Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F24: Graphene SpintronicsInvited
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Sponsoring Units: GMAG Chair: Roland Kawakami, Ohio State University Room: New Orleans Theater C |
Tuesday, March 14, 2017 11:15AM - 11:51AM |
F24.00001: Advances in graphene spintronics Invited Speaker: bart van wees I will give an overview of the status of graphene spintronics, from both scientific as technological perspectives. In the introduction I will show that (single) layer graphene is the ideal host for electronic spins, allowing spin transport by diffusion over distances exceeding 20 micrometers at room temperature. I will show how by the use of carrier drift, induced by charge currents, effective spin relaxation lengths of 90 micrometer can be obtained in graphene encapsulated between boron-nitride layers[1]. This also allows the controlled flow and guiding of spin currents, opening new avenues for spin logic devices based on lateral architectures. By preparing graphene on top of a ferromagnetic insulator (yttrium iron garnet (YIG)) we have shown that we can induce an exchange interaction in the graphene, thus effectively making the graphene magnetic[2]. This allows for new ways to induce and control spin precession for new applications. Finally I will show how, by using two-layer BN tunnel barriers, spins can be injected from a ferromagnet into graphene with a spin polarization which can be tuned continuously from -80{\%} to 40{\%}, using a bias range from -0.3V to 0.3V across the barrier[3]. These unique record values of the spin polarization are not yet understood, but they highlight the potential of Van der Waals stacking of graphene and related 2D materials for spintronics. [1] J. Ingla Aynes et al., Eighty --eight percent directional guiding of spin currents with 90 micrometer relaxation length in bilayer graphene using carrier drift, Nano Lett. 16, 4825 (2016) [2] C. Leutenantsmeyer et al., Proximity induced room-temperature ferromagnetism in graphene probed with spin currents, 2D Mater. 4, 014001 (2017) [3] M.Gurram et al., Giant electric field controlled spin polarization in ferromagnet/bilayer boron nitride/graphene tunneljunctions, submitted to Nat. Comm. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:27PM |
F24.00002: Magnetic proximity control of spin currents and giant spin accumulation in graphene Invited Speaker: Simranjeet Singh Two dimensional (2D) materials provide a unique platform to explore the full potential of magnetic proximity driven phenomena. We will present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to the ferromagnetic-insulator (FMI) magnetization in graphene/FMI heterostructures. Owing to clean interfaces, a strong magnetic exchange coupling leads to the experimental observation of complete spin modulation at low externally applied magnetic fields in short graphene channels. We also discover that the graphene spin current can be fully dephased by randomly fluctuating exchange fields. This is manifested as an unusually strong temperature dependence of the non-local spin signals in graphene, which is due to spin relaxation by thermally-induced transverse fluctuations of the FMI magnetization. Additionally, it has been a challenge to grow a smooth, robust and pin-hole free tunnel barriers on graphene, which can withstand large current densities for efficient electrical spin injection. We have experimentally demonstrated giant spin accumulation in graphene lateral spin valves employing SrO tunnel barriers. Nonlocal spin signals, as large as 2 mV, are observed in graphene lateral spin valves at room temperature. This high spin accumulations observed using SrO tunnel barriers puts graphene on the roadmap for exploring the possibility of achieving a non-local magnetization switching due to the spin torque from electrically injected spins. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 1:03PM |
F24.00003: A two-dimensional spin field-effect switch Invited Speaker: Felix Casanova The integration of the spin degree of freedom in charge-based electronic devices has revolutionised both sensing and memory capability in microelectronics. Further development in spintronic devices requires electrical manipulation of spin current for logic operations. The mainstream approach followed so far, inspired by the seminal proposal of the Datta and Das spin modulator [1], has relied on the spin-orbit field as a medium for electrical control of the spin state [2-4]. However, the still standing challenge is to find a material whose spin-orbit coupling (SOC) is weak enough to transport spins over long distances, while also being strong enough to allow their electrical manipulation. In our recent work [5], we demonstrate a radically different approach by engineering a van der Waals heterostructure from atomically thin crystals [6], and which combines the superior spin transport properties of graphene with the strong SOC of MoS$_{\mathrm{2}}$, a transition metal dichalcogenide with semiconducting properties. The spin transport in the graphene channel is modulated between ON and OFF states by tuning the spin absorption into the MoS$_{\mathrm{2}}$ layer with a gate electrode [5]. Our demonstration of a spin field-effect switch using two-dimensional (2D) materials identifies a new route towards spin logic operations for beyond CMOS technology. Furthermore, the van der Waals heterostructure at the core of our experiments opens the path for fundamental research of exotic transport properties predicted for transition metal dichalcogenides [7], in which electrical spin injection has so far been elusive. [1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990). [2] H.C. Koo et al., Science 325, 1515 (2009). [3] J. Wunderlich et al., Science 330, 1801 (2010). [4] P. Chuang et al., Nat. Nanotechnol. 10, 35 (2015). [5] W. Yan et al., Nat. Commun. 7, 13372 (2016). [6] A. K. Geim and I. V. Grigorieva, Nature 449, 419 (2013). [7] Y. Song and H. Dery, Phys. Rev. Lett. 111, 026601 (2013). [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:39PM |
F24.00004: Graphene: A membrane with steadily improving charge and spin transport properties Invited Speaker: Bernd Beschoten Long electron spin lifetimes are an important prerequisite for enabling advanced spintronic devices. In this respect the 1-ns benchmark is of high technological interest as it marks the threshold at which manipulation of spins with electrical high frequency technology becomes feasible (1 ns $\leftrightarrow$ 1 GHz). For a long time, the measured spin lifetimes were shorter than 1 ns. Here we report on a major improvement in device fabrication which pushes the spin lifetimes to 12.6 ns in single layer graphene spin transport devices at room temperature which results in spin diffusion lengths as long as 30.5 $\mu$m [1]. This is accomplished by the fabrication of Co/MgO-electrodes on a Si/SiO$_2$ substrate and the subsequent dry transfer of a graphene/hexagonal boron nitride (hBN) stack on top of this electrode structure where a large hBN flake is needed in order to diminish the ingress of solvents along the hBN-to-substrate interface. We demonstrate that the spin lifetime does not depend on the contact resistance area products in these devices, indicating that spin absorption at the contacts is not the predominant source for spin dephasing which may pave the way towards probing intrinsic spin properties of graphene. In the second part, we summarize our effort to replace natural by synthetically grown graphene [2]. We report on an advanced transfer technique that allows both reusing the copper substrate of the CVD graphene growth process and making devices with carrier mobilities as high as three million cm$^2$/(Vs) [3] thus rivaling exfoliated "natural" graphene. This material quality allows truly ballistic experiments with electron mean free paths exceeding 28 $\mu$m which brings novel electron-optic devices into reach. [1] M. Dr\"{o}geler et al., Nano Lett. 16, 3533 (2016). [2] L. Banszerus et al., Sci. Adv. 1, e1500222 (2015). [3] L. Banszerus et al., Nano Lett. 16, 1387 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 2:15PM |
F24.00005: Atomic-scale control of graphene magnetism by using hydrogen atoms Invited Speaker: Ivan Brihuega Incorporating magnetism to the long list of graphene capabilities has been pursued since its first isolation in 2004. In this talk I will show how we use a scanning tunneling microscope to explore and manipulate graphene pi-magnetism at an atomic level. Our work shows how the absorption of single H atoms on graphene magnetizes the graphene regions around them. In contrast to common magnetic materials, where the magnetic moments are localized in a few angstroms, the induced graphene magnetic moments extend over several nanometers and present an atomically modulated spin texture. Our measurements also prove that the induced magnetic moments couple strongly at very long distances following a particular rule: magnetic moments sum-up or neutralize critically depending on the relative H-H adsorption sites [1]. [1] H. Gonzalez-Herrero, J. M. Gomez-Rodriguez, P. Mallet, M. Moaied, J. J. Palacios, C. Salgado, M.M. Ugeda, J. Y. Veuillen, F. Yndurain, I. Brihuega, Science, 352, 437 (2016) [Preview Abstract] |
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