Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D8: Spin Transport in Carbon-based Materials |
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Sponsoring Units: GMAG Chair: Jing Shi, University of California, Riverside Room: Portland Ballroom 255 |
Monday, March 15, 2010 2:30PM - 3:06PM |
D8.00001: Recipes for lateral spin transport between magnetic contacts, advantage of carbon-based materials. Invited Speaker: After the presentation of magneto-transport results [1] on metallic carbon nanotubes (CNT) between LSMO electrodes (MR $\approx $ 60-70{\%}, [V$_{AP}$ -- V$_{P}$] $\approx $ 60mV), I will discuss the general problem of spin transport in a nonmagnetic lateral channel between spin-polarized contacts in both the diffusive and ballistic regimes. In the diffusive regime, a treatment by the classical drift-diffusion equations applied to a multi-terminal structure is used to calculate what can be expected for the output signal with local or non-local voltage probes. A general result is that the output signal ($\Delta $R = $\Delta $V/I where $\Delta $V is the local or non-local output voltage), directly related to the spin accumulation splitting in the channel, scales with the smallest of the relevant spin and interfaces resistances. In the best situation, that is with only tunnel contacts having the same (large) resistance R$_{T}$ and separated by less than the spin diffusion length ($\lambda )$ in a lateral channel limited to the zone of the contacts, the signal $\Delta $R increases in proportion of R$_{T}$ as long as the dwell time is smaller than the spin lifetime. $\Delta $R can be thus much larger than the spin resistance of the channel (product of its resistivity by the ratio $\lambda $/section ). This explain why, in the experiments of Ref.[1] on CNT, $\Delta $R can be as large as 90 M$\Omega $, that is of the order of the tunnel contact resistances and much larger than the spin resistance of the CNT (smaller signals in experiments with CNT or graphene are often due the leak of spin accumulation in lateral channels extending too far outside the contacts). The relative disadvantage for semiconductors comes from the too long dwell time due to much smaller electron velocities than in metallic CNTs (and graphene). We will conclude by a similar analysis of the ballistic regime and a discussion of experiments with graphene. \\[4pt] [1] Hueso et al, Nature 445, 410 (2007). [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:42PM |
D8.00002: Electronic spin transport, spin precession and spin relaxation in graphene field effect transistors Invited Speaker: After an introduction of spintronics in graphene I will describe our recent measurements of spin transport in graphene field effect transistors. Using a non-local geometry, with seperated ferromagnet injector and detector circuits, we were able to study the injection, transport, relaxation, precession and detection of carrier spins in great detail, in both the metallic electron and hole regimes as well as at the Dirac charge neutrality point. We found that: a) Carriers can carry spins in graphene, with typical spin relaxation lengths of 1 to 2 micrometers [1]. b) The spin relaxation times were found to be in the order of 100 to 200 ps, and the spin relaxation of spins directed perpendicular to the graphene plane was found to be slightly faster that spins oriented parallel to the plane [2]. c) Spin transport occurs by diffusion. It was found however that carrier drift, induced by applying large electric fields in the graphene layer could speed up or slow down the transport of spins [3]. d) We found that the spin relaxation is most likely limited by the carrier impurity potential scatterering, probably by the Elliot Yafet mechanism. No appreciable change was observed in graphene nanoribbons down to 100 nm width [4]. e) By changing the diffusion coefficient by changing the carrier density we were able to observe that an increase in the diffusion constant, and thus in the momentum scattering time, is accompanied by a similar increase in in the spin relaxation time [5]. When extraploating these resulst to high mobility (suspended) graphene, this implies that spin relaxation lengths approaching 100 micrometers might be possible at room temperature. Also it was found that the diffusion constants for charge and spin are similar within less than 10{\%}. f) Finally I will present recent results where we compare spin transport in single and N-layer graphene (with N ranging for 2 to 20) We find an increase in the spin relaxation time when the number of layers is increased. Possible mechanisms will be discussed. [1] N. Tombros \textit{et al}., Nature 448, 571 (2007) [2] N. Tombros \textit{et al.}, Phys. Rev. Lett. 101, 046601 (2008) [3] C. Jozsa \textit{et al.}, Phys. Rev. Lett. 100, 236603 (2008), C. Jozsa \textit{et al}., Phys. Rev. B79, 081402 (2009) [4] M. Popinciuc \textit{et al.}, to be published in Phys. Rev. B, arXiv: 0908.1039 [5] C. Jozsa \textit{et al}., to be published in Phys. Rev. B, arXiv: 0910.1054 [6] T. Maassen, in preparation. [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 4:18PM |
D8.00003: Molecular Spintronics Invited Speaker: In organic molecules and molecular solids the weak spin-orbit and hyperfine interactions result in extremely long spin-lifetimes reaching up to the second mark. However the same are characterized by a generally poor mobility, so that the spin-diffusion lengths are rather short. These peculiar characteristics position organic molecules in a unique space within Spintronics and one should envision applications where the spins are manipulated close to where they are injected [1]. In this contribution I will review the current state of the art of the theory of spin-transport and manipulation in organic molecules. I will start the discussion by presenting a new mechanism, the electrostatic spin crossover effect, for manipulating electrically the magnetic state of a molecules without calling for current-driven spin-transfer torques [2]. This is based on the fact that the different spin states of a molecule Stark-shift differently and it is mostly effective when inversion symmetry is broken. Then I will move to discuss the consequences of such an effect on the transport properties of a molecule presenting two magnetic centers and demonstrate that there exist a critical voltage at which the current becomes temperature-independent [3]. Finally I will present results for spin-transport in Mn$_{12}$ and demonstrate that the magnetic state of the molecule can be read electrically with a single $I$-$V$ read-out obtained by using non-magnetic electrodes [4]. \\[4pt] [1] G. Szulczewski, S. Sanvito and J.M.D. Coey, Nature Materials {\bf 8}, 693 (2009).\\[0pt] [2] N.~Baadji, M.~Piacenza, T.~Tugsuz, F.~Della~Sala, G.~Maruccio and S.~Sanvito, Nature Materials {\bf 8}, 813 (2009).\\[0pt] [3] S.K.~Shukla and S. Sanvito, Phys. Rev. B, in press; also at arXiv:0905.1607.\\[0pt] [4] C.D.~Pemmaraju, I.~Rungger and S.~Sanvito, Phys. Rev. B {\bf 80}, 104422 (2009). [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:54PM |
D8.00004: Coherent spin dynamics in organic electronic devices Invited Speaker: Organic semiconductors, such as pi-conjugated polymers, offer exciting opportunities for the development of novel device architectures. While much of the earlier work has focused on exploiting the unique processing conditions of these materials -- solution-based, flexible plastics -- organic electronics also provides access to a range of physical parameters not found in many inorganic systems. The spin-degree of freedom is particularly intriguing in organic semiconductors, which are characterized by weak spin-orbit coupling and medium to strong hyperfine interactions. Primary photoexcitations exhibit strong ($\sim $0.7 eV) exchange interactions, leading to phosphorescent triplet states shifted to lower energy with respect to the singlets [1]. Organic semiconductors exhibit strong magnetic field dependencies in charge carrier recombination and transport, and concomitantly in conductivity [2,3], which generally indicate extremely weak spin-lattice relaxation [2]. Spin dephasing is also very slow, so that spin excitations maintain phase coherence over timescales in excess of microseconds. This phenomenon allows the observation of time-domain spin Rabi flopping in the device current, by exploiting the technique of electrically-detected magnetic resonance [4]. Most recently, spin beating due to the coherently-coupled nutation of electron and hole spins has been observed, providing a direct visualization of hyperfine coupling. Coherent organic spin electronics may ultimately lead to new device concepts besides providing a deeper understanding of fundamental material properties, which are crucial to minimizing degradation. \\[4pt] [1] PRL 89, 167401 (2002). \\[0pt] [2] Nature Mat. 4, 340 (2005). \\[0pt] [3] Nature Mat. 7, 598 (2008). \\[0pt] [4] Nature Mat. 7, 723 (2008). [Preview Abstract] |
Monday, March 15, 2010 4:54PM - 5:30PM |
D8.00005: Spin polarized tunneling and injection in organic semiconductors Invited Speaker: In recent years, organic spintronics has emerged as a hot area of research leading to advances in their fundamental understanding with a potential for technological development [1]. Organic semiconductors (OS) with their good optical properties, combined with the possibility of weak spin scattering have provided high impetus to this field. The strong excitonic and polaronic mechanisms of charge transport in OS have been widely studied. However, their influence on spin transport processes is an open area that is just beginning to be explored. Given this scenario, another area that needs attention is to achieve efficient spin injection into OS. This is possible using conventional spin sources such as ferromagnetic metals or using ferromagnetic insulators such as a spin filter. The mechanism of tunneling provides many interesting attributes for spin injection. Recent studies showing large tunneling magnetoresistance in OS has given some significant results [2]. The fact that OS exhibit strong electron-phonon coupling provide additional information to our spin injection study at the FM/OS interface. The use of inelastic tunneling spectroscopy has shed interesting insights on the morphological ordering of the organic molecules at these interfaces. The study performed on OS, rubrene, corroborated with structural and transport measurements reveal the importance of these findings on spin injection efficiency [3]. In addition, exploring the (FM/OS) interface magnetism using polarized neutron reflectometry and SQUID has shown complex behavior. These studies provide valuable input for the optimization of our approach for spin injection and transport by improving the device structure, leading to enhanced spin signals. In addition, the use of spin filter as the injection source is currently being explored and shall be presented. \\[4pt] [1] Dediu et al., Nat. Mater. \textbf{8} (2009); V. Vardeny, Organic spintronics, Taylor {\&} Francis, 2010.\\[0pt] [2] Santos et al. Phys. Rev. Lett., \textbf{98} (2007); Shim et al., Phys. Rev. Lett. \textbf{100} (2008).\\[0pt] [3] Raman et al., Phy. Rev. B (2009). [Preview Abstract] |
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