$_{60}$-based spin valves.

Carbon-based materials offer a unique playground for spin transport studies by merging relatively small spin relaxation mechanisms with the potential chemical versatility of some organic molecules. However, how the spin travels inside such materials is far from understood. In this work, we present magneto-transport studies in vertical spin valves containing a C$_{60}$ non-magnetic spacer and simple ferromagnetic (Co and Py) electrodes. Large magnetoresistance values (up to 5{\%}) are recorded at room temperature for fullerene thickness up to 30 nm. Remarkably, magnetoresistance is also present at relatively high bias (1 Volt), highlighting the robustness of the spin transport in this material. By choosing such a simple carbon system we are also able to introduce a simple multi-step tunneling model, which explain the electronic transport data and which is compatible with coherent spin transport over long distances.   

Spin-orbit coupling in graphene: from single layers to graphite

The spin-orbit interaction in graphene is full of contrasts. First, this relativistic interaction destroys the ideal relativistic ``touching cones'' electronic dispersion at the K points. A finite, albeit small, gap appears, giving a finite mass to the electrons. Then, while the spin-orbit splitting in the carbon atom is about 10 meV, the electronic states at the K points have a gap of only about 24 micro eV. Finally, it turns out that this quintessential sp material has its spin-orbit coupling derived almost exclusively from d orbitals. In this talk I will give first-principles [1] and tight-binding [2] perspectives on the spin-orbit coupling in graphene in the presence of a transverse electric field. The field, which would normally come from the substrate or the gates, breaks the space inversion symmetry and gives the extrinsic (Bychkov-Rashba) splitting of the states. It also brings interesting band-structure topologies, from gapped at low electric fields (the topological insulator phase), through a mixture of genuine touching Dirac cones and parabolic bands (the intrinsic and extrinsic spin-orbit strengths equal), to gapples, dominated by the extrinsic effects [1]. The intrinsic coupling is dominated by d orbitals, while the extrinsic by the field induced hybridization of the s and p orbitals. It turns out that similar physics holds for bilayer and trilayer graphene, ultimately also in graphite. I will also discuss the problem of the spin relaxation in graphene. The main issue is that conventional theories predict microseconds for the spin relaxation time, while experiments seem to consistently yield 100 ps. One possibility [3] is that that spin relaxation in graphene is due to adatoms that pull out the carbon-like spin-orbit coupling of the p electrons and lead to the larger spin relaxation of the Dyakonov-Perel type.\\[4pt] [1] M. Gmitra et al., Phys. Rev. B 80, 235431 (2009);\\[0pt] [2] S. Konschuh et al., Phys. Rev. B (in press);\\[0pt] [3] C. Ertler et al., Phys. Rev. B(R) 80, 041405 (2009).   

Ferromagnetism in Mn-implanted HOPG

20 keV energy Mn ions were implanted on HOPG samples at 300$^{\circ}$C. SQUID magnetometer measurements show ferromagnetic ordering and magnetic hysteresis at very low temperatures. Mn K-edge XAS spectra show presence of Mn and O in our sample and XMCD data shows ferromagnetic ordering of Mn at 14 K, but not at 300 K. SIMS data show presence of Mn and O with carbon, besides other elements. Raman spectroscopy results indicate disorder graphite phase and high resolution TEM images confirm amorphous Mn-implanted region with presence of nanocrystallites.   

Organic spin-valves based on fullerene C60

Recent work suggests that the spin-transport length in organic semiconductors is limited by hyperfine coupling. Therefore, to potentially overcome this limitation, we fabricated spin-valves based on C60 for which the hyperfine coupling is minute. However, our devices do not show a significantly larger spin-diffusion length. This suggests that either a mechanism other than hyperfine coupling causes the loss of spin-polarization, or that in thick devices an increasing conductivity mismatch limits spin-injection.   

On the Role of Spin-Orbit Coupling in the Spin Response of C$_{60}$-based Spintronics Devices

We report comprehensive studies of the spin response in C$_{60}$-based spintronics devices such as spin valves and diodes. The buckeyball C$_{60}$ molecules are composed of $\sim $99{\%} $^{12}$C carbon atoms having spinless nuclei with zero hyperfine interaction. Therefore it was believed that the spin diffusion length in C$_{60}$-based spin-valves is large, and the magnetoresistance (MR) in C$_{60}$ diodes is negligible small. Surprisingly, we obtained a small spin diffusion length which we believe to be due to a relatively strong spin-orbit (SO) coupling in the material. We also found that the MR in C$_{60}$ diodes is relatively small, with characteristic magnetic field response dominated by the SO coupling with strength, $\xi \approx \quad \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} \quad \mu $eV, more than ten times larger than the HFI constant\textbf{. }This was verified by measuring the response of $^{13}$C-rich C$_{60}$ diodes.   

Kondo effect of magnetic impurities on nanotubes

The effect of magnetic impurities on the ballistic conductance of nanocontacts is, as suggested in recent work, amenable to ab initio study [1]. Our method proceeds via a conventional density functional calculation of spin and symmetry dependent electron scattering phase shifts, followed by the subsequent numerical renormalization group solution of Anderson models -- whose ingredients and parameters are chosen so as to reproduce these phase shifts. We apply this method to investigate the Kondo zero bias anomalies that would be caused in the ballistic conductance of perfect metallic (4,4) and (8,8) single wall carbon nanotubes, ideally connected to leads at the two ends, by externally adsorbed Co and Fe adatoms. The different spin and electronic structure of these impurities are predicted to lead to a variety of Kondo temperatures, generally well below 10 K, and to interference between channels leading to Fano-like conductance minima at zero bias.\\[4pt] [1] P. Lucignano, R. Mazzarello, A. Smogunov, and E. Tosatti, Nature Materials 8, 563 (2009).   

Electric-field control of magnetism in graphene quantum dots: A route to spin field effect transistors

Graphene is a promising candidate for all-carbon electronics because of its outstanding electrical, mechanical, and thermal properties. Also, the relentless drive for miniaturization leads to the use of ever smaller graphene fragments; at nanoscopic dimensions ($<$ 10nm), edge states become more relevant. Edge states are important because they lie in the vicinity of the Fermi level and hence are relevant to transport properties. Furthermore, edge states exhibit magnetism. We have employed ab-initio electronic structure and Landauer-B\"{u}ttiker transport calculations to study the magnetoelectro effects of graphene patches. We will present results of (1) how specific geometries (such as ``diamond'' shape) favor specific magnetic states, (2) how those magnetic states can be controlled by an external electric field [1], and (3) we will demonstrate how a graphene fragment containing different edge geometries can be employed as a spin-polarized field effect transistor.\\[4pt] [1] Agapito et al., PRB Rap. Com. 82, (2010)   

Evidence for magnetic behavior in chemically modified graphene

Although graphene has exceptional electronic and structural properties, there is very little experimental evidence that graphene by itself shows strong electron-electron correlations. In fact, in spite of a large amount of theoretical work, recent experiments have shown that pure and clean graphene shows no signs of correlated many-body states such as magnetism or superconductivity. We will report on the observation of room temperature magnetism in mechanically exfoliated, chemically modified, graphene. The effect has been found using both magnetic force microscopy and magnetization tests. It was shown that the graphene can be brought back to a non-magnetized state by removing the surface chemistry. The search for correlated electronic states in graphene generates an enormous interest because of its low dimensionality, which is prone to strong quantum and thermal effects, and also because it would open up doors for a plethora of technological applications, from permanent two-dimensional magnets to spintronics.   

QMC study of molecules for spintronics and photoswitching

A combination of QMC and quantum chemistry (CAS-SCF) techniques are used to study two large molecules: azobenzene (AB) imporatnt as a photoswitch and vanadbenzene (VB), frequently used in spintronics [1]. In AB higher singlet state, S$_{2}$ the fingerprint of AB in excitation spectra has been calculated in addition to the low singlet states S and S$_{1}$ [2]. We have also calculated the lowest triplet T$_{1}$ vertical excitation, identified by EELS [3] as well as adiabatic T$_{1}$ excited state. All calculated energies are in excellent agreement with available experiments [3, 4]. In VB we focus initially on PES for dissociation and excited state of the vanadium cation. \\[4pt] [1] V.V. Maslyuk et al., Phys.Rev.Lett. 97, 201, (2006). [2] M. Dubecky et al., J.Chem.Phys, accepted (2010). [3] M. Allan, private communication. [4] J.-{\AA}. Andersson, R. Petterson, L. Tegn\'{e}r, J. Photochem. \textbf{20}, 17 (1982).