Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D32: Focus Session: Spin Dependent Physics in Organic Materials and Graphene |
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Sponsoring Units: GMAG DMP Chair: Tiffany Santos, Argonne National Laboratory Room: 336 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D32.00001: Magnetism by nonmagnetic defects in a 2D BN sheet: \textit{ab initio} studies Ru-Fen Liu, Ching Cheng This study attempts to resolve: 1) whether a long-range magnetic order can be established in a sp material, 2) whether the magnetic properties can be controlled through defects in the previous systems. Through studying different defects concentrations in a 2D BN sheet by the first-principles methods, we found that, despite most of the defects doped BN sheet only lead to formations of local moments, there are systems exhibiting long-range ordered magnetic moment with estimated T$_{C}\approx $70K[PRB 76, 014405 (2007)]. Our latest results demonstrate that a ferromagnetic free electron gas due to Stoner instability is possible to develop[RFL{\&}CC, to be published]. This result is contrary to the direct transition from paramagnetism to Wigner crystal in a 2D uniform electron gas established recently [N.D. Drummond{\&}R.J. Needs, to be published]. A contrastive study between this Stoner magnet, i.e. a defect system with N atoms replaced by O impurities (O$_{N})$, and a metallic ferromagnetic systems with localized moments (Si$_{B})$ throughout all the considered defect concentrations is studied by their band structure, partial DOS's and the defect-concentration-dependent magnetization energies. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D32.00002: Magnetoresistance in hydrogen-doped graphene nanoribbons. F. Munoz-Rojas, D. Soriano, J. Fernandez-Rossier, J. J. Palacios Recent works have focused on hydrogen-doped graphene, both in the diluted and highly doped concentrations. It is known that a single hydrogen atom on top of a carbon atom in graphene has a magnetic moment. In the case of a low concentration of hydrogen dopants, it is believed that the ground state features local moments with zero total spin. Application of a strong enough magnetic field can spin polarize the system, in analogy with diluted magnetic semiconductors. In this work we study whether this spin order changes the resistance of the system. We study the relation between conductance and spin order for hydrogen-doped graphene armchair nanoribbons. We use both mean field Hubbard model and density functional theory calculations and compare results from both approaches. For the latter one, B3LYP hybrid functional is used. The conductance is calculated for the diluted limit. We use the Landauer formalism with the Green's Function Approach for the conductance calculation. We find that the conductance in these systems is significantly affected by spin order. Thus, we predict magnetoresistance in graphene ribbons doped with Hydrogen. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D32.00003: Spin dependent Transport in Thin Graphite and Few Layer Graphene Christopher Malec, Dragomir Davidovic Few layer graphene as well as thin graphite samples are measured by local and non-local spin injection techniques at 4.2 K. Both spin valve, and spin precession measurements are performed. Spins remain coherent over micron length scales. Latest results will be discussed. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D32.00004: Magnetism and magnetic interactions in graphene and graphite Invited Speaker: Magnetic materials and nanostructures based on carbon and other light elements provide a number of attractive opportunities for future information technologies such as spintronics and quantum information processing. In this talk, I review the first-principles studies of the magnetism induced by defects and edges in graphene and graphite. We show that in graphene the single-atom defects (e.g. vacancies and hydrogen chemisorption) induce the spin-polarized defect states [1,2]. The coupling between the magnetic moments is either ferromagnetic or antiferromagnetic, depending on whether the defects correspond to the same or to different sublattices of the graphene lattice, respectively. These results explain the recent experimental observations of high-temperature ferromagnetism in proton-irradiated graphite. Similarly, the zigzag edges of graphene are predicted to induce localized magnetic moments which can serve as a basis for novel spintronic devices. We address the question of the spin correlation length at finite temperatures in this one-dimensional magnetic system and establish the limitations of the proposed spintronic devices [3]. Finally, I consider the hyperfine interactions (i.e. the magnetic interactions between the spins of electrons and nuclei) in carbon nanostructures and materials [4]. Possible approaches for achieving long electron spin decoherence times in graphene-based nanostructures are discussed. \newline [1] O. V. Yazyev and L. Helm, Phys. Rev. B {\bf 75}, 125408 (2007). \newline [2] O. V. Yazyev, Phys. Rev. Lett. {\bf 101}, 037203 (2008).\newline [3] O. V. Yazyev and M. I. Katsnelson, Phys. Rev. Lett. {\bf 100}, 047209. \newline [4] O. V. Yazyev, Nano Lett. {\bf 8}, 1011 (2008). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D32.00005: Determining the spatial and spin anisotropy of reduced-dimensionality Cu-based magnets using EPR, ultra-high-field magnetization and simulations Susan Cox, Ross McDonald, John Singleton, Pinaki Sengupta, Paul Goddard, Stephen Blundell, Janez Bonca, Samir El Sawish, Jamie Manson, John Schlueter Pulsed-field magnetization experiments (up to 85 T) and electron paramagnetic resonance (EPR) experiments ($10-110$~GHz) are reported on a family of organic Cu-based two-dimensional (2D) Heisenberg magnets. The low-$T$ $M(H)$ relationship is concave, with a sharp transition to a saturation value at a critical field $H_{\rm c}$. Monte-Carlo simulations including a finite interlayer exchange energy quantitatively reproduce the data. Thus, one can obtain accurate values for both intra- and interlayer exchange energies. The EPR spectra show pronounced changes in effective $g$ factor, linewidth and zero- field intercept at temperatures, fields and frequencies of the same energy scale as the dominant exchange parameter. The EPR results are modeled using finite-cluster- size methods, and the data are well matched by an easy-plane spin anisotropy in the range $0.01-0.05$. Thus, EPR measurements allow the spin orientation dependence of the exchange interaction to be determined. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D32.00006: Ferromagnetic transition coupled to magnetoelastic interactions J.L. Musfeldt, L.I. Vergara, T.V. Brinzari, L.C. Tung, Y.J. Wang, J.A. Schlueter, J.L. Manson We investigate the magneto-infrared response of CuHF$_2$(pyz)$_2$BF$_4$, a quasi-two-dimensional Heisenberg antiferromagnet, in order to probe the microscopic aspects of magnetoelastic coupling through the field-driven antiferromagnetic to ferromagnetic transition. The ferromagnetic transition is accompanied by substantial changes in the out-of-plane pyrazine ring distortion and bending modes giving rise to an overall softer lattice. The size of these field-induced distortions tracks the bulk magnetization demonstrating that the ferromagnetic transition is coupled to magnetoelastic interactions in this material. We discuss these results in terms of local structural distortions and the effect on in-plane superexchange interactions. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D32.00007: Magnetic phase diagram of a 2D quantum Heisenberg antiferromangetic compound Cu(pz)$_{2}$(ClO$_{4})_{2}$ Fan Xiao, Nat Fortune, Christopher Landee, Mark Turnbull Cu(pz)$_{2}$(ClO$_{4})_{2}$ is a 2D quantum Heisenberg antiferromagnet with an exchange strength of 17.5(5) K and a zero-field ordering temperature of 4.25 K. The ordering temperature has been found to be affected by an applied field. The phase diagram of Cu(pz)$_{2}$(ClO$_{4})_{2}$ is determined by measuring the magnetization and the in-field specific heat. The behavior of the ordering temperature can be interpreted as a field induced 2D Heisenberg to 2D XY crossover. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D32.00008: Gapped quantum spin-liquid state in a frustrated triangular magnet $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ Minoru Yamashita, Norihito Nakata, Yuichi Kasahara, Takahiko Sasaki, Naoki Yoneyama, Norio Kobayashi, Satoshi Fujimoto, Takasada Shibauchi, Yuji Matsuda Unveiling the nature of quantum-spin-liquids (QSL) states, quantum fluctuation-driven disordered ground states, has been a central challenge in condensed matter physics. Especially the nature of the low-lying spin excitations and the presence/absence of the ``spin gap'' have been of great interest. Recently, NMR measurements have shown that a QSL state is realized in $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ with a nearly isotropic 2D triangular lattice structure. Here we report on our thermal-transport measurements in this compound down to 80~mK. We find that the QSL state has a full gap of $\sim 0.5$~K ($\sim J/500$) and the gap is hardly affected by magnetic fields up to 10~T [1], which sharply contradict recent reports of heat capacity measurements reporting a finite $\gamma$-term. We will discuss some possibilities to explain the tiny spin gap in this triangular system. \\[0pt] [1] M. Yamashita et al., Nature Physics (in press). [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D32.00009: Neutron scattering measurements of spin excitations in the spin ladder compound (pip)$_2$CuBr$_4$* A.T. Savici, C.L. Broholm, G.E. Granroth, S.E. Nagler, K.P. Schmidt, G.S. Uhrig, D.M. Pajerowski, M.W. Meisel, D.R. Talham, C.M. Brown Recent theoretical and experimental work on S=1/2 ladders has
been inspired, in part, by the realization of a novel quantum
spin liquid state. Bulk magnetization measurements indicate that
(C$_5$H$_{12}$N)$_2$CuBr$_4$ (BPCB) is a two-leg spin ladder with
stronger coupling along the rung ($J_\bot$) than along the leg
direction ($J_\|$) [1]. Here we report neutron spectroscopy
measurements performed on a deuterated BPCB. We show that
$J_\bot$ and $J_\|$ are consistent with the previous
measurements. No dispersion in the inter-ladder direction means
that the ladders are magnetically isolated, likely due to
frustrated inter-ladder exchange. We show that any diagonal
exchange is $ |
Monday, March 16, 2009 4:42PM - 4:54PM |
D32.00010: Pressure-dependent $\nu$(CC) and $\nu$(CN) Raman modes of the molecule-based magnets M[TCNE](NCMe)$_2$X Alexander E. Midgley, Konstantin Pokhodnya, C. Olson, A.N. Caruso, Michael B. Kruger M[TCNE] (M=V, Fe, Mn, Co, Ni; TCNE=tetracyanoethylene) molecule-based magnets demonstrate high magnetic ordering due to a strong antiferromagnetic interaction between the unpaired d- and \mbox{p-electrons } of the metal ions and the ligands; however, the type of bonding involved in the superexchange mechanism remains unclear. The Raman active C=C vibration depends solely upon the degree of charge transfer from the metal ion to the $\pi$* antibonding orbital of the ligand, therefore the strength of the vibration is only sensitive to backbonding. Raman spectra of the M[TCNE](NCMe)$_2$X (M=Fe, Mn, Ni; X=FeCl$_4$, SbF$_6$) molecule-based magnets were collected in a diamond anvil cell at pressures up to 36 kbar. The observed pressure-induced strengthening of the $\nu$(CC) and $\nu$(CN) Raman modes provides a clearer picture of the type and degree of backbonding, which will ultimately help build a model of how superexchange is occurring in these systems. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D32.00011: Bonding, Backbonding and Spin-Polarized Molecular Orbitals: Basis for Magnetism and Semiconducting Transport in V[TCNE]$_{x\sim 2}$ Jeffrey Kortright, Derek Lincoln, Ruth Shima Edelstein, Arthur Epstein V[TCNE]$_{x\sim 2}$ films exhibit magnetic order up to 400 K, magneto-resistance, and photo-induced magnetism. Yet the spin-polarized interactions between the TM and molecular species underlying these properties have remained elusive, in part because of its structural disorder. Using element-specific x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V $L$ edges, and the C and N $K$ edges we have gained new insight into these mechanisms [1]. We find evidence for covalent bonding between the V $e_{g}$ and TCNE \textit{$\sigma $} MO states, and a weaker interaction between V $t_{2g}$ and TCNE \textit{$\pi $} MO states, consistent with a generalized bonding/backbonding model with V octahedrally coordinated by N in \textit{$\sigma $}-bridging positions between TCNE radical anions. C and N XAS and MCD reveal spin-polarized splitting of the former LUMO of neutral TCNE, indicating that a direct exchange interaction underlies these properties. This indicates an active role of TCNE$^{\bullet -}$ in the magnetic properties of extended V[TCNE]$_{x\sim 2}$ and related systems, which is distinctly different from superexchange models generally used to describe magnetic Prussian blue analogs. [1] Phys. Rev. Lett. \textbf{100}, 257204 (2008). [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D32.00012: XPS and UV/Vis MCD studies of M[TCNE] organic-based magnets Saad Janjua, Konstantin Pokhodnya, Marcus Driver, Anthony Caruso M[TCNE] (M = V, Fe, Mn, Ni; TCNE = tetracyanoethylene) organic-based magnets provide a systematic means of studying magnetic superexchange by varying the 3d t$_{2g}$ and e$_{g}$ filling. X-ray photoemission binding energy spectra of core electrons C (1s), N (1s) and M (3p) were used to study valency and bond type, giving a correlation between binding energy and transition temperature. UV/Vis Magnetic Circular Dischroism studies were conducted to investigate lowest unoccupied state and onsite Coulomb repulsion for both M 3d and TCNE $\pi $*. This talk will focus on providing empirical evidence of the near Fermi edge spin polarized electron structure in the context of magnetic exchange [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D32.00013: ABSTRACT WITHDRAWN |
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