Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H16: Focus Session: Spins in Carbon-Based Materials-- Spin Valves and Interfaces |
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Sponsoring Units: GMAG DMP Chair: Minn-Tsong Lin, Taiwan University Room: D173 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H16.00001: Spin filtering effect of ferromagnetic metal-organic interfaces Invited Speaker: The study of the spin properties of organic semiconductors (OSC) is recently receiving great attention. Being characterized by moderate spin-relaxation lengths, one of the most promising routes to employ OSC for spintronics applications is probably to exploit the high spin injection achievable across ferromagnetic metal-organic interfaces [1,2]. Combined with the extreme flexibility and tunability of OSC, it is expected that such hybrid interfaces will constitute a fundamental building block for advanced spintronics devices, where spin-injection is controlled by fine-tuning of the interface physical ad chemical properties. An example has been recently presented in [3], where doping of the OSC copper phthalocyanine (CuPc) has been successfully used to tune the spin functionality of a cobalt-CuPc interface. In particular, the presence of a spin-polarized hybrid interface state, acting as a spin-filter at the interface, has been used to enhance the efficiency of spin injection to values above 100{\%}. In order to exploit such great potential of hybrid organic-inorganic interfaces, fundamental knowledge about their spin-dependent properties is essential. Besides the cobalt-CuPc interface, we have studied the iron-CuPc, cobalt- tris[8-hydroxyquinoline]aluminium (Alq3) and iron-Alq3 interfaces. We applied several complementary experimental techniques, namely spin polarized scanning tunnelling microscopy and spectroscopy together with spin polarized ultraviolet photoemission spectroscopy and spin- and time-resolved two-photon photoemission. We found evidence for spin-polarized interface states and show that they act as a spin-filter for electrons crossing the interface between the ferromagnetic metal and the OSC. Correspondingly, we observed a pronounced spin-dependency of the lifetime of electrons injected in the above mentioned hybrid spin-polarized interface states. \\[4pt] [1] M. Cinchetti et al., Nature Materials 8, 115-119 (2009); \\[0pt] [2] H. Ding et al., Phys. Rev. B 78, 075311 (2008); \\[0pt] [3] M. Cinchetti et al., Phys. Rev. Lett. 104, 217602 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H16.00002: Spin valve effect and high field magnetoresistance in hybrid magnetic tunnel junction of V(TCNE)$_{x~2}$/rubrene/ La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ Jung-Woo Yoo, Bin Li, C.Y. Chen, V.N. Prigodin, A.J. Epstein, H.W. Jang, C.W. Bark, C.B. Eom Molecule/organic-based magnets, that allow chemical tuning of electronic and magnetic properties, are a promising new class of magnetic materials for future spintronics [1]. V (TCNE:tetracyanoethylene)$_{x}$ ($x \sim$ 2) is the room temperature organic-based magnetic semiconductor ($T_c \sim$ 400 K). It has ferrimagnetic coupling between the spins in the TCNE$^{-}$ anions and spins in V$^{\rm II}$ leading highly spin- polarized valence and conduction bands. In this talk, we present realization of an organic-based magnetic as an electron spin polarizer in the standard spin valve device geometry [2]. The room temperature organic-based magnet, V(TCNE)$_x$ was successfully incorporated into the standard magnetic tunnel junction devices in tandem with LSMO (La$_{2/3}$Sr$_{1/3}$MnO$_3$) film. Beside spin valve effect, the device exhibits large negative high-field magnetoresistance, which may be associated with anomalous field dependent Fermi level shift in LSMO.\\[4pt] [1] A.J. Epstein, MRS Bull. ${\b 28}$, 492 (2003)\\[0pt] [2] Yoo et al., Nature Materials ${\b 9}$, 638 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H16.00003: Organic Spin Valves with Characteristics of Inelastic Tunneling and Hopping Transport Minn-Tsong Lin, Kai-Shin Li, Yin-Ming Chang, Santhanam Agilan, Jhen-Yong Hong, Jung-Chi Tai, Wen-Chung Chiang, Keisuke Fukutani, P.A. Dowben We report on the inelastic scattering characteristics of an organic-based spin valve with a thin organic barrier of 3,4,9,10-perylene-teracarboxylic dianhydride (PTCDA) dusted with alumina at organic/ferromagnetic interfaces. Spin injection with magnetoresistance up to 12\% at room temperature was achieved. In the inelastic tunneling spectrum, the observation of characteristic vibrational loss peak of organic spacer provides direct evidence of the interplay between the spin-polarized electrons and the organic molecules. The spin-dependent transport mechanism can be further described with a model of combined tunneling and hopping processes as verified by experiments as a function of organic layer thickness. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H16.00004: Coulomb blockade magnetoresistance in organic spin transport device Dali Sun, X.-G Zhang, Paul C. Snijders, Hangwen Guo, Zheng Gai, T. Zac Ward, Jian Shen Using buffer-layer-assisted growth, we successfully fabricated organic spin transport devices with a discontinuous granular magnetic layer centered in an organic spacer film. The Coulomb blockade magnetoresistance (MR) effects were observed, as predicted by X.-G. Zhang \textit{et al} (Phys. Rev. B. 81, 155122, 2010). The spin-dependent Coulomb blockade voltage arises from the coupled magnetic dots inside the organic material and correlate with the observed MR effect. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H16.00005: Spin-polarized tunnel injection and extraction effects on magneto-resistance in organic semiconductor spin valves Mohammad Yunus, P. Paul Ruden, Darryl L. Smith Experimental evidence of large magneto-resistance has been reported for organic spin valves. An organic spin valve consists of a conjugated hydrocarbon semiconductor sandwiched between two ferromagnetic contacts. Tunnel injection of charge carriers from a ferromagnetic contact can be strongly spin-polarized. The process is modeled as tunneling through a thin interfacial layer into localized molecular states of the organic semiconductor near the equilibrium Fermi level, and subsequent thermally activated hopping of the charge carriers out of these localized states into the bulk of the semiconductor, where the transport can be described by macroscopic device equations. The extraction of charge carriers follows an analogues process at the collecting contact. We explore the consequences of parallel or anti-parallel alignment of contact magnetizations on the spin-polarization and the magneto-resistance associated with the spin-polarized current in the device. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H16.00006: Spin-orbit coupling, spin relaxation, and spin diffusion in organic solids: applications to Alq$_3$ and CuPc Zhi-Gang Yu We develop a systematic approach of quantifying spin-orbit coupling (SOC) and a rigorous theory of carrier spin relaxation caused by the SOC in disordered organic solids. The SOC mixes up- and down-spin in the polaron states and can be characterized by an admixture parameter $\gamma^2$. The spin mixing effects spin flips as polaron hops from one molecule to another even through the interaction that facilitate hopping is spin-indepdent. The spin relaxation time is $\tau_{sf}= \bar{R}^2/(16 \gamma^2 D)$ and the spin diffusion length is $L_s=\bar{R}/4|\gamma|$, where $\bar{R}$ is the mean polaron hopping distance and $D$ the carrier diffusion constant. We show that the SOCs in tris-(8-hydroxyquinoline) aluminum (Alq$_3$) and in copper phthalocyanine (CuPc) are particularly strong, due to the orthogonal arrangement of the three ligands in the former and Cu $3d$ orbitals in the latter. The theory quantitatively explains the recent observed spin diffusion lengths in Alq$_3$ from muon measurements and in CuPc from two-photon photoemission. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H16.00007: Triplet exciton controlled current in an organic semiconductor William Baker, Dane McCamey, Kipp Van Schooten, John Lupton, Christoph Boehme Organic materials like MEH-PPV have relatively low spin-orbital coupling leading to long spin lifetimes and good spin-selection rules. As a result, the rates of recombination of an ensemble of polaron pairs can be changed by resonant manipulation of either the P$^{+}$ or the P$^{-}$ spins, leading to a flopping between the singlet to triplet manifolds - this can be observed as a small change in the device current [1]. In this study we have used pulsed electrically detected magnetic resonance with electron-rich OLED devices to investigate the possibility of other spin-dependent processes like an exciton-polaron interaction. We expect that devices with excess electrons will show signals from free electrons interacting with long lived triplet excitons. Coherent Rabi nutation experiments were carried out from room temperature down to 5K. At room temperature only a single polaron-pair resonance is observed at g$\sim $2.003. However, as the temperature is decreased a signal at g$\sim $4(triplet exciton resonance) is observed along with a second signal at g$\sim $2.003 corresponding to the rotation of a single polaron.\\[4pt] [1] D. R. McCamey, \textit{et. al.} Nature Mater. \textbf{7}, 723, (2008) [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H16.00008: STM studies of Lanthanide Phthalocyanine molecules on metallic and thin-insulating surfaces Fadi El Hallak, Ben Warner, Michael Waters, Joris van Slageren, Cyrus F. Hirjibehedin Molecules deposited onto surfaces are of interest because of their potential use as nano-scale electronic components. More recently, the magnetic properties of these systems have also become accessible. Using scanning tunneling microscopy (STM), it is possible to study both sets of properties, and to examine the local environment of the molecules. For example, large magnetic anisotropies have been observed for transition metal Phthalocyanine (Pc) molecules on thin insulators, which decouple the spin from the underlying metal. We present STM imaging and spectroscopy data on lanthanide double-decker Pcs. We explore the different binding configurations and study how these can influence the properties of these molecules on surfaces. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H16.00009: STM Studies of Iron Phthalocyanine on Fe(110) Films Andreas Sandin, Dan Dougherty, J. E. (Jack) Rowe We have observed molecular-scale-resolution arrays of Iron Phthalocyanine (FePc) molecules which we adsorbed at room temperature on thin ($\sim $5-10 ML) films of Fe(110). These molecular layers were grown in a UHV Omicron/AFM/STM/ multi-probe system at NC State in the Physics Department at pressures of $\sim $10$^{-10}$ torr. Our results indicate a strong inter-molecular interaction that produces well-ordered films at monolayer coverage. For lower coverage ($\sim $0.2 -- 0.6 ML) the FePc-Fe substrate interaction strongly dominates and the STM image morphology has only small clusters of 2-6 molecules. Our data clearly shown that the FePc molecules are lying flat on the surface in the ordered $\sim $1 ML samples since we see evidence of the carbon-ring ligands in some images. We discuss the possibility of spin-dependent effects between the molecular Fe and the substrate Fe as an example of potential molecular-modified spin-based devices. Initial STM-spectroscopy including both I vs. V and Z vs. V results are consistent with our structural observations. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H16.00010: Scanning Tunneling Microscopy and Spectroscopy Studies of a Model Organic Spintronic Interface: Alq3 on Cr(001) Zhengang Wang, Alex Pronschinske, Daniel Dougherty Scanning tunneling microscopy was used to observe coverage-dependent structure during growth of the first monolayer of Alq3 on a Cr(001) surface. No long range molecular ordering is observed, though molecules tend to form randomly oriented chain-like aggregates even at the lowest coverages. This illustrates that the well-known amorphous nature of Alq3 films begins even in the first layer, but that the disorder in films have subtle local correlations relevant to electronic and spintronic device modeling. Scanning tunneling spectroscopy was used to locate the LUMO-derived transport state above the Fermi level and correlate its position with local film structure. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H16.00011: STM studies of a novel organic/inorganic interface: TCNE/GaAs(110) N.M. Santagata, J.A. Gupta Recent efforts in the field of spintronics have focused on the integration of organic molecular magnets with inorganic semiconductors. Little is known, however, about the interfacial chemistry and physics that occurs between the organic spin injector and the inorganic device structure. We are therefore studying tetracyanoethylene/GaAs(110) as a model system to gain a basic understanding of the properties that emerge upon integration of these materials. Utilizing low temperature (7 K) ultrahigh vacuum scanning tunneling microscopy we are able to identify both bonding geometries and bonding sites for isolated TCNE molecules on the unreconstructed GaAs(110) surface. Scanning tunneling spectroscopy can provide a detailed look at the interfacial electronic structure, including alignment of individual molecular orbitals with respect to the band structure of the underlying substrate. Single transition metal--TCNE complexes can be realized and investigated via atomic/molecular manipulation. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H16.00012: Magnetic Field Effects Generated by Inter-molecular Excited States in Organic Semiconductors Liang Yan, Ming Shao, Bin Hu It has experimentally found that an external magnetic field can change electroluminescence, electric current, and photocurrent, generating magnetic field effects (MFEs) in non-magnetic organic semiconductors. Our photoluminescence studies have found that the intermolecular excited states are accountable for the MFEs while the intra-molecular excited states exhibit negligible MFEs. Our experimental studies further indicated that inter-molecular excited states can exhibit tunable spin-orbital coupling and exchange interaction based on materials mixing. We observed that tuning inter-molecular spin-orbital coupling and exchange interaction can largely modify the MFEs through spin-dependent formation and intersystem crossing in inter-molecular excited states. Therefore, the use of inter-molecular excited states presents a new mechanism to generate magnetic responses in non-magnetic organic semiconductors. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H16.00013: Charge transport through single alkanedithiol molecules on an ultrathin insulating film: Influence on an atomic Kondo resonance Taeyoung Choi, Jay Gupta Studies of charge/spin transport through single molecules are important for understanding organic-based electronic and memory devices. We have realized a single molecule wire comprising an alkanedithiol molecule and a single Co atom contact using a low temperature scanning tunneling microscope. This wire is formed on an ultrathin insulating layer (Cu2N on Cu(100)). A Kondo resonance observed on isolated Co atoms on Cu2N indicates minimal contact to the Cu substrate. However, increased contact to Cu is achieved by connecting the Co atom via the alkanedithiol molecule. A change in the Kondo lineshape on the Co atom indicates an open conduction channel through the molecule. ~This result provides an opportunity to study charge/spin transport through single molecules with atomically precise contacts. We acknowledge financial support from NSF CAREER Award No. DMR-0645451 and NSF MRSEC-0820414. http://www.physics.ohio-state.edu/$\sim $jgupta. [Preview Abstract] |
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