Session D14: Focus Session: Spins in Carbon - Spins in Graphene

Spin transport in graphene

Conventional electronic transistors involve the control of electronic charge at the nanoscale to realize memory, logic and communication functions. All these electronic charges, however, also carry a spin that remains unutilized in present commercial devices. This has motivated the search for new materials that propagate spin-polarized currents over large distances. Among the most promising materials for spintronics has been graphene. Micron-scale spin relaxation lengths have been previously demonstrated in single-layer graphene. Recently, we showed that bilayer graphene is a far more interesting candidate for spintronics. By fabricating spin valves on bilayer graphene we have achieved at room temperature spin relaxation times up to 2 nanoseconds, which are an order of magnitude higher than for single layer graphene [1]. Furthermore, the spin-relaxation time scales inversely with the mobility of BLG sample. This indicates the importance of D'yakonov-Perel' spin scattering in BLG. Last not but least, the presence of an electric field tunable band gap in bilayer graphene makes it particularly appealing. Our work provides fundamental insight into the unique properties of bilayer graphene for spintronic applications. Remarkably, a similar difference between single layer and bilayer graphene is also observed in large area graphene grown by the CVD method on copper. These results demonstrate the potential of CVD graphene in realistic spintronics devices [2]. \\[4pt] [1] T - Y. Yang et al., Observation of Long Spin-Relaxation Times in Bilayer Graphene at Room Temperature, PRL (2011). \\[0pt] [2] A. Avsar et al., Towards Wafer Scale Fabrication of Graphene Based Spin Valve Devices, Nano Lett. (2011).   

Spin Transport in Epitaxial Graphene on SiC (0001)

Long spin lifetimes in graphene make it an ideal candidate for the channel material in future spintronic devices. The long spin lifetimes arise due to the small intrinsic spin orbit coupling and low hyper-fine interaction of the electron spins with the carbon nuclei. Spin lifetimes are measured in epitaxially grown graphene on SiC from IBM. The spin lifetime is measured with non-local Hanle measurements to observe spin precession in the graphene. Spin lifetimes are then extracted by fitting to the solution of the Bloch equation with a diffusion term. A comparison will be presented between an injection and detection contact structure with a bare Co/graphene interface and a Co/HfO$_2$/graphene interface, where it was found that the HfO$_2$ interface results in an increased spin lifetime. Temperature dependent spin lifetimes and Hall mobilities were measured, and a comparison of the two measurements will also be presented to provide insight into the spin scattering mechanism in epitaxial graphene on SiC.   

Spin Dependent Scattering from Gated Obstacles in Graphene Systems

We study scattering of Dirac fermions in the presence of both intrinsic and Rashba spin orbit interactions (SOIs). We use the analytical form of eigenstates in a system with cylindrical symmetry to calculate useful quantities for the scattering of Dirac particles such as phase shifts, and both transport and total cross sections, as well as the corresponding scattering times. At low energies the scattering from a gated obstacle in the absence of SOIs is anisotropic and predominantly forward $[1]$. In contrast, for energies close to the intrinsic SOI amplitude, the scattering becomes \emph{isotropic}, which can be seen as arising from the effective Dirac mass generated by the SOI interaction. In the presence of Rashba fields we find that the spin-flip scattering is isotropic while it remains anisotropic and predominantly forward for spin-preserving scattering, leading to persistent spin polarization in the forward direction. At high energies, we find a series of resonances in the elastic scattering times, associated with particle trajectories orbiting the obstacle and characterized by long lifetimes. The Rashba SOI is found to double the number of long lived states in both spin-preserving and spin-flip scattering channels. [1]M.Monteverde et al., PRL \textbf{104},126801 (2010)   

Tunnel spin injection into graphene through ALD-grown tunnel barrier

Graphene is a promising material for spintronics devices because of its long spin relaxation time due to weak spin-orbit interaction and hyperfine interaction. For the spintronics, it is very essential to develop a reliable method to inject spin polarized electrons into graphene from ferromagnetic electrodes. In this study, between ferromagnetic electrodes and graphene we fabricated a new type of Al$_{2}$O$_{3}$ tunnel barrier grown by atomic layer deposition (ALD). Before ALD of Al$_{2}$O$_{3}$, we functionalized the surface of graphene with a self-assembled monolayer of 3, 4, 9, 10 perylene tetracarboxylic acid (PTCA) to improve adhesion and growth of Al$_{2}$O$_{3}$. Using ALD-Al$_{2}$O$_{3}$/PTCA composite barrier, large nonlocal magnetoresistance of 30 $\Omega $ was observed at 45 K. Nonlocal magnetoresistance reached maximum around charge neutrality point, and $I-V$ characteristics of the contacts are nonlinear. These results indicate the achievement of tunnel spin injection into graphene, revealing potentially high performance of ALD-Al$_{2}$O$_{3}$/PTCA tunnel barrier [1]. [1] T. Yamaguchi et al., J. Magn. Magn. Mater. (2011), doi: 10. 1016/j.jmmm. 2011. 09. 031   

Spin waves in finite graphene ribbons

In a previous work we have shown that the spin excitations of a graphene zigzag ribbon have a dispersion relation predominantly linear for large wave lengths, due to the antiferromagnetic coupling between the magnetizations of the opposite edges. Although the excitations are weakly damped in electrically neutral nanoribbons, the damping can be enhanced by the application of gate voltages. This allows control of the spin relaxation times by purely electrostatic means. Here we investigate spin excitations and also electronic transport in finite zigzag ribbons, connected to graphene-like metallic leads. The ground state is described self-consistently within a mean-field scheme. The spin excitations are extracted from transverse dynamic susceptibility. As a general result we found conductance gaps populated with localized states that are swept out as the length of the conductor increases. The magnetic moment is site dependent, differently from the infinite case, and diverse spin wave excitation modes are exhibited. We study the spin wave behavior and the dynamic susceptibility dependence on the coupling intensity between ribbon and leads. We analyze the role played by the coupling on the spin wave life times and the effects of external doping.   

GMAG Dissertation Award Talk: Spin Injection and Relaxation in Graphene

Graphene is a unique and promising candidate for spintronics due to its high mobility, low intrinsic spin-orbit and hyperfine couplings, which should lead to long spin lifetimes and relaxation lengths. Experimentally, the gate-tunable spin transport has been achieved at room temperature. However, the spin injection efficiency has been low and the spin lifetime is still much shorter (50- 200 ps) than expected theoretically ($\sim $micro seconds). To fulfill the potential of graphene for spintronics, two major advances are needed to be accomplished; enhance the spin injection efficiency and extend the spin lifetime. In this talk, I will focus on the contributing results for these advancements in graphene spintronics during my Ph. D. study. First, I develop a method to grow atomically smooth MgO tunnel barrier using Ti seeding layer prior the MgO growth on graphene. Then tunneling spin injection into graphene is achieved. The nonlocal spin signal is observed to be as high as 130 ohms at 300 K, with a spin injection efficiency of 30{\%}. Second, using tunneling contacts to suppress the contact-induced spin relaxation, we observed the spin lifetimes as long as 771 ps at 300 K, 1.0 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene (BLG). Furthermore, contrasting spin relaxation mechanisms are found in SLG and BLG. Third, the spin lifetimes on the same SLG spin valve with tunable mobilities are investigated. These results are important advances for graphene to be used for spin computing or spin logic applications.   

Magnetism in neutron irradiated graphene samples

Recent work in our lab has shown that graphene can become ferromagnetic by way of the addition of hydrogen. The graphene shows a weak but distinct magnetization loop at room temperature. Magnetic force microscopy shows that the magnetic effect can be added and subsequently removed by exposure to a cold hydrogen plasma followed by annealing at 400 Deg C. An outstanding question has been whether the effect observed is due to the interaction of the hydrogen with the graphene, or the addition of defects from the hydrogen. The role of the hydrogen vs. defects was studied using raman microscopy and magnetic force microscopy on graphene samples exposed to neutron irradiation for comparison of a sample containing defects without adatoms.   

Magnetic Moment Formation in Hydrogenated and Defected Graphene

Recent experimental observations of magnetic moment formation and magnetic ordering in graphene and graphite have excited both theorists and experimentalists. Magnetic ordering in carbon based materials would provide an alternate material to the conventional d and f metals employed in current technologies and could contribute to new applications in nanotechnology, spintronics, medicine, and telecommunications. While still a young and controversial field, previous experimental and theoretical works suggest the presence of magnetic moments in carbon materials is attributed to impurities, boundaries, reduced dimensionality, or defects. In this study, we perform spin transport measurements on graphene devices in order to investigate magnetic moment formation in doped graphene. The graphene surface is modified inside an ultrahigh vacuum chamber through a variety of methods including hydrogen adsorption, Ar sputtering, and molecular beam deposition of transition metals. We observe signatures of paramagnetic moment formation associated with dopants and defects in graphene.   

Quantum Monte Carlo study of magnetic impurity in bilayer grephene

It is expected to observe many different properties in bilayer graphene when compared with single layer graphene due to the differences in crystal structure. Additionally, bilayer system offers a freedom of inducing a gap in the energy band by applying a shift in the electrochemical potential to two graphene layers. In this work, we study the magnetic properties of an Anderson magnetic adatom in Bernal stacking bilayer graphene and compare the results with those of single layer counterpart. Several different cases such as different adatom position and different potential bias of two layers are studied using the quantum Monte Carlo method. In all the cases, we find that the impurity local magnetic moment can be switched between relatively large and small values by tuning the chemical potential. We apply MaxEnT method to compute impurity spectral density and find its behavior to differ from that of an impurity in a single layer graphene. We also calculate various correlation functions and make comparisons.