Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B16: Focus Session: Spin-Dependent Physics in Graphene |
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Sponsoring Units: GMAG DMP Chair: Minn-Tsong Lin, National Taiwan University Room: 318 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B16.00001: Magnetic Moment Formation in Graphene Detected by Scattering of Pure Spin Currents Adrian Swartz, Kathy McCreary, Jen-Ru Chen, Wei Han, Jaroslav Fabian, Roland Kawakami Graphene's 2D nature and high surface sensitivity have led to fascinating predictions for induced spin-based phenomena through careful control of adsorbates, including the extrinsic spin Hall effect, band gap opening, and induced magnetism. By taking advantage of atomic scale control provided by MBE, we have investigated deposition of adsorbates and their interactions with graphene. Spin transport measurements performed in-situ during systematic introduction of atomic hydrogen demonstrated that hydrogen adsorbed on graphene forms magnetic moments that couple via exchange to the injected spin current. The observed behavior is quantitatively explained utilizing a phenomenological theory for scattering of pure spin currents by localized magnetic moments. Lattice vacancies show similar behavior, indicating that the moments originate from so called pz-orbital defects. On the other hand, experiments with charge impurity scatterers such as Mg and Au, are noticeably absent of features related to magnetic moment formation. Furthermore, we observe gate dependent effective exchange fields due to the spin-spin coupling between conduction electrons and magnetic moments, which are of interest for novel phenomena and spintronic functionality but have not been seen previously in graphene. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B16.00002: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 11:39AM - 11:51AM |
B16.00003: Spin Transport Measurements in Hydrogenated Graphene Devices Gavin Koon, Jayakumar Balakrishnan, Barbaros Oezyilmaz Graphene with all its extraordinary properties still fall short when it comes to manipulation of electron spins. Chemically modified Graphene has been explored by many to further enhance Graphene properties, tailoring it to suit desired application purposes. Here we study the effects of hydrogenation rate on graphene spin transport, spin relaxation time and length in this defected system. These findings are important for future theoretical and experimental studies on other adatoms modified Graphene.~ [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B16.00004: Magnetic Insulators-Induced Proximity Effects in Graphene Ali Hallal, Hongxin Yang, Damien Terrade, Xavier Waintal, Stephan Roche, Mairbek Chshiev Due to its very long spin diffusion lengths up to room temperature, emergence of magnetism in graphene has attracted a lot of research interest in the field of spintronics. Several methods have been proposed to magnetize graphene, from edge magnetism, to depositing magnetic atoms or molecules, and using ferromagnetic substrate. We present first-principles calculations of spin-dependent properties in graphene induced by its interaction with ferromagnetic insulator EuO, and show that this proximity effect results in spin polarization of graphene $\pi$ orbitals by up to 24\% together with large exchange splitting bandgap of about 36 meV. Moreover, the position of the Dirac cone is shown to depend strongly on the graphene-EuO interlayer distance. These findings pave the way towards the possible engineering of graphene spin gating by proximity effect especially in a view of recent experiments on successful growth of Europium oxide on top of graphene. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B16.00005: Spin pumping at Permalloy/graphene interfaces Simranjeet Singh, Brett Barin, Ajit Kumar Patra, Barbaros \"Ozyilmaz, Enrique delBarco We present evidence of large spin relaxation effects in CVD graphene observed by means of ferromagnetic resonance (FMR) measurements of Permalloy/graphene (Py/Gr) bilayers. A substantial increase of the FMR linewidth in the Py/Gr bilayer, as compared to the Py layer, is interpreted in terms of an enhancement of the Gilbert damping in the ferromagnetic layer as a consequence of spin pumping at the Py/Gr interface, which is driven by the Py magnetization dynamics (i.e. precession of the magnetization induced by the microwave stimulus at resonance). The remarkable increase in the FMR linewidth compares with observations in other bilayer systems in where thick layers (thicker than the spin diffusion length) of heavy metals with strong spin-orbit interaction are employed as the non-magnetic layer. Our results indicate that spin relaxation in CVD graphene must be greatly enhanced in order to account for the losses of angular momentum by the ferromagnet. We will also present a comparative study of the Gilbert damping in Py/NM films employing highly ordered pyrolitic graphene as the non-magnetic layer, for which a more moderate broadening of the FMR linewidth is observed. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B16.00006: Probing Spin Orbit Interaction in Single Layer Graphene via Electronic Transport Sergio Ulloa, Mahmoud Asmar An important effect on the dynamics of spins in materials is the spin orbit interaction (SOI), which may reflect or arise from intrinsic symmetries in the lattice structure, or via broken symmetries (Rashba interaction) in the system. Resonant scatterers, limiting electron mobility in graphene, are realized by impurities such as hydrogen atoms, molecules, clusters of impurities, vacancies, or metallic islands deposited on (or grown under) the surface of graphene. Resonant scatterers can also generate or enhance the Rashba SOI in graphene samples. We have developed analytical spinor solutions of the Dirac equation that include spin dependent observables, and use these to examine the role of SOI on scattering cross sections.By making use of the ratio of the total to transport cross section in the system at low energy, we predict a strong enhancement in the scattering isotropy in the presence of the intrinsic SOI. Similarly, we see fundamental changes in resonant scatterers in the presence of the Rashba SOI, which also lead to enhanced isotropy. We will discuss how these results have implications on the better characterization of impurities in graphene samples, and how typical experimental results can provide quantitative estimates of the SOI present in the system. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B16.00007: Rashba Spin Orbit Interaction and Birefringent Electron Optics in Graphene Mahmoud Asmar, Sergio Ulloa Analogies between geometrical optics and electron trajectories have resulted in a number of interesting proposals for device applications, where material interfaces play a similar role to that of transparent interfaces in physical optics. Optical birefringence in materials arising from crystal anisotropies are manifested as different group velocities for different polarizations of light. By making use of analytical solutions of the Dirac equation, and extending the partial wave component method of scattering to include spin dependent observables, we show that an equivalent phenomenon to optical birefringence in electron optics is feasible in two dimensional graphene. The electronic birefringence arises from the intrinsic graphene structure and requires the presence of Rashba spin-orbit interaction. The different group velocities depend on the chirality of the electronic states, mimicking the light polarization dependence of the group velocities in optical birefringent materials. In circular regions containing large Rashba interaction and reversed charge density (Veselago lenses), we predict the formation of sets of double caustics and cusps, where the spacing between the two different chiral cusps is proportional to the strength of the Rashba interaction in the system. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B16.00008: Colossal spin-orbit coupling in functionalized graphene Jayakumar Balakrishnan, Gavin Koon, Barbaros Oezyilmaz Graphene's low intrinsic spin orbit (SO) interaction strongly limits the realization of several functional spintronics devices. It is therefore quite desirable to develop methods to tune this SO coupling strength. Among the different approaches, the functionalization of graphene seems to be more promising from an application perspective. Recent theoretical and experimental results on functionalized graphene have shown interesting magnetic properties. Here, we will show our preliminary spin-transport results on such functionally modified graphene and discuss the various possibilities it holds for future graphene-based spintronics applications. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B16.00009: Spin waves in graphene nanoribbon devices Francisco Culchac, Andrea Latg\'e, Rodrigo Capaz, Antonio Costa We investigate spin excitations and electronic properties of graphene nanoribbon devices with zigzag edges. The magnetic region of the device is coupled to nonmagnetic metallic leads. The ground state of the magnetic region is described self-consistently within a mean-field scheme. Spin excitations are extracted from the transverse dynamic spin susceptibility. Several standing-wave modes can be identified. We study the behavior of these modes as the coupling between the magnetic region and the leads is varied. A central point found is that for a finite zigzag nanoribbon, spin excitations are damped at all finite energies. The signature of antiferromagnetic correlations is still present in the predominantly linear relationship between the standing-mode energy and the mode wave vector. The effect of an external doping is also considered and, as in the infinite case, it is found that ferromagnetic order along the ribbon's edges becomes unstable at modest doping levels. We also show the behavior of the spin excitations in the infinite zigzag nanoribbons when an electric field is applied in the transversal direction. As it is well known, this system presents a half-metallic state. A reduction of the spin-wave lifetime is found for increasing electric field intensities. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B16.00010: Spin transport studies in encapsulated CVD graphene Ahmet Avsar, Jun You Tan, Yuda Ho, Gavin Koon, Barbaros Oezyilmaz Spin transport studies in exfoliated graphene on SiO2/Si substrates have shown spin relaxation times that are orders of magnitude shorter than the theoretical predictions. Similar to the charge transport case, the underlying substrate is expected to be the limiting factor. The recent work Zomer, P. J. et al. [1] shows that spin transport over lengths up to 20um is possible in high mobility exfoliated graphene devices on boron nitride (BN) substrates. Here we discuss our initial attempts to repeat such spin transport experiments with CVD graphene on BN substrates. The effect of encapsulation of such devices with an extra BN layer will be also discussed. [1] Zomer, P. J.; Guimaraes, M. H. D.; Tombros, N.; van Wees, B. J. ArXiv:1209.1999, 2012 [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B16.00011: Suppression of spin relaxation due to weak localization in multilayer graphene spin valves Takehiro Yamaguchi, Satoru Masubuchi, Kazuyuki Iguchi, Rai Moriya, Tomoki Machida Graphene is a promising material for spintronics devices because of its long spin diffusion length. In addition, graphene is a fascinating system where quantum interference phenomena such as weak localization and Fabry-Perot interference can be observed because of its long phase coherent length at low temperature. Therefore, graphene is an ideal system for exploring the physics of spin transport and spin relaxation under the phase coherent system. In this study, we fabricated multilayer graphene spin valve devices [1] and investigated temperature dependence of spin transport and spin relaxation properties. Spin relaxation time obtained by Hanle effect with nonlocal geometry was found to start increasing below 70 K and reach 2.5 ns at 2 K. Under the same temperature range, we also found weak localization emerged. These results suggest the correlation of spin relaxation and phase coherent transport in graphene [2]. [1] T. Yamaguchi et al., J. Magn. Magn. Mater. 324, 849 (2012), [2] T. Yamaguchi et al., submitted [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B16.00012: Long Electron Spin Lifetimes in Armchair Graphene Nanoribbons Matthias Droth, Guido Burkard Armchair graphene nanoribbons (aGNR) are promising as a host material for electron spin qubits because of their potential for scalability and long coherence times [1]. The spin lifetime $T_1$ is limited by spin relaxation, where the Zeeman energy is absorbed by lattice vibrations [2], mediated by spin-orbit and electron-phonon coupling. We have calculated $T_1$ by treating all couplings analytically and find that $T_1$ can be in the range of seconds for several reasons: (i) Van Vleck cancellation; (ii) weak spin-orbit coupling; (iii) low phonon density; (iv) vanishing coupling to out-of-plane modes due to the electronic structure of the aGNR. Owing to the vanishing nuclear spin of $^{12}$C , $T_1$ is a good measure for overall coherence. These results and recent advances in the controlled production of graphene nanoribbons [3] make this system interesting for classical and quantum spintronics applications.\\[4pt] [1] B. Trauzettel, D. V. Bulaev, D. Loss, and G. Burkard, Nature Phys. 3, 192-196 (2007).\\[0pt] [2] M. Droth and G. Burkard, Phys. Rev. B 84, 155404 (2011).\\[0pt] [3] X. Zhang et al., arXiv:1205.3516 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B16.00013: Giant magnetic anisotropy of 5d dopants in graphene and boron nitride monolayer Jun Hu, Ruqian Wu Searching for novel magnetic nanostructures is urgent due to the need for the miniaturization of spintronics devices. One of the main bottlenecks for this is the low blocking temperature (\textless 10 K) in most magnetic nanoentities studied so far. In this work, we predict that extremely high blocking temperature can be achieved in graphene or boron nitride monolayer by embedding 5d transition metal (TM) atoms, based on density functional theory calculations. For example, the size of the magnetocrystalline anisotropy energy (MAE) of Re/graphene or Re/BN can be larger than 20 meV for each Re atom, sufficient for room temperature magnetic recording and spintronics applications. We provide physical insights for the further development of nanostructures with larger MAE. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B16.00014: Magneto-Resistance in thin film boron carbides Elena Echeverria, Guangfu Luo, J. Liu, Wai-Ning Mei, F.L. Pasquale, J. Colon Santanta, P.A. Dowben, Le Zhang, J.A. Kelber Chromium doped semiconducting boron carbide devices were fabricated based on a carborane icosahedra (B$_{10}$C$_{2}$H$_{12})$ precursor via plasma enhanced chemical vapor deposition, and the transition metal atoms found to dope pairwise on adjacent icosahedra site locations. Models spin-polarized electronic structure calculations of the doped semiconducting boron carbides indicate that some transition metal (such as Cr) doped semiconducting boron carbides may act as excellent spin filters when used as the dielectric barrier in a magnetic tunnel junction structure. In the case of chromium doping, there may be considerable enhancements in the magneto-resistance of the heterostructure. To this end, current to voltage curves and magneto-transport measurements were performed in various semiconducting boron carbide both in and out plane. The I-V curves as a function of external magnetic field exhibit strong magnetoresistive effects which are enhanced at liquid Nitrogen temperatures. The mechanism for these effects will be discussed in the context of theoretical calculations. [Preview Abstract] |
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