Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y16: Focus Session: Spins in Carbon-Based Materials -- Magnetoresistance, Magneto-Electric Effect |
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Sponsoring Units: GMAG DMP Chair: Bin Hu, University of Tennessee Room: D173 |
Friday, March 25, 2011 8:00AM - 8:36AM |
Y16.00001: Quantum Linear Magnetoresistance and Extraordinary Magnetoresistance in Graphene Invited Speaker: Graphene, a single atomic layer of hexagonally arranged carbon atoms, presents the optimal platform to study rarely-observed magnetoresistance (MR) effects because of its temperature-independent mobility and linear band structure with zero band gap. Linear magnetoresistance (LMR), which is characterized as a large, non-saturating linear MR, is one such unusual effect. Normally, the resistance of a conductor in an applied magnetic field increases quadratically with field and then saturates at a relatively low value. Models that explain LMR behavior have been proposed that include both quantum and classical origins, but most systems studied thus far can be explained by a purely classical model. However, we find that quantum LMR is observed in multilayer epitaxial graphene grown on SiC at temperatures as high as 300 K and with a magnitude greater than 200{\%} at 12 Tesla (T). In addition, a phenomenon closely related to classical LMR called extraordinary magnetoresistance (EMR) and characterized by even larger MR, can be realized in metal-shunted graphene devices. Here, due to the different magnetic-field-dependent resistances of the metallic shunt, graphene, and shunt-graphene interface, current flows easily through the shunt in zero and low magnetic field, while in high magnetic field, more current flows around the shunt and is redistributed in the graphene. Devices made from chemical vapor deposition (CVD) graphene grown on copper and transferred to a SiO$_{2}$/Si substrate with Ti/Au shunts display gate-tunable longitudinal MR of $\sim $600{\%} at 12 T and also show promise for use as Hall sensors. Graphene magnetoresistance devices have many possible applications including magnetic field sensors and magnetic read-heads. In contrast with the many proposed electronic uses for graphene, which necessitate the creation of a band-gap, graphene magnetoresistance devices that exploit LMR or EMR provide a use for as-grown or deposited graphene. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y16.00002: Frequency dependence of organic magnetoresistance Fujian Wang, James Rybicki, Ran Lin, Kent Hutchinson, Jia Hou, Markus Wohlgenannt Organic magnetoresistive (OMAR) devices show a large enough magnetoresistive response (typically 10{\%}) for potential applications as magnetic field sensors. However, applications often require sensing high frequency magnetic fields, and the examination of the frequency-dependent magnetoresistive response is therefore required. Analysis of time constants that limit the frequency response may also shed light on the mechanism behind the OMAR effect, because different OMAR mechanisms occur at different time scales In our experiments, the AC magnetic field is supplied by a coil with a ferrite core which is driven by a function generator The AC magnet shows a frequency response that is almost flat up to 1MHz. We found that the OMAR frequency limit is about 10 kHz for a typical organic semiconductor device and at least 100 kHz for devices made from a doped polymer film. We also performed capacitance and conductance vs. frequency measurements to understand the origin of the observed limit frequencies. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y16.00003: Room Temperature Ferromagnetic Polymer and the Correlated Anomalous Magnetoresistance Phenomenon Jinsong Huang, Bin Yang, Jeffrey Shield Organic magnetoresistance (OMAR) has been observed in organic semiconductor devices where resistance can change in a relatively small external magnetic field at room temperature. Since a weak magnetic field is involved, the hyperfine interaction (HFI) is employed to explain OMAR in the reported literatures. None of these issues consider the magnetic properties of the organic semiconductors themselves. However, the we recently discovered that polymer semiconductors, such as poly(3-hexylthiophene) P3HT, can have room temperature (RT) ferromagnetic properties in their crystalline phase and when mixed with phenyl-C61-butyric acid methyl ester (PCBM). Here, we will report the possible correlation between the ferromagnetic property of the P3HT:PCBM and anomalous OMAR phenomenon including the anisotropic and hysteretic OMAR behavior. The magnetic property of the polymer including the anisotropic and photo induced change of magnetism will be also discussed to explore the possible mechanism of the room temperature ferromagnetism.~ [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y16.00004: Magnetic fringe field control of electronic transport in an organic film Markus Wohlgenannt, Fujian Wang, Ferran Macia, Andrew Kent, Michael Flatte Random nuclear hyperfine fields in organic materials dramatically affect electronic transport properties such as the electrical (photo)conductivity and electroluminescence. The influence of these nuclear hyperfine fields can be overwhelmed by a uniform external applied magnetic field. As a result, in applied magnetic fields of about 10mT the kinetics of exciton formation, bipolaron formation, and carrier hopping are all modified, leading to changes in room-temperature electrical transport properties in excess of 10 {\%} in many materials. Here we demonstrate a new method of controlling the electronic transport in an organic film, using the spatially-varying magnetic fringe fields of an unsaturated ferromagnetic electrode. The effect of these magnetic fringe fields is hysteretic, anisotropic, and depends sensitively on the distance of the organic material from the ferromagnetic electrode; all these effects appear in the magnetic-field dependences of electronic transport in these films. Such structures, which do not rely on spin injection or spin-valve behavior, may provide a simple approach to integrating magnetic metals and organics for hybrid spintronic devices. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y16.00005: Spin-Boson Theory of Organic Magnetoresistance Chang-Qin Wu The discovery of room-temperature, low-field magnetoresistance (MR) in organic light-emitting devices was one of major achievements of spintronics in the last decade. Compared to its inorganic counterpart, a sizable organic MR (OMR) is relatively easy to be obtained, showing extensive potential in magnetically controlled applications. Yet, after years of intense research, a comprehensive understanding of this magnetic field effect out of these nonmagnetic materials is still lacking. In this work, we present a spin-boson theory for magnetotransport in organic semiconducting materials, on the basis of a coupling between charge carriers' spin and a local bosonic environment, which is shown to be an irreducible ingredient in understanding of the anomalous OMR. Among those compose this environment triplet excitons play a basic role. The incoherent hopping rate between molecules is calculated to give out the basic behavior of OMR. The underlying mechanism is revealed from the calculation of entanglement, represented by the von Neumann entropy, between the carrier's spin and bosons. We also obtain the dependence of OMR on both of the bias voltage and the spin-boson coupling. The results obtained from the theory are in good agreement with experiments. [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y16.00006: Electronic and transport properties of Cobalt-based valence tautomeric molecules and polymers Yifeng Chen, Arrigo Calzolari, Marco Buongiorno Nardelli The advancement of molecular spintronics requires further understandings of the fundamental electronic structures and transport properties of prototypical spintronics molecules and polymers. Here we present a density functional based theoretical study of the electronic structures of Cobalt-based valence tautomeric molecules Co$^{III}$(SQ)(Cat)L Co$^{II}$(SQ)$_{2}$L and their polymers, where SQ refers to the semiquinone ligand, and Cat the catecholate ligand, while L is a redox innocent backbone ligand. The conversion from low-spin Co$^{III}$ ground state to high-spin Co$^{II}$ excited state is realized by imposing an on-site potential U on the Co atom and elongating the Co-N bond. Transport properties are subsequently calculated by extracting electronic Wannier functions from these systems and computing the charge transport in the ballistic regime using a Non-Equilibrium Green's Function (NEGF) approach. Our transport results show distinct charge transport properties between low-spin ground state and high-spin excited state, hence suggesting potential spintronics devices from these molecules and polymers such as spin valves. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y16.00007: Spin polarized transport properties of impurity induced Carbon nanostructures Serkan Caliskan, Mehmet Canturk We study spin polarized transport on structures consisting of Carbon wires including impurities. We perform first principle calculations on these structures using the nonequilibrium Green Function formalism combined with the density functional theory. The different impurity induced Carbon nanostructures are found to depend strongly on the geometrical disorder. Through the conductance, transmission spectra, density of states and current-voltage characteristics the numerical results of spin polarized calculations are discussed. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y16.00008: Spin transport in organic semiconductor single crystals H.-Jae Jang, Oleg Kirillov, Kurt Pernstich, William Rippard, Katelyn P. Goetz, Oana D. Jurchescu, David Gundlach, Mariona Coll Bau, Brad R. Conrad, Christina Hacker, Curt A. Richter Organic semiconductors have been attracting much attention as potential spin transport media due to their weak spin-orbit and hyperfine interactions that promise long spin lifetimes. However, to date most studies have focused on amorphous, or polycrystalline thin-film based organic semiconductors. In addition, short transport distances equal to or less than a couple of hundred nanometers have been measured despite the prediction of long spin transport distance. We have investigated spin injection and transport in high purity single-crystal organic semiconductors, especially rubrene(5,6,11,12-tetraphenylnaphthacene). We will present and discuss our experimental results obtained in both vertical and lateral transport geometries. Great care is needed to understand and avoid possible spurious effects in these studies. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y16.00009: Correlation of electric polarization and magnetic ordering in cobalt chloride thiourea Eundeok Mun, Jason Wilcox, Jamie Manson, Brian Scott, Paul Tobash, Eric Bauer, Vivien Zapf The coupling between electricity and magnetism in magneto-electric multiferroics has been intensively investigated in a wide range of transition metal oxides. Recently the material classes have been extended to organo-metallic insulators (sometimes known as metal-organic frameworks or molecular magnets) such as NiCl$_{2}$-4[SC(NH$_{2})_{2}$], which provides a new arena for designing magneto-electric multiferroics. We have grown single crystals of cobalt chloride thiourea, CoCl$_{2}$-n[SC(NH$_{2})_{2}$], which forms two different crystal structures with n = 2 and 4. The compound CoCl$_{2}$-2[SC(NH$_{2})_{2}$] has a triclinic crystal structure with strong magnetic anisotropy and $\sim $ 3 $\mu _{B}$/Co ion, indicating \textbf{S} = 3/2 Co spins, and the compound CoCl$_{2}$-4[SC(NH$_{2})_{2}$] has a tetragonal structure with almost no magnetic anisotropy and 1 $\mu _{B}$/Co ion, indicating \textbf{S} = 1/2 Co spins. We will present details of the magnetic field-induced electric polarizations and magnetic properties of these compounds. [Preview Abstract] |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y16.00010: Magneto-electric multiferroic behavior in a metal-organic framework Vivien S. Zapf, Pinaki Sengupta, Cristian Batista, Farzana Nasreen, Frederik Wolff-Fabris, Armando Paduan-Filho We will discuss strong magneto-electric coupling in the metal-organic compound NiCl$_{2}$-4SC(NH$_{2})_{2 }$(also known as a metal-organic framework or molecular magnet). Magneto-electric multiferroic behavior is traditionally investigated in transition-metal oxides, however we are expanding the field to metal-organics, which are designable materials with soft lattices and electrically polar organic elements. In this material we observe a magnetic field-induced change in the electric polarization of 50 $\mu $C/m$^{2}$ driven by ordering of the Ni S = 1 spins. We can model it in terms of a combination of exchange striction and crystal electric fields, and Quantum Monte Carlo simulations of these effects provide an excellent fit to the data. We find that the induced electric polarization is a sum of $<$S$_{z}^{2}>$ and the nearest neighbor correlation function $<$S$_{i}$ dot S$_{j}>$ The presence of electrically polar thiourea molecules [SC(NH$_{2})_{2}$] amplifies the effect of small magnetically induced crystal distortions on the electric polarization. [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y16.00011: Manipulating singlet-triplet equilibrium in organic biradical materials O. Gunaydin-Sen, J. Fosso-Tande, P. Chen, J.L. White, J.L. Musfeldt, R.J. Harrison, T.L. Allen, P.M. Lahti, J. Cherian, T. Tokumoto, S. McGill We investigated the tunability of the singlet-triplet equilibrium population in 1,4-phenylenedinitrene via magneto-optical spectroscopy. Both temperature- and magnetic field-induced spectral changes in this organic biradical are sensitive to magnetic energy scales, specifically the spin gap, demonstrating the important interplay between charge, and magnetism in this system. These measurements also establish the value of local-probe photophysical techniques for extraction of magnetic properties data in systems where a traditional Curie law analysis has intrinsic limitations. *This work is supported by the National Science Foundation. [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y16.00012: Reversible mechanism for spin crossover in transition-metal cyanides Mukul Kabir, Krystyn J. Van Vliet Spin transitions generally occur in compounds of octahedrally coordinated 3$d$ transition metal ions. These transitions can be induced by external perturbations such as light, heat, pressure, magnetic field, and chemical substitution. Transition metal cyanides are one such material, which exhibit {\em reversible} spin transition while perturbed with light at $T < $10 K. Here we report the first-principles (DFT+U) study of anhydrated KCoFe(CN)$_6$. We find that the complete spin transition from the low spin ground sate ($S=0$) to a high spin ($S=2$) state takes place due to intra-atomic and inter-atomic charge transfers in two steps. In the first step a $d$-electron is transferred from Fe to Co through cyanide ligand, which is followed by the $d$-electron rearrangement in the Co. This spin transition is strongly correlated with the internal lattice, and we find as large as 10\% extension of the Co$-$N bond via a Jahn-Teller active (tetragonally distorted) lattice in the intermediate spin ($S=1$) state. The calculated energy required for this transition is in agreement with experiments. We further predict that this spin transition in such materials can be induced, and further tuned, by external pressure to enable realization of such reversible transitions at ambient temperatures. [Preview Abstract] |
Friday, March 25, 2011 10:48AM - 11:00AM |
Y16.00013: Psuedo-entanglement between nuclear spins in photoexcited functionalized fullerenes Vasileia Filidou, Stephanie Simmons, Harry L. Anderson, G. Andrew D. Briggs, Arzhang Ardavan, Steven Karlen, Feliciano Giustino, John J.L. Morton Optically excited triplet electron spins can be used to polarise, manipulate, couple and measure nuclear spins. Here we present photoexcited pulsed magnetic resonance experiments for the characterization of functionalized fullerene structures with homo and hetero nuclear spins. We use density functional theory in order to predict the hyperfine interaction between the photoexcited triplet and various nuclear spins in the structure, and then use magnetic resonance (ENDOR) to investigate these values experimentally. In addition to the hyperfine coupling strength, we measure the relevant relaxation rates and initial hyperpolarisation of the triplet in order to understand the possible degree of entanglement of nuclear spins through the optically excited mediator spin. We measure an increased nuclear-nuclear coupling in the presence of the triplet which permits fast nuclear controlled-NOT gates.These operations, in conjunction with the transfer of electron polarisation to the nucleus, allow the demonstration of nuclear-nuclear pseudo-entanglement, measured using quantum state tomography. [Preview Abstract] |
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