Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X31: Organic-based Magnetism and Spintronics |
Hide Abstracts |
Sponsoring Units: DPOLY FIAP Chair: Arthur Epstein, Ohio State University Room: LACC 503 |
Friday, March 25, 2005 8:00AM - 8:12AM |
X31.00001: Charge and Spin Delocalization in Novel Porphyrin Oligomers P. Frail, K. Susumu, M.J. Therien, P.J. Angiolillo, J.M. Kikkawa We discuss a new class of \textit{meso}-to-\textit{meso }ethyne-bridged (porphinato)zinc(II) oligomers with potential for TFT, FET, photovoltaic, magnetic, and spintronic applications on both a single molecule and bulk level. As the series progresses from dimer through heptamer, intramolecular electronic delocalization results in red-shifted optical transitions (2.0 eV-1.4 eV) and motional narrowing of the EPR signal. The former is consistent with potentiometrically determined HOMO-LUMO gaps, while the latter indicates spin delocalization lengths that span the molecular length scales of these structures (20 - 75 Angstroms). Generally, solubilizing appendages impose steric restrictions which can largely prevent such properties from transferring to the bulk phase. Through systematic modification of these appendages, the intermolecular resistance can be lowered dramatically. Resistivities measured for undoped oligomers can vary over 2-5 orders of magnitude for a given conjugation length. These large changes in resistivity correlate with the nature of the $\pi -\pi $ interactions made possible in the bulk phase. The doped species of these systems will also be discussed along with photoconductive properties. This work partially supported by DARPA/ONR N00015-01-0831. [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X31.00002: Large Magnetoresistance (\%10 at 10 Mt, 300k) in Semiconducting Polymer Thin Film Devices Omer Mermer, Govindarajan Veeraraghavan, Thomas Francis, Markus Wohlgenannt, Yugang Sheng, Duc Nguyen We have performed comprehensive magnetoresistance (MR) study on a set of polymer sandwich devices made from different pi-conjugated polymers, namely Poly(9, 9-dioctylfluorenyl -2,7 diyl) (PFO), Poly(3-hexylthiophene-2,5-diyl) (RR-P3HT) and Poly(3-octylthiophene-2,5-diyl) (RRa-P3HT). The measurements were performed at different voltages and temperatures, ranging from 10K to 300K, and at weak magnetic fields, $B$ $< 100\textit{mT}$. This magnetoresistance effect we discovered is amongst the largest of any bulk material. We find that the effect is related to hole current in the devices. We observed large negative and positive magnetoresistance dependent on material and operating conditions. A peculiar transition occurs between negative and positive MR region. We compare the results obtained in devices made from different materials with the goal of providing a detail picture of the polymer MR effect. We discuss our results in the framework of known MR mechanisms and find that none of the existing models can explain our results. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X31.00003: Addressing individual metal ion centers in supramolecules by STS M.S. Alam, S. Str\"omsd\"orfer, V.V. Dremov, P. M\"uller, A.M. Ako, R.W. Saalfrank, M. Ruben, L.K. Thompson, J.-M. Lehn As the information of STM measurements arises from electronic structure, separating information on the topography is not straightforward for complex molecules. Scanning tunneling spectroscopy (STS) measurements give information about the molecular energy levels, which are next to the molecules Fermi level. Using a home built STM working under ambient conditions, we succeeded to combine high resolution topography mapping with simultaneous current-voltage characteristics (STS) measurements on single molecules deposited on highly oriented pyrolytic graphite surfaces. We present our recent results on grid-type molecules [Co$_{4}$L$_{4}$] (L=4,6-bis(2',2''-bipyridyl-6-yl)pyrimidine) and [Mn$_{9}$L$_{6}$] (L=2POAP-2H) as well as on ring-shaped Fe ion chains [Fe$_{6}$Cl$_{6}$L$_{6}$] (L=1-Ecosyliminodiethanol). Small, regular molecule clusters as well as separated single molecules were observed. We found a rather large contrast at the expected location of the metal centers in our molecules, i.e. the location of the individual metal ions in their organic matrix is directly addressable by STS. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X31.00004: Atomic, electronic and spin-density structure of cobalt/polythiophene/cobalt magnetic tunnel junction Ivan Oleynik Electron and spin transport through ferromagnet/organic molecule/ferromagnet junctions depends critically on the properties of the electrical contacts of organic molecules with metallic electrodes. We performed first-principles density functional theory investigation of atomic and electronic structure of cobalt/polythiophene/cobalt magnetic tunnel junction (MTJ). We will present results on geometry and energetics of metal/organic interfaces, distribution of electron charge and spin densities, local atomic charges and spins, and local densities of states on individual atoms. We will discuss the spin-dependent electronic properties of cobalt/polythiophene/cobalt MTJ in connection with its spin-dependent transport characteristics. [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X31.00005: Photoinduced magnetism in chemical vapor deposited V(TCNE)$_{x}$, $x\sim2$ films J.W. Yoo, R. Shima Edelstein, P.I. Pokhodnya, A.J. Epstein, Joel S. Miller We previously reported[1] that Mn(TCNE)$_{x}$, $x\sim2$, exhibits strong photo-induced magnetism. Here, we extend our study of photo-induced magnetism to V(TCNE)$_{x}$, $x\sim2$ which is a magnetic semiconductor with $T_{C}$ $>$ 350 K, can be grown as a film by a CVD process[2] and can be incorporated in spinvalve system[3]. We report that the excitation of (TCNE) $\pi$ $\rightarrow$ $\pi_{\ast}$ band with blue light (457.9nm) at low temperature(5 K) and low magnetic field(10 Oe) leads to reduction in the magnetization. The photo-excited effects are more substantial at low magnetic field but the saturated magnetization remains the same as that of the ground state. The photo-excited metastable state has a lifetime $>$ 10$^{6}$ s at low temperatures. Only after warming above $\sim$ 100 K does the system completely relax to the state before illumination. Possible origins of the observed effects will be discussed. $^{\ast}$Supported in part by AFOSR Grant No. F49620-03-1-0175 and DOE Grant No. DE-FG02-01ER45931. [1] D. A. Pejakovic $\it{et}$. $\it{al}$., Phys. Rev. Lett. $\bf {88}$, 057202 (2002) [2] K. I. Pokhodnya $\it{et}$. $\it{al}$., Adv. Mater. $\bf{12} $, 410 (2000) [3] J. Bergeson $\it{et}$. $\it{al}$., this Bulletin. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X31.00006: Magnetic properties of a molecule-based Fe[TCNE]2 magnet Konstantin Pokhodnya, Arthur J. Epstein, Michael Bonner, Joel S. Miller The temperature dependence of magnetization $M(T)$ of Fe[TCNE]$_{2}$ {\textbullet}xCH$_{2}$Cl$_{2}$, (x$\sim $0.3-1; TCNE = tetracyanoethylene) molecule-based magnet$^{1}$ demonstrates two phase transitions (at $T_{1}$ $\sim $ 95-100 K and at $T_{2} \quad \sim $10-25 K) whose temperatures depend upon the degree of disorder. The position of the ac susceptibility peak related to the high-temperature transition is almost frequency independent, while the low-temperature peak demonstrates a shift per frequency decade (\textit{$\phi $} = d$T$/$T_{1})$/d(log $\omega )$ of 0.03 characteristic for spin-glasses. A coercive field $H_{cr}$ (400 Oe at 2 K) decreases exponentially on warming and vanishes when approaching $T_{1}$. This behavior may be qualitatively described in terms of the Heisenberg mean-field model developed for the systems with competing ferrimagnetic (FM) and spin-glass ordering. At $T_{1}$ the system passes from PM to FM state (the $M$ component parallel to $H$ orders while the perpendicular $M_{tr}$ freely rotates). On further cooling, the system begins to acquire a static $M_{tr}$ component (non-vanishing $H_{cr})$. At $T_{2}$, a crossover between weak and strong irreversibility occurs and a spin- or a cluster-glass state is achieved. [1] K. Pokhodnya, N. Petersen, and J. S. Miller. \textit{Inorg. Chem.} \textbf{41}, 1996 (2002) [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X31.00007: Effect of incommensurate transverse magnetic anisotropy on tunneling rate in Mn12-acetate Kyungwha Park, Mark Pederson, Noam Bernstein, Tunna Baruah, Steven Richardson Recent density-functional calculations on the single-molecule magnet Mn$_{12}$-acetate have revealed that there are two types of transverse magnetic anisotropy such as that induced by disordered solvent and the coupling between electronic and vibrational degrees of freedom. The former breaks fourfold symmetry of an isolated molecule so that it contributes to the second-order transverse anisotropy, while the latter contributes to the fourth-order transverse anisotropy. The hard axis of the second-order anisotropy induced by solvent disorder does not necessarily align with that of the fourth-order anisotropy caused by the coupling, which complicates the tunneling rate. In this talk, we determine the hard axes of the second-order and the fourth-order anisotropy from first-principles calculations. Using our calculated hard axes and anisotropy parameters, we calculate the tunneling rates as a function of the energy levels and the direction and magnitude of the transverse magnetic field, in order to examine the revival or suppression of the tunneling-rate oscillations. We also explore how small isotope defects influence the tunneling rates. [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X31.00008: Pulsed Field Studies of Unconventional Magnetoresistance in Q1D Conductors Harukazu Yoshino, Zeynel Bayindir, Joydeep Roy, Ben Shaw, Heon-ick Ha, Andrei Lebed, M.J. Naughton We built a single-axis sample rotator for use in a 45T pulsed magnetic field system. We used this to measure the magnetoresistance of the quasi-one dimensional organic conductor (TMTSF)$_2$ClO$_4$ in the interference-commensurate ``L-N" angle regime. We were able to obtain the angle-dependent magnetoresistance and observe the L-N oscillations. We were further able to study the field-dependence of the magnetoresistance at very high field, thus extending the work of Ha, {\it et al.}, to several times higher field. In our talk, we compare the recent theory of Lebed, which suggests unconventional $B$-dependence at various resistance maxima and minima (vs. angle), associated with unusual dimensional crossovers, to our pulsed field data. [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X31.00009: Unconventional Field Dependent Magnetoresistance in Q1D Conductors Heon-Ick Ha, Andrei Lebed, M.J. Naughton We report on magnetoresistance measurements in the organic metal (TMTSF)$_2$ClO$_4$ at specific crystal orientations in a magnetic field. In the regime of interference-commensurate ``L-N-type" oscillations, field sweeps at resistance minima and maxima (vs. angle) show qualitatively different magnetoresistance behavior. At minima, $R(B)$ tends to saturation at large $B$ (to 10T at 0.1K). At maxima, $R$ follows an empirical formula containing only linear and quadratic terms, with the former coefficient decreasing and the latter increasing as the field is tilted away from the highly conducting axis. We interpret these two regimes of behavior (maxima and minima) in terms of dimensional crossovers based on Lebed's recent theory. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X31.00010: Giant Nernst Effect in (TMTSF)$_2$PF$_6$ Weida Wu, Paul Chaikin Here we present a detailed study of the Nernst effect in (TMTSF) $_2$PF$_6$, where giant resonant-like Nernst oscillations were found when a magnetic field is aligned with magic angles (inter- chain directions).\footnote{W. Wu et al, PRL 91, 056601 (2003)} The amplitude of Nerst signal is order of 100 $\mu$V/K at H$\sim$7.5T and T$\sim$1K. The Nernst resonance at H//c’ rises gradually as T is lowered, reaches a peak at T$\sim$1K and then falls sharply to zero around 150mK. The Nernst signal is a highly non-linear function of H. This can be partially explained by the large magneto-resistance. The Nernst resonance has a weak pressure dependence. The sign-change of Nernst effect upon rotation through the magic angles suggests that the transport in (TMTSF) $_2$PF$_6$ is effectively coherent only in planes and only when these magic angle planes are parallel to the applied field. [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X31.00011: Theory of FICDW-FISDW Phases in Organic Superconductors Natalia Bagmet, Andrei Lebed We present analytical theory of magnetic Field-Induced Charge-Density-Wave(FICDW) phases (which takes into account FICDW/FISDW mixing effects) and calculate metal-FICDW/FISDW phase transition temperature. We compare our theoretical results with experiments on observations of FICDW/FISDW mixed phases (by Brooks's and Kartsovnik's groups) which appear at low temperatures in inclined magnetic fields in low-dimensional conductor alpha-(ET)2KHg(SCN)4 . [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X31.00012: NMR evidence for very slow spin density fluctuations in the organic metal $(TMTSF)_2ClO_4$ Fan Zhang, Yosuke Kurosaki, Jun Shinagawa, Barakat Alavi, Stuart E. Brown $(TMTSF)_2ClO_4$ is a quasi-one dimensional organic conductor and superconductor with $T_c=1.4K$, which undergoes an anion ordering transition at $T_{AO}=25K$ with wavevector $\vec{Q}=(0,1/2,0)$. We have investigated the spin-echo decay rates for the $^{77}Se$ nuclear spins in the low-temperature relaxed state and in the vicinity of a pronounced peak in $T_2^{-1}$ near $T=30K$[1]. By varying the magnetic field orienation, we attribute this increase to the slowing down of fluctuations in the local spin density. The linewidth at low temperatures appears to be dominated by disorder effects, rather than by distinct hyperfine fields of an ordered phase. The inhomogenous broadening of the spectra below $T=15K$ is consistent with a freezing of domain wall motion.[2] \newline \newline [1]M. Takigawa and G. Saito, J. Phys. Soc. Jpn. {\bf 55}, 1233 (1986). \newline [2]This research was supported by the National Science Foundation under grant No. DMR-0203806. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 10:36AM |
X31.00013: Magnetic excitations probed by $^{77}$Se T$_1$ at Magic angles in (TMTSF)$_2$PF$_6$ Stuart Brown, Weida Wu, Jun Shinagawa, Paul Chaikin We report $^{77}$Se spin-lattice relaxation T$_1$ measurements of (TMTSF)$_2$PF$_6$ at different field orientations with B=7.3T (f=60MHz) and P=8kbar. We want to see whether T$_1$ changes at the Lebed magic angles and whether the field induced spin density wave (FISDW) critical temperature changes for field applied along the magic angles. Our temperature range was 300mK to 20K. In situ resistance measurements (R$_{zz}$) were used to identify the experimental conditions and give precise sample alignment information. Our T$_1$ data show no significant change as magnetic field is rotated through the magic angles, i.e. no effect in the spin channel. Unexpectedly we also find that the FISDW transition temperature, seen by both T$_1$ and transport (R$_{zz}$) is not enhanced at the magic angles. [Preview Abstract] |
Friday, March 25, 2005 10:36AM - 10:48AM |
X31.00014: Probing Spin Pairing of Superconductiing State in (TMTSF)$_2$ClO$_4$ with $^{77}$Se Knight Shift Measurement J. Shinagawa, C. Parker, F. Zhang, B. Alavi, S.E. Brown We report measurements of the $^{77}Se$ Knight shift at low temperatures and $\vec{B}(=1.38T)\parallel \vec{a}$ in the organic superconductor (TMTSF)$_2$ClO$_4$. Recent observations of an upper critical magnetic field H$_{c2}$ exceeding the spin paramagnetic limit by a factor of order two led to suggestions that this system may be a candidate for a triplet pairing state [1]. The Knight shift is proportional to the spin susceptibility, which does not vanish for the case of triplet pairing. We were limited by heating effects to a range of temperatures $T>.8T_c(1.38T)$, but nevertheless observed a change in the NMR frequency consistent with a 10-15 percent reduction in the spin susceptibility relative to the normal state. We discuss the implications of these results. This research is supported by NSF grant DMR-0203806. [1] J.I. Oh and M.J. Naughton, Phys.Rev.Lett. $\bf92$, 067001 (2004) [Preview Abstract] |
Friday, March 25, 2005 10:48AM - 11:00AM |
X31.00015: Study of the Fermi velocity and scattering time by periodic orbit resonance in the quasi-one-dimensional conductor (TMTSF)$_2$ClO$_4$ S. Takahashi, S. Hill, S. Takasaki, J. Yamada, H. Anzai (TMTSF)$_2$ClO$_4$ belongs to the family of quasi-one- dimensional (Q1D) Bechgaard salts, and is one of the most widely studied organic conductors because of its variety of exotic ground states, which may be tuned by applying pressure, magnetic field, or by employing different cooling processes (relaxed and quenched states). These phases include: metallic, insulating, p-wave superconducting, and field-induced-spin-density-wave. We recently observed periodic orbit resonances (POR) in (TMTSF)$_2$ClO$_4$ (a form of Q1D cyclotron resonance).$^1$ Studies of the angle-dependence of the POR enable us to determine the Fermi velocity, $v_F$, and the scattering time, $\tau$. In this talk, we will discuss the dependence of $v_F$ and $\tau$ on the cooling process, and their relation to the superconducting critical temperature.\\ \\ $^1$S. Takahashi {\it et al.}, cond-mat/0404545 (2004) [Preview Abstract] |
|
X31.00016: Comparison of Organic and Inorganic Semiconductors for Spin Injection and Transport J.D. Albrecht, P.P. Ruden, D.L. Smith We present a theoretical description of spin injection, extraction, and transport in structures with a ferromagnetic metal injector, a thin semiconductor layer in which diffusive transport occurs, and a ferromagnetic metal collector. Transport layers composed of conjugated polymers, small molecule organic molecular crystals, and inorganic semiconductors are compared. Spin injection implies driving the semiconductor out of local (spin) equilibrium and requires a spin-selective injection process. Tunneling through a Schottky barrier or a thin interfacial insulator layer may provide that selectivity. Since carrier mobilities (and other relevant parameters) for the three systems differ over orders of magnitude, the conditions for achieving spin injection also differ. We present results for spin injection and transport, and we derive design criteria for spin injectors accounting for the varied fabrication and process issues relevant to the device technologies. [Preview Abstract] |
|
X31.00017: Angular Magnetoresistance Oscillations in Organic Conductors Heon-Ick Ha, Andrei Lebed, Michael Naughton We demonstrate that electron wave functions change their dimensionality at some commensurate directions of a magnetic field (related to the so-called Magic Angles) in conductors with open [quasi-one-dimensional (Q1D)] sheets of Fermi surface. These 1D $\rightarrow$ 2D dimensional crossovers lead to delocalization of wave functions and are responsible for angular magnetoresistance oscillations. As an example, we show that suggested theory is in excellent qualitative and quantitative agreements with the very recent experimental data obtained on (TMTSF)2ClO4 conductor by Heon-Ick Ha and Michael Naughton. [Preview Abstract] |
|
X31.00018: Observation of Spin-flop Transition in Antiferromagnetic Organic Molecular Conductors using AFM Micro-cantilever Madoka Tokumoto, Shigeo Hara, Hisashi Tanaka, Takeo Otsuka, Hayao Kobayashi, Akiko Kobayashi A series of (BETS)$_{2}$Fe$_{1-x}$Ga$_{x}$Cl$_{1-y}$Br$_{y}$ salts is a good candidate for a systematic study of $\pi $-d interaction between the conduction electrons and local magnetic moments in organic conductors. Some of them show antiferromagnetic ground state at low temperatures. A torque magnetometry is useful for precise determination of the easy axis as well as the spin-flop field. In this work we will report on the measurements of spin-flop transitions in antiferromagnetic organic molecular conductors including $\lambda $-(BETS)$_{2}$FeCl$_{4}$[1], using a commercial self-sensing piezo-resistive microcantilever for Atomic Force Microscopy (AFM) developed by Seiko Instruments Inc. We have succeeded in observation of spin-flop transitions of tiny single crystals including $\lambda $-(BETS)$_{2}$FeCl$_{4}$ as small as less than 1$\mu $g[2]. The results are consistent with the capacitive magnetic torque study[3]. [1] M. Tokumoto et al. Synth. Metals 86, 2161 (1997). [2] M. Tokumoto et al., ICMM2004, Polyhedron in press. [3] T. Sasaki et al., Synth. Metals 120, 759 (2001). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700