Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q8: New Developments in Organic Spintronics |
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Sponsoring Units: GMAG Chair: Valy Vardeny, University of Utah Room: Ballroom C4 |
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q8.00001: Spin Injection/detection using organic-based magnetic semiconductor Invited Speaker: This abstract not available. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q8.00002: Magnetic Field Effect in Organic Devices: the Role of Hyperfine, Exchange and Spin Orbit Interactions Invited Speaker: Recently we have observed a novel phenomenon in both magneto-electroluminescence (MEL) and magneto-conductance (MC) in a variety of organic light emitting diodes that consists of a sign reversal at very small magnetic fields ($B\le $1 mT), dubbed hereafter ultra-small magnetic field effect (USMFE) [1]. Similar response has been obtained in MC($B)$ of unipolar organic diodes [2]. As $B$ is reduced below the zero crossing field, the magnitude of the obtained MEL and MC increases to a maximum value at $B=B_{m}$, before diminishing at $B$=0. We found that $B_{m}$ is isotope dependent: it is lower when the protons in the organic material are replaced by deuterons having a smaller nuclear magnetic moment and reduced hyperfine interaction (HFI), and is higher when the $^{12}$C atoms (nuclear spin I=0, no HFI) are replaced by $^{13}$C atoms (I=$\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $, with finite HFI). We also found that $B_{m}$ scales with the half width at half maximum, $\Delta $B, of the high field response. From the MEL($B)$ and MC($B)$ responses, the marked isotope effect, and voltage and temperature dependencies we explain the USMFE as well as the width $\Delta $B, as due to loosely coupled pairs of polarons (either with same or opposite charges) of which spins are intermixed via the HFI [1,2]. The model captures the sign reversal and its dependence on the HFI strength. The role of the HFI anisotropy, exchange interaction between the polaron pair spins, and spin orbit interaction effect on the USMFE will be discussed. *Supported by the Israel Science Foundation grant 745/08, and NSF grant DMR 08-03325. **In collaboration with T. D. Nguyen, B. R. Gautam, and Z. V. Vardeny, University of Utah. \\[4pt] [1] T. D. Nguyen, G. Hukic-Markosian, F. Wang, L. Wojcik, X-G. Li, E. Ehrenfreund, Z. V. Vardeny, \textit{Nature Materials} \textbf{9}, 345 (2010). \\[0pt] [2] T. D. Nguyen, B. R. Gautam, E. Ehrenfreund, Z. V. Vardeny, Phys. Rev. Lett. \textbf{105}, 166804 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q8.00003: Molcular spntronics: tailoring spin polarization with molecules Invited Speaker: Organic/molecular spintronics is a rising research field at the frontier between spintronics and organic chemistry. Organic molecules and semiconductors were first seen as promising for spintronics devices due to the expected long spin lifetime. An exciting challenge is now to find opportunities arising from chemistry to develop new spintronics functionalities that were unavailable with inorganic materials. Here one can hope to control the spin dependent transport by using the chemical versatility brought by molecules and molecular engineering. Starting from the use of Alq3 and Phthalocyanine molecules we will show how the ferromagnetic metal/molecule hybridization can strongly influence the interfacial spin properties: from spin polarization enhancement to its sign control. CNRS/Thales team: C. Barraud, P. Seneor, R. Mattana, S. Tatay, K. Bouzehouane, S. Fusil, C. Deranlot, F. Petroff, A. Fert in collaboration with ISMN, Bologna, Italy \& IPCMS, Strasbourg, France [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q8.00004: Spin relaxation in organic semiconductors Invited Speaker: Intriguing magnetic field effects in organic semiconductor devices have been reported: anomalous magnetoresistance in organic spin valves and large effects of small magnetic fields on the current and luminescence of organic light-emitting diodes. Influences of isotopic substitution on these effects points at the role of hyperfine coupling. We performed studies of spin relaxation in organic semiconductors based on (i) coherent spin precession of the electron spin in an effective magnetic field consisting of a random hyperfine field and an applied magnetic field and (ii) incoherent hopping of charges. These ingredients are incorporated in a stochastic Liouville equation for the dynamics of the spin density matrix of single charges as well as pairs of charges. For single charges we find a spin diffusion length that depends on the magnetic field, explaining anomalous magnetoresistance in organic spin valves. For pairs of charges we show that the magnetic field influences formation of singlet bipolarons, in the case of like charges, and singlet and triplet excitons, in the case of opposite charges. We can reproduce different line shapes of reported magnetic field effects, including recently found effects at ultra-small fields. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 2:15PM |
Q8.00005: Coherent spin spectroscopy in organic thin film semiconductor devices Invited Speaker: With the emergence of organic spintronics and renewed interest in magnetoresistive effects, there is much need to illuminate the properties of spins in molecular electronic materials. Examples include spin-relaxation times, spectral diffusion times, spin dephasing times and spin interactions. In this presentation, an overview is given about the concepts of pulsed, electrically and optically detected magnetic resonance spectroscopy as techniques to manipulate and observe and thus characterize these fundamental properties of electron and nuclear spins in organic semiconductors [1]. By coherent (pulsed) magnetic resonant perturbation of spin states one may cause the spins to coherently propagate in a defined manner [2]. Spin-dependent charge carrier-transport or -recombination allow the observation of this coherent spin motion through electrical or optical measurements in working devices, such as organic light-emitting diodes. The ubiquitous presence of hydrogen nuclei gives rise to strong hyperfine interactions, which appear to provide the basis for many of the magnetoresistive effects observed in these materials. Since hyperfine coupling influences resonantly driven quantum spin beating in electrically or optically detectable electron-hole pairs, an extraordinarily sensitive probe for hyperfine fields in such pairs is given [3]. This allows scrutinizing the various existing models for these electronic processes. Qualitative as much as quantitative insights are gained into some of the physical intricacies of organic semiconductor device fabrication such as the influence of contact materials on spin-orbit coupling. \\[4pt] [1] D. R. McCamey, et al. Nature Mat. 7, 723, (2008). \\[0pt] [2] C. Boehme et al. Phys. Stat. Sol B. 246, 11-12, 2750 (2009).\\[0pt] [3] D. R. McCamey, et al. Phys. Rev. Lett. 104, 017601 (2010). [Preview Abstract] |
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