Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W29: Focus Session: Spin-Dependent Physics in Carbon-Based Materials |
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Sponsoring Units: GMAG DMP Chair: Olaf van’t Erve, Naval Research Laboratory Room: 206A |
Thursday, March 5, 2015 2:30PM - 3:06PM |
W29.00001: Extrinsic spin Hall effect in graphene Invited Speaker: Tatiana Rappoport The intrinsic spin-orbit coupling in graphene is extremely weak, making it a promising spin conductor for spintronic devices. In addition, many applications also require the generation of spin currents in graphene. Theoretical predictions and recent experimental results suggest one can engineer the spin Hall effect in graphene by greatly enhancing the spin-orbit coupling in the vicinity of an impurity. The extrinsic spin Hall effect then results from the spin-dependent skew scattering of electrons by impurities in the presence of spin-orbit interaction. This effect can be used to efficiently convert charge currents into spin-polarized currents. I will discuss recent experimental results on spin Hall effect in graphene decorated with adatoms and metallic cluster [1,2] and show that a large spin Hall effect can appear due to skew scattering. While this spin-orbit coupling is small if compared with what it is found in metals, the effect is strongly enhanced in the presence of resonant scattering, giving rise to robust spin Hall angles. I will present our single impurity scattering calculations done with exact partial-wave expansions [3] and complement the analysis with numerical results from a novel real-space implementation of the Kubo formalism for tight-binding Hamiltonians [4].\\[4pt] [1] J. Balakrishnan et al., Nat. Phys. 9, 284 (2013).\\[0pt] [2] J. Balakrishnan et al., Nat. Commun. 5, 4748 (2014). \\[0pt] [3] A. Ferreira, T. G. Rappoport, M. A. Cazalilla, A. H. Castro Neto, Phys. Rev. Lett. 112, 066601 (2014).\\[0pt] [4] Jose H. Garcia, Lucian Covaci and Tatiana G. Rappoport, arXiv:1410.8140. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W29.00002: Inverse Spin Hall Voltage in Organic Semiconductors with Tunable Spin-Orbit Coupling Dali Sun, Kipp van Schooten, Hans Malissa, Marzieh Kavand, Chuang Zhang, Christoph Boehme, Z. Valy Vardeny Spin-current in organic semiconductors that are generated via the process of `spin-pumping' from FM substrates subjected to resonant microwave absorption has attracted recently great interest, since this scheme circumvents the impedance mismatch that exists in the ``spin injection'' technique between the organic semiconductors and FM injector. Because of the weak spin-orbit coupling (SOC) in most organic semiconductors (OSECs), the obtained inverse spin Hall effect (ISHE) in these materials is very subtle. In this work we measured ISHE in a variety of OSECs having tunable SOC ranging from strong SOC (pi-conjugated polymers that contain intrachain Pt atoms) to weak SOC polymers (such as DOO-PPV). We found that the ISHE response in these compounds increases with the SOC, in spite of the decrease in the spin diffusion length. [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W29.00003: Neutral currents probed by non-local transport in graphene with 5 \textit{d} metal adatoms Yilin Wang, Xinghan Cai, Shudong Xiao, Wenzhong Bao, Janice Reutt-Robey, Michael Fuhrer Adsorption of adatoms on graphene has been theoretically proposed as an effective means to enhance spin-orbit coupling in graphene. Here we use the non-local measurement geometry to detect neutral currents (spin, valley, energy) through their Hall and inverse Hall effects. Single-layer graphene devices are probed \textit{in-situ} in ultra-high vacuum while depositing 5$d$ heavy metal atoms (Ir, Au) at a temperature of 7 K. Surprisingly, we detect a non-local signal in pristine devices as well as metal-atom modified devices, with a consistent dependence of the signal on length and gate voltage for several devices. Changes in the non-local signal upon deposition of 5$d$ metal atoms appear governed by charge carrier mobility (reduced with increasing metal atom concentration) and are difficult to understand with spin Hall/inverse spin Hall effects alone due to increased spin-orbit coupling in graphene. We will discuss other possible origins of the non-local signal. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W29.00004: Theory of spin relaxation in bilayer graphene. Denis Kochan, Susanne Irmer, Martin Gmitra, Jaroslav Fabian We present a new spin relaxation mechanism based on resonant scattering off local magnetic moments. We apply this mechanism to mono [1] and bilayer graphene and show that it can account for the ultrafast spin relaxation observed in spin injection experiments. In particular, the model explains the opposite dependence of the spin relaxation on the carrier density in mono and in bilayer graphene. We also show that for bilayer graphene the model explains the observed temperature induced changes in the carrier density dependence. \\[4pt] [1] D. Kochan, M. Gmitra, J. Fabian; Phys. Rev. Lett. 112, 116602 (2014). [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W29.00005: Spin-orbit coupling in fluorinated graphene Susanne Irmer, Tobias Frank, Sebastian Putz, Martin Gmitra, Denis Kochan, Jaroslav Fabian We theoretically study spin-orbit coupling effects of fluorine chemisorbed on graphene. Both dense and dilute limit reveal a giant local enhancement of spin-orbit coupling by a factor of 1000 in the vicinity of the adatom---spin-orbit strength of about 10 meV. We present results of fully converged first-principles calculations and analyze them by a tight-binding Hamiltonian based on symmetry arguments. Our work covers different limits of fluorine concentration from dense to intermediate to dilute coverage. We find that fluorine's native spin-orbit coupling exceeds the effect of the sp$^{3}$ distortion of the lattice. Moreover, we identify fluorine as a weak resonant scatterer giving rise to resonant signatures in the band structure off the Dirac point by about 0.3 eV. Our findings are important for studies on relaxation and transport. Details can be found in the following manuscript: http://arxiv.org/abs/1411.0016 [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W29.00006: Simultaneous magnetic force microscopy and electrical transport measurements of a graphene non-local spin valve Michael Page, Andrew Berger, Hua Wen, Vidya Bhallamudi, Roland Kawakami, P. Chris Hammel Non-local signals in graphene spin valves depend on the magnetization states of the ferromagnetic electrodes. Currently, determining the relative influence of each magnetic electrode relies on fitting the non-local signal to the one-dimensional spin diffusion model. We report imaging of the magnetization states of the spin valve electrodes using a custom magnetic force microscope, while simultaneously acquiring the non-local spin signal electrically. This allows direct correlation of the non-local signal features to the switching of the individual electrodes and determination of the relative contribution to the signal by the participating electrodes. We also image the formation and motion of domain walls near the graphene transport channel and correlate these with features in the non-local signal. This measurement technique supports the one-dimensional spin diffusion model and provides information necessary for reliable switching behavior in spin valves with magnetic electrodes. [Preview Abstract] |
(Author Not Attending)
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W29.00007: Spintronics in hybrid organic/inorganic heterojunctions Shayan Hemmatiyan, Erik McNellis, Jairo Sinova In this work, we present the results of extensive analytical and numerical calculations to investigate spin and charge transport inside organic semiconductors and also at the interface with ferromagnetic metals. Based upon these calculations, we will describe the underlying spin relaxation mechanisms with reference to the spin dependent characteristic parameters e.g. spin relaxation time and spin diffusion length of the organic semiconductors in connection with the ferromagnetic electrodes. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W29.00008: Bias Dependence of Tunneling Spin Injection into Graphene Tiancong Zhu, Hua Wen, Walid Amamou, Zhisheng Lin, Jing Shi, Roland Kawakami Bias dependence of spin injection into a spin channel typically exhibits unusual behavior, which has been challenging to understand. In this study, we investigate the bias-dependence of tunneling spin injection into graphene with lateral spin-valve geometry. Co/MgO/Graphene is used as tunneling barrier contact and lock-in measurement is performed. By applying a DC bias to AC spin injection current, we observe a strong non-linearity of bias-dependent non-local voltage on both the electron and hole side of graphene. The non-local voltage also flips sign when a large negative DC bias is applied. We extracted the interfacial spin polarization as a function of DC bias. The data analysis suggests that the unusual behavior of bias-dependent tunneling spin injection in graphene is mainly due to the spin polarization changing at the ferromagnetic/graphene interface. To better understand this behavior, we also compare our data with several other existing models on bias-dependent spin injection. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W29.00009: Tunable Magnetic Proximity Effects in Graphene Junctions Predrag Lazic, Kirill Belashchenko, Igor Zutic The characteristic length of the magnetic proximity effects exceed the thickness of a graphene layer leading to an important, but typically overlooked, modifications of equilibrium and transport properties, as well as the implications for graphene spintronics [1,2]. Using the first-principles studies that integrate a real space density functional theory (GPAW) [3] with the state-of-the art boundary elements electrostatic code based on the Robin Hood method [4], we explore tunable electronic structure and magnetic proximity effects in the ferromagnet/insulator/graphene junctions. We show that the inclusion of a finite-size gate electrodes and van der Walls interaction lead to nontrivial effects that could also be important in other two-dimensional materials beyond graphene.\\[4pt] [1] P. Lazic et al., Phys. Rev. B 89 085429 (2014).\\[0pt] [2] W. Han et al., Nature Nanotech. 9, 794 (2014).\\[0pt] [3] J. Enkovaara et al., J. Phys. Cond. Matter 22, 253202 (2010).\\[0pt] [4] P. Lazic et al., J. Comp. Phys. 213, 117 (2006). [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W29.00010: Giant Perpendicular Magnetic Anisotropy of Graphene-Co Heterostructures Hongxin Yang, Ali Hallal, Mairbek Chshiev We report strongly enhanced perpendicular anisotropy (PMA) of Co films by graphene coating via \textit{ab-initio} calculations.[1] The results show that graphene coating can improve the surface anisotropy of Co film up to twice large of the bare Co case and keep the film effective anisotropy being out-of-plane till 25 {\AA} of Co, in agreement with experiments[2,3]. Our layer resolved analysis reveals that PMA of Co (Co/Gr) films mainly originates from the adjacent 3 Co layers close to surface (interface) and can be strongly influenced by graphene. Furthermore, orbital hybridization analysis uncovers the origin of the PMA enhancement which is due to graphene-Co bonding causing an inversion of Co 3$d_{\mathrm{z2}}$ and 3$d_{\mathrm{x2-y2}}$ Bloch states close to Fermi level. Finally, we propose to design Co-graphene heterostructures which possess a linearly increasing surface anisotropy and a constant effective anisotropy. These findings point towards a possible engineering graphene-Co junctions with giant anisotropy, which stands as a hallmark for future spintronic information processing.$^{\mathrm{\thinspace }}$[1] H. X. Yang, et al. to submit. [2] C. Vo-Van, et al$.$ New J. Phys.~12, 103040 (2010). [3] N. Rougemaille et al. Appl. Phys. Lett. 101, 142403 (2012). [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W29.00011: Spin and charge transport across cobalt/graphene interfaces Mairbek Chshiev, Alan Kalitsov, Oleg Mryasov We report ballistic calculations of in-plane and out-of-plane spin and charge transport through graphene attached to the hcp-Co electrodes. Our calculations are based on the Keldysh non-equilibrium Green Function formalism and the tight binding Hamiltonian model tailored to treat both lateral and vertical device configurations. We present results for (i) vertical device that consists of a one-side fluorinated C$_{\mathrm{4}}$F graphene sandwiched between two hcp Co electrodes and (ii) lateral device consisting of pristine graphene/C$_{\mathrm{4}}$F graphene bilayer with two top hcp-Co electrodes Our calculations predict large magnetoresistance with small resistance-area product and significant deviation from sinusoidal behavior of spin transfer torque for the vertical device configuration. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W29.00012: Blocking of spin transport between Ni$_{80}$Fe$_{20}$ and Cu by a graphene interlayer Will Gannett, Mark W. Keller, Tom Silva, Hans Nembach, Ann Chiaramonti Debay By chemical vapor deposition on epitaxial thin films of Cu(111), we are able to produce continuous, large-grain monolayer graphene (Gr). We then sputter deposit Ni$_{80}$Fe$_{20}$ (Py) in a different chamber to create large area Cu/Gr/Py samples. We are able to avoid damaging the graphene during this process by varying the Py deposition angle, and we confirm this with Raman spectroscopy. Ferromagnetic resonance measurements with a vector network analyzer show no change in damping with varying Py thickness, while Py deposited directly on Cu(111) shows the typical increase in damping associated with spin pumping between Py and Cu. We interpret these results in terms of spin pumping, interfacial conductivity, and magnetic proximity effects. [Preview Abstract] |
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