Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F6: Focus Session: Spin-Dependent Physics in Carbon-Based Materials II |
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Sponsoring Units: GMAG DMP Chair: Tatiana Rappoport, Universidade Federal do Rio de Janeiro Room: 108 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F6.00001: Electrical spin injection into graphene through hexagonal boron nitride tunnel barrier Takehiro Yamaguchi, Yoshihisa Inoue, Satoru Masubuchi, Sei Morikawa, Masahiro Onuki, Kenji Watanabe, Takashi Taniguchi, Rai Moriya, Tomoki Machida Two-dimensional crystals such as graphene, h-BN, and transition metal dichalcogenides are emergent material system and receiving much attention for spintronics applications. Particularly, these 2D crystals have significant advantages when they are used as a tunnel barrier. 1) These materials can be exfoliated with a monolayer thick resolution. 2) A single-crystalline flake can be fabricated. 3) A wide range of band gaps are available. However, up to now, spin polarized tunneling through these materials has not been fully explored experimentally. Here, we demonstrate spin polarized tunneling through one monolayer thick of hexagonal boron nitride (h-BN) layer and used it for electrical spin injection into graphene [1]. A NiFe/ML h-BN/bilayer graphene/h-BN structure is fabricated using a micromechanical cleavage and dry transfer technique. I-V curve across h-BN exhibits non-linear characteristics and suggests the successful fabrication of tunnel barrier. A spin signal is observed in non-local magnetoresistance measurement. Spin diffusion constant and spin relaxation time are obtained from the Hanle measurement.\\[4pt] [1] T. Yamaguchi, Y. Inoue, et al., Applied Physics Express 6, 073001 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F6.00002: Effect of contacts on spin lifetime measurements in Graphene Evan Sosenko, Vivek Aji Current spintronic devices favor Graphene's high carrier mobility, however spin precession measurements using the Hanle effect in nonlocal spin valve devices have yielded spin lifetimes between 100 ps and 1 ns. These are orders of magnitude smaller than what is observed in ESR measurements or expected theoretically. In this talk, I revisit the issue of contact induced losses, and establish the extent to which it accounts for this discrepancy. We use the standard approach of solving the Block equations augmented by boundary conditions characterizing the device. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F6.00003: Homoepitaxial Graphene Tunnel Barriers Adam Friedman, Olaf van 't Erve, Connie Li, Jeremy Robinson, Berend Jonker Tunnel barriers are key elements for spintronic devices. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, interface stability, and electronic states that severely complicate fabrication and compromise performance. Graphene is the perfect tunnel barrier: It is an insulator out-of-plane, possesses a defect-free, linear habit, and is impervious to interdiffusion. Nonetheless, true tunneling between two stacked graphene layers is not possible except under extreme circumstances. However, two stacked graphene layers can be decoupled using chemical functionalization, which would allow tunneling between the two layers and the realization of an all graphene electronic tunneling device. Here, we demonstrate a homoepitaxial tunnel barrier device in which graphene serves as both the tunnel barrier and the high mobility transport channel. Beginning with bilayer graphene, we fluorinate the top layer to decouple it from the bottom layer, so that it serves as a single monolayer tunnel barrier for both charge and spin injection into the lower graphene transport channel. We demonstrate high spin injection efficiency and lateral transport of spin currents in non-local spin-valve structures and determine spin lifetimes with the non-local Hanle effect. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F6.00004: Tailoring Graphene Spintronics from first Principles Igor Zutic, Predrag Lazic, Guilherme Matos Sipahi, Nicolae Atodiresei, Roland Kawakami, Kirill Belaschenko, Branislav Nikolic Graphene/ferromagnet junctions provide large spin signals and important opportunities for spintronic devices [1,2]. However, for critical studies of such structures it is crucial to establish accurate predictive methods that would yield atomically-resolved information of interfacial properties and incorporate van der Walls interactions. We formulate a computationally-inexpensive model to study spin injection and proximity effects [3] and apply our finding to magneto-logic gates [2] using Ni(111) or Co(0001) as the ferromagnetic electrode. We show that spin polarization maps can be a versatile tool to tailor materials properties for graphene spintronics and explore their relation to computationally more demanding nonequilibrium transport codes [4]. [1] W. Han et al., Phys. Rev. Lett. 105, 167202 (2010); I. Neumann et al., Appl. Phys. Lett. 103,112401 (2013). [2] H. Dery et al., IEEE Trans. Electron Dev. 59, 259 (2012). [3] G. M. Siphai et al., J. Phys. Cond. Matter (in press); P. Lazic et al., preprint. [4] K. K. Saha, et al., Phys. Rev. B 85, 184426 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F6.00005: Cr2O3 Films for Magnetoelectric Gate Applications Sean Stuart, Edward Sachet, J.P. Maria, J.E. (Jack) Rowe, Marc C. Ulrich, Dan Dougherty The magnetoelectric properties of Cr2O3 have been extensively studied, including recent reports of a robust electrically switched magnetic surface state. We have identified Cr2O3 as a material whose magnetoelectric properties would enable voltage controlled switching of the exchange interaction with graphene, as in the Field Effect Transistor proposed by Semenov et al. (Appl. Phys. Lett. 91, 153105). We used pulsed laser deposition to grow thin Cr2O3 films directly on HOPG and sapphire. Atomic force microscopy for films grown on HOPG show closely packed Cr2O3 islands, with a smooth surface interrupted by grain boundaries. X-Ray Diffraction shows that the film has a (0001) texture for films grown at 650 deg. C, which is the ideal orientation for magnetoelectric gating. X-Ray photoelectron spectroscopy on incomplete films suggest strong chemical interactions between the graphite and Cr2O3. Films grown on sapphire have improved crystallinity and surface morphology, which allow for measurement of the surface magnetization by magnetic force microscopy after magneto-electric annealing. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F6.00006: Spin Transfer Torque in Graphene Chia-Ching Lin, Zhihong Chen Graphene is an idea channel material for spin transport due to its long spin diffusion length. To develop graphene based spin logic, it is important to demonstrate spin transfer torque in graphene. Here, we report the experimental measurement of spin transfer torque in graphene nonlocal spin valve devices. Assisted by a small external in-plane magnetic field, the magnetization reversal of the receiving magnet is induced by pure spin diffusion currents from the injector magnet. The magnetization switching is reversible between parallel and antiparallel configurations by controlling the polarity of the applied charged currents. Current induced heating and Oersted field from the nonlocal charge flow have also been excluded in this study. Next, we further enhance the spin angular momentum absorption at the interface of the receiving magnet and graphene channel by removing the tunneling barrier in the receiving magnet. The device with a tunneling barrier only at the injector magnet shows a comparable nonlocal spin valve signal but lower electrical noise. Moreover, in the same preset condition, the critical charge current density for spin torque in the single tunneling barrier device shows a substantial reduction if compared to the double tunneling barrier device. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F6.00007: Defect-Mediated Spin Relaxation and Dephasing in Graphene Invited Speaker: Joshua Folk This talk will describe a series of transport measurements that disentangle mechanisms of spin and orbital phase relaxation in graphene. The measurements are based on well-known quantum interference phenomena--weak localization and universal conductance fluctuations. We show that a careful analysis of the in-plane magnetic field and temperature dependences of these effects can separately quantify spin-orbit and magnetic scattering rates; this technique works especially well in graphene due to its single-atom thickness. Spin relaxation in exfoliated graphene on SiO$_2$ is found to be dominated by magnetic scattering (scattering off of magnetic defects), with a smaller contribution from spin-orbit interaction. A similar measurement performed in graphene on SiC suggests that both magnetic scattering and spin-orbit interaction are a factor of 10 stronger than in exfoliated graphene. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F6.00008: Graphene spin relaxation via resonant scattering off magnetic impurities Denis Kochan, Martin Gmitra, Jaroslav Fabian We will present phenomenological theory, based on first-principles calculations, of the exchange splitting and spin relaxation in graphene with hydrogen adatoms. The phenomenological modeling includes a symmetry based tight-binding model with the adatom interaction and local exchange couplings that are fitted to the first-principles electronic band structure data in the ferromagnetic ground state of hydrogenated graphene. We will show that resonant scattering and the exchange interaction with the paramagnetic impurities at the adatom site can explain the experimentally observed short spin relaxation times, providing a competitive mechanism to that based on spin-orbit coupling. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F6.00009: Inter-valley scattering and spin transport in graphene Sergio E. Ulloa, Mahmoud M. Asmar Electron scattering in graphene is characterized by a highly anisotropic behavior due to the helical nature of its charged carriers. This anisotropy has been experimentally verified in [1], as the ratio of transport to elastic times is found to take a constant value of ~2, consistent with the single valley Dirac equation description at low energies. It was also shown theoretically in [2] that the presence of spin orbit interactions (SOIs) transforms the intra-valley scattering process to be increasingly isotropic for stronger SOI. In this work we analyze the effects of inter-valley scattering on the electronic and spin transport of electrons in graphene. By considering the most relevant terms allowed by time reversal symmetry in the Dirac Hamiltonian, and using partial wave decomposition, we obtain full spin-dependent scattering amplitudes in the system. Here, we present the scattering in the absence and presence of SOIs, where we extract critical strengths of the inter-valley mixing terms that could lead to drastic changes in previous results [1,2]. We also obtain estimates of the critical parameter features of impurities for which the single valley description of graphene fails. [1] M. Monteverde, et al., PRL 104, 126801 (2010).[2] M. M. Asmar and S. E. Ulloa, arXiv:1311.1271 [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F6.00010: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F6.00011: Ferromagnetism on graphene multilayers by hydrogen adsorption Juan J. Palacios, Mohammed Moaied, Jose V. Alvarez, Maria J. Caturla A remarkable theoretical prediction for graphene is that, in theory, it can be permanently magnetized by the adsorption of H atoms. Unfortunately, this will only be possible if the adsorption is selectively realized in such a way that all H atoms occupy the same sublattice so that the contributions of the H-induced local magnetic moments add up due to the expected ferromagnetic coupling in this situation. Inspired by recent experiments, I will show that such selectivity can be naturally achieved on the graphite surface. Due to the sublattice broken symmetry on the surface, a spontaneous arrangement of the hydrogen atoms where all end up adsorbed on the same sublattice takes place at room temperature in a reasonable time scale. First-principles calculations combined with kinetic Monte Carlo simulations and model Heisenberg-like Hamiltonians derived from them give a complete account of the emergence of this novel ferromagnetism. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F6.00012: Magnetism of Adatom on Bilayer Graphene and its Control: A First-principles Perspective Tanusri Saha-Dasgupta, Dhani Nafday We present first-principles investigation of the electronic and magnetic properties of adatom on bilayer graphene within the framework of density functional theory. In particular, we study the influence of an applied gate-voltage which modifies the electronic states of the bilayer graphene as well as shifts the adatom energy states relative to that of the graphene energy states. Our study carried out for a choice of three different adatoms, Na, Cu and Fe, shows that the nature of adatom-graphene bonding evolves from ionic to covalent, in moving from alkali metal, Na to transition metal, Cu or Fe. This leads to the formation of magnetic moments in the latter cases (Cu, Fe) and its absence in the former (Na). Application of an external electric field to bilayer graphene, completely changes the scenario, switching on a magnetic moment for Na adatom, and switching off the magnetic moments for Cu, and Fe adatoms. Our results have important implications for fundamental studies of controlled adatom magnetism and spintronics application in nanotechnology. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F6.00013: Substrate Effects on Adsorbate-induced Magnetism in Graphene Pratibha Dev, Thomas Reinecke Using density functional theory, we show that substrates play an important role in the properties of layered systems such as graphene. In particular, we focus on the effects of a copper substrate on magnetic properties associated with functionalized graphene. Local magnetic moments are created in freestanding graphene by decorating it with an unequal number of fluorine (hydrogen) adatoms in the two sublattices. However, when the functionalized graphene is placed on copper, the local moments completely disappear. We attribute this to several interconnected effects -- doping by the substrate, increased distortion relative to the freestanding case and broadening of the defect states. We show that the interactions with the substrate and the formation of local magnetic moments can be modified by using multiple layers of graphene. This work also shows the importance of including the effects of the immediate environment in determining the properties of functionalized, layered structures such as graphene. [Preview Abstract] |
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