Organic Spintronics


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Organic Spintronics

  1. 1. Organic Spintronics Zeev Valy Vardeny University of Utah; Salt Lake City
  2. 2. <ul><li>Collaborators: </li></ul><ul><li>Physics Department, University of Utah </li></ul><ul><li>(1) Professors : Jing Shi (UC Riverside); Tho Nguyen; Brian Saam </li></ul><ul><li>(2) Post doctors : Z. H. Xiong (Prof. at USTC); Burtman (Industry) </li></ul><ul><li>(3) Graduate Students : F. Wang (UoI), B. Gautam </li></ul><ul><li>(4) Staff members : L. Woijcik, R. Polson </li></ul><ul><li>Other Institutions </li></ul><ul><li>(i) Prof. X. J. Li; USTC, China </li></ul><ul><li>Prof. Eitan Ehrenfreund; Technion, Israel </li></ul>Presently supported by the NSF-MRSEC program at the UoU (9/2011)
  3. 3. 1. “Giant magnetoresistance in organic spin-valves”, Z. H. Xiong, D. Wu, Z. V. Vardeny, and J. Shi, Nature 427, 821 (2004). 2. “Spin-valves of organic semiconductors; the case of Fe/Alq 3 /Co”, F. Wang et al ., Synth. Metals (2005). 3. “High-field magnetoresistance of organic light emitting diodes based on LSMO”, D. Wu, Z. H. Xiong, Z. V. Vardeny, and J. Shi, Phys. Rev. Lett . 95 , 016802 (2005). 4. “Spin Dynamics in Organic Spin-Valves”, F. Wang, C. G. Yang, and Z. V. Vardeny, Phys. Rev. B 75, 245324 (2007). 5. “Organic Spintronics strikes back”, Z. V. Vardeny, Nature Materials 2, 91 (2009). 6. “Isotope effect in magneto-transport of π -conjugated films and devices ”, T. D. Nguyen et al., Nature Materials 9, 345 (2010). 7. “ Organic Spintronics ”, book edited by Z. V. Vardeny, Francis & Taylor, April 2010. 8. ““Magnetoconductance Response in Organic Diodes at Ultra-small Fields”, T. D. Nguyen et al ., Phys. Rev. Lett . 105 , 166804 (2010). The work presented here can be found in:
  4. 4. <ul><li>Outline: </li></ul><ul><li> -conjugated semiconductors; photophysics, OLED devices and spin-physics </li></ul><ul><li>Spintronics ; an introduction </li></ul><ul><li>Spin-valve devices and their applications </li></ul><ul><li>LSMO/Alq 3 /Co organic spin-valve devices; two FM electrodes </li></ul><ul><li>Spin ½ relaxation process; isotope effect and hyperfine interaction </li></ul><ul><li>Organic spin valves based on C 60 ; spin-orbit interaction? </li></ul><ul><li>High field GMR response in organic diodes; one FM electrode </li></ul><ul><li>Magnetic field effect in OLEDs; no FM electrode </li></ul>
  5. 5. σ = 10 -9 S/cm (insulator) σ = 38 S/cm (conducting plastic) 2000 Nobel prize in Chemistry In the beginning … H. Shirakawa, A.G. MacDiarmid, and A. J. Heeger first reported polymer conduction from oxidized (“doped”) polyacetylene (CH) x J. Chem. Soc., Chem. Commun. 1977 , 578. Alan McDiarmid dances the Mauri’s ‘Haka’ during the ‘Nobel’ ceremony in Stockholm, 2000
  6. 6. Luminescence properties of DOO-PPV Singlet excitons with binding energy of about 0.5 eV are responsible for the photoluminescence band. PL quantum efficiency : ~ 30% in thin film at RT . C. X. Sheng, Ph.D. thesis, University of Utah (2005)
  7. 7. Organic semiconductors for light-emission Whereas the original polymer, polyacetylene is non-luminescent , more recently luminescent polymers have been in the focus of the scientific study and applications. PL-quantum efficiencies up to 60% in thin films [mL-PPP]; originating from singlet excitons. Oligomers Polymers Debut of organic light emitting diodes; Tang, 1987
  8. 8. Advantages of ‘organic electronics’ <ul><li>Physical flexibility; ‘plastic’ </li></ul><ul><li>Low-cost manufacture: </li></ul><ul><li>ink printing, </li></ul><ul><li>roll-to-roll coating </li></ul><ul><li>large scale production </li></ul><ul><li>Light weight; portable </li></ul><ul><li>Low energy consumption; </li></ul><ul><li>low drive bias voltage </li></ul><ul><li>high brightness </li></ul><ul><li>variety of colors; leading </li></ul><ul><li>to white light emission </li></ul>
  9. 9. Primary photoexcitations in  -conjugated polymers The 1D localization leads to considerable Coulomb correlation; thus the photophysics is dominated by excitons . Low lying singlet and triplet excitons are separated by ~ 0.7 eV of exchange energy continuum Ground-state singlet triplet ~2.5eV ~1.8eV E b  0.5 eV Singlet GS triplet singlet <ul><li>Singlet energies measured by optical absorption </li></ul><ul><li>Triplet energies measured by the weak phosphorescence </li></ul>optical absorption X Theory ; Mazumdar, Abe, Bredas Experiment ; Baessler, Friend, Vardeny
  10. 10. Spintronics Dictionary <ul><li>Electron spin, hole spin ; these are spin ½ charge carrying excitations. </li></ul><ul><li>Singlet exciton ; e-h bound pair in s = 0 combined spin state configuration. </li></ul><ul><li>Triplet exciton ; e-h pair in s = 1 combined spin state. </li></ul><ul><li>Spin relaxation time ; the time in which the prepared spin sense stays put. </li></ul><ul><li>Spin injection ; carrier injection with preferred spin sense. </li></ul><ul><li>Spin diffusion length ; the distance spin carriers diffuse before spin flip occurs. </li></ul><ul><li>This may be very different from carrier diffusion length; </li></ul><ul><li>L s  L drift </li></ul>
  11. 11. A New possible device: Spin-OLED <ul><li>Regular OLED with non-magnetic electrodes: QE max ~ 25% of SE </li></ul>Triplet excitons: no EL Singlet luminescence <ul><li>OLED with ferromagnet (FM) electrodes : QE from 0 to 50% with H </li></ul>Only triplet excitons (state 1 or 2) are formed; ELQE=0 Excitons in state 3 and state 4 are formed; ELQE = 50% NM 1 NM 2 Organic e h exciton FM 1 FM 2 Organic e h M 1 M 2 Parallel M FM 1 FM 2 Organic e h M 1 M 2 Anti-parallel M
  12. 12. Resistance mismatch problem for spin injection into semiconductors Parallel magnetization FM1 FM1 SEC FM2 SEC FM2 FM1 FM1 SEC FM2 SEC FM2 R r R r R r r R R R R R P =  = R SC /2R M Schmidt, Rashba , Smith; 2000-2001 SEC FM1 FM2 Large  kills MR  Is large since R sc is large Anti-parallel magnetization R R  R/R = R R - R = P 2 /(1+  (1-P 2 )) 2 r r r r +
  13. 13. Solutions to the problem of spin injection into SEC <ul><ul><li>1. Reduce resistance mismatch (reduce  ) </li></ul></ul>2. Injector with 100% spin polarization (half-metallic ferromagnets) 3. Ferromagnet semiconductor injector (higher R for FM1 and FM2) 4. Appropriate tunnel barrier at interfaces (higher R for Int.1 and Int.2) 5. Spin filters; such as MgO  = R sc /R electrode FM1 FM2 Int1 Int2 Semiconductor
  14. 14. Band structure diagram of two ferromagnets P is the spin polarization degree at the Fermi level (%) LSMO spin polarization is ~100% due to a large gap between majority and minority carriers
  15. 15. Spintronics using inorganic semiconductors <ul><li>Optically injected electron spins can travel coherently over several microns in GaAs ( Awschalom 1997 ) </li></ul><ul><li>Electrical injection/optical detection was demonstrated in inorganic spin- LEDs (Ohno, 1999; Jonker, 2002) </li></ul><ul><li>No spin-valves (electrical injection and </li></ul><ul><li>detection) has been demonstrated so far </li></ul><ul><li>T c of FM SEC injectors is too low (110K </li></ul><ul><li>for GaMnAs) </li></ul><ul><li>Interface quality is hard to control </li></ul><ul><li>It requires MBE growth </li></ul><ul><li>Recently 60% spin polarization has been </li></ul><ul><li>obtained (APS meeting 2010) </li></ul>Does not work for OSEC; Since PL emission is from excitons with weak S-L coupling
  16. 16. Possible electrical injection/detection in organic semiconductors <ul><li>Two well-accepted schemes using ferromagnetic (FM) injectors: </li></ul><ul><ul><li>Hanle effect </li></ul></ul><ul><ul><li>Polarizer-analyzer effect ( spin-valve effect) </li></ul></ul>R parallel < R anti-parallel Thickness, d<  s H Electron spin precesses in the plane. As H changes, the resistance changes, if the electrode separation is less than the spin diffusion length,  s in the active layer. Parallel magnetization Analyzer Polarizer Anti-parallel magnetization Analyzer Polarizer
  17. 17. The spin-valve device <ul><li>Practical GMR structures: FM/NM/FM for tunneling spin-valves </li></ul><ul><li>Thickness of few nm < spin diffusion length (~10 nm at RT) </li></ul><ul><li>Magnetization parallel and anti-parallel configuration </li></ul><ul><li>Typical GMR ratio ~ 5-10% (record for tunnel junctions: 80%) </li></ul><ul><li>Typical coercive fields ~ 10 Oe: high-sensitivity for MRAM </li></ul><ul><li>Used in current-in-plane geometry to get adequate signal </li></ul><ul><li>2007 Nobel Prize in Physics awarded to A. Fert and P. Gr ü nberg </li></ul>Parallel: low R Anti-parallel: high R
  18. 19. Moodera, Myazaki (1995) Spin-valves with metallic interlayer
  19. 20. MRAM applications for spin valves 1MB prototype chip shown by Motorola in June 2002 Write Mode <ul><li>Program initially funded by DARPA in 1996 </li></ul><ul><li>Commercial chips are available from 2008 </li></ul><ul><li>2007 Nobel Prize in Physics </li></ul>Isolation Transistor “ ON” Bit Line Digit Line Read Mode Sense Current Isolation Transistor “ OFF” Program Current H e Bit Line Digit Line Program Current H h
  20. 21. Advantages of Organic Spintronics <ul><li>Long spin relaxation time for injected spin ½ polarons </li></ul><ul><ul><li>weak spin-orbit coupling; light elements (H SO ~ Z 4 ), and  -electron angular momentum properties (L z =0) </li></ul></ul><ul><ul><li>weak hyperfine interaction;  -electron orbital properties </li></ul></ul><ul><li>Variable tunnel barrier height using different electrodes; interface resistance manipulation (Campbell & Smith, 1998; Gillin, 2011) </li></ul><ul><li>Introduction of carriers by external in-situ doping after fabrication </li></ul><ul><li>Light emission activity </li></ul><ul><li>Possibility of all-organic spin devices using organic ferromagnet electrodes for spin-injection (J. Miller/Epstein) </li></ul>
  21. 22. Spintronics Debut in Organics <ul><li>Dediu et al. Solid State Com. 122, 181 (2002) ; </li></ul><ul><li>Magnetoresistance  spin-valve effect </li></ul><ul><li>Also does not prove spin injection into the OSEC </li></ul>Zero-field High field
  22. 23. Vertical spin-valves in our group <ul><li>Bottom electrode : La 0.7 Sr 0.3 MnO 3 (LSMO) on SrTiO 3 substrate </li></ul><ul><ul><li>High spin-polarization (P  100%) </li></ul></ul><ul><ul><li>Stable in air (unlike FM metals) </li></ul></ul><ul><li>Organic spacer : Alq 3 , C 60 , polymers </li></ul><ul><li>Top electrode : Co, Fe; capped with Al layer </li></ul><ul><ul><li>Differential coercivity (Hc 1  Hc 2 ) </li></ul></ul><ul><ul><li>High Curie-temperature </li></ul></ul><ul><ul><li>Flexibility in deposition </li></ul></ul>SrTiO 3 La 0.7 Sr 0.3 MnO 3 Co, Ni, or Fe Organic No nanolithography is required
  23. 24. Organic spin-valves fabricated in our group The spin-valve device is a vertical sandwich of LSMO/Alq 3 /Co/Al configuration Xiong; 2004 . . . . . . . . . . . . LSMO Alq 3 Co ~ 3-5nm
  24. 25. Differential Coercivity Bottom electrode: La 2/3 Sr 1/3 MnO 3 ; top electrode: Co <ul><li>Parallel and anti-parallel magnetization configurations can be controlled </li></ul><ul><li>Polarizer/analyzer experiment can be performed </li></ul>Hysteresis loops from the two electrodes measured using MOKE
  25. 26. First ever organic spin-valve obtained in our research group (April 2004) <ul><li>12% GMR observed at low fields (to be compared with 5-10% in most metallic spin-valves to date). </li></ul><ul><li>Inverse GMR that is consistent with the band structure properties of the two FM electrodes; </li></ul><ul><li>P 1 (LSMO)  1 </li></ul><ul><li>P 2 (Co)  -0.3 </li></ul>Field (kOe) Xiong et al ., Nature , 2004 GMR of LSMO/Alq 3 /Co at 11K is over 12% Co LSMO
  26. 27. Fe/Alq 3 /Co devices; two ‘conventional’ FM electrodes Spin valve response also obtained using Fe and Co; two “conventional” FM electrodes; but only ~ 4% F. Wang et al ., 2005
  27. 28. Spin valves with small molecules and polymers have been also shown by many other groups : Brown University, RI; Ab ö Akademie, Finland; Bologna; Alabama, OSU, MIT, ISU, Weizmann Institute, Drezden, U. Paris, U. of London, etc. NPD: another small molecule material Spin valves with other organic materials F. Wang et al . 2006
  28. 29. Alq 3 spin-valve at ‘optimum conditions’ <ul><li>Alq 3 thickness 130 nm </li></ul><ul><li>Temperature 11K </li></ul><ul><li>Bias 20 mV </li></ul>40% GMR value GMR spin-valve response MOKE response of the FM electrodes Hc 1 (LSMO)  20 Oe Hc 2 (Co)  100 Oe Nature 2004 Xiong et al .,
  29. 30. 1. GMR; OSEC film thickness dependence Modified spin-valve equation: spin polarization; p 1 p 2 = 0.3
  30. 31. Carriers diffuse and drift within the organic layer and spin polarization decays over a distance  s ; the spin diffusion length p 1 p 2 = 0.3 d 0 = 85 nm  s = 45 nm Spin diffusion length in organic semiconductors e E F E F Ferromagnet 1 Ferromagnet 2 Organic Interfaces
  31. 32. 2. GMR; bias voltage dependence <ul><li>Maximum GMR at V  0 </li></ul><ul><li>Asymmetry respect to V </li></ul><ul><li>‘ Zero-field anomaly’ at V  0 </li></ul><ul><li>Why GMR decreases with V? </li></ul>
  32. 33. 3. GMR; temperature dependence <ul><li>Low- and high-field magnetoresistance </li></ul><ul><li>Spin-valve GMR at low field </li></ul><ul><li>Non spin-valve MR effect at high field; </li></ul><ul><li>due to the LSMO electrode injection </li></ul><ul><li>(iii) The HFMR shows the superior LSMO films </li></ul>Spin-valve GMR response (low field) vs. temperature Magnetoresistance vs. temperature The spin-valve GMR response decreases at high T, and is much steeper than M s (T) of the LSMO electrode; SL relaxation?
  33. 34. Alq 3 purified α -NPD CVB Organic spin-valves at UoU ; different OSEC materials I and V are the injected current and biasing voltage across the device and H is the external in-plane magnetic field . OSV measurements LSMO Co/Al CVB V I H
  34. 35. CVB 50mV at 12K Magnetoresistance response; LSMO/CVB/Co spin-valves Analysis using: the modified Jullière model :  R/R = 2 P 1 P 2 D/(1 + P 1 P 2 D); D = exp[-(d-d 0 )/  s ] Wang, Yang, Li, & Vardeny Phys. Rev. B 75, 245324 (2007)
  35. 36. The MR vs. temperature in organic spin-valves <ul><li>The MR value of three different LSMO/OSEC/Co spin valve devices vs. temperature, T normalized at T = 14 K. </li></ul><ul><li>The MR value decreases with T . </li></ul><ul><li>It diminishes at T ~ 220 K. </li></ul><ul><li>Why the ‘quasi-universal’ T- dependence? </li></ul>
  36. 37. Organic Spintronics strikes back Z. V. Vardeny; Nature Materials 8 , 91, 2009 Work done: Drew et al ., Nature Materials 8 , 109 (2009) Proof of spin injection into organic semiconductors; Muons spin rotation for measuring ‘local’ magnetic field
  37. 38. Spin diffusion length vs. temperature Drew et al , Nature Materials 8 , 109 (2009) Is this the reason for the GMR temperature dependence?
  38. 39. Molecular Electronics with Self-Assembled Monolayers e H LSMO Cobalt Ralph 2006; Burtman 2007
  39. 40. SAM spin-valve Fabrication Approach to molecular spin-valves (Burtman and Ndobe; 2006)
  40. 41. Spin-valves of SAM diodes; Isolated conducting molecules Single molecule spin valve with giant TMR of 500% at low temperatures
  41. 42. <ul><li>What mechanism determines the spin relaxation rate in organic semiconductors? </li></ul><ul><li>Small spin orbit coupling (low Z); is it? </li></ul><ul><li>Small hyperfine interaction ( π -electron wave-function); is it? </li></ul><ul><li>Spin ½ impurities? </li></ul><ul><li>Diffusion of magnetic atoms from FM electrodes? </li></ul><ul><li>This is one of the main questions posed for the NSF/MRSEC program at the UoU (funded on 09/15/2011) </li></ul>
  42. 43. One idea: replace proton hydrogen atoms with deuterium atoms A 0 : hyperfine coupling constant The ratio between the hyperfine constant, A 0 of proton and deuterium is ~6.5 H H H H H H H H Hydrogenated DOO-PPV Hydrogen atoms closest to backbone carbons are the main source of HFI; nuclear spin: ½ Deuterated DOO-PPV The chemist: Leonard Wojcik Deuterium atoms have nuclear spin: 1
  43. 44. Nguyen et al. Nature Materials 2010 Photoluminescence and Raman spectra of H- and D-polymers <ul><li>The same PL spectra + PL quantum efficiency (12%)-> similar conjugation length </li></ul><ul><li>Raman scattering spectra and NMR -> all 1 H near the backbone carbon atoms on the polymer chains were indeed replaced by 2 H (D) atoms. </li></ul>Raman-active vibrational modes: at ~1300 cm -1 (CH-CH stretching) at ~1500 cm -1 (CH=CH stretching) [m(CD)/m(CH)] 1/2  1.037 “ square root of the mass ratio rule”:
  44. 45. Properties of electron spin resonance (ODMR)  B D  0.7 mT  B H  1.2 mT  B depends on the hyperfine coupling constant + wavefunction extent of the polaron on the polymer chain + inhomogeneous broadening  SL (H)/  SL (D)~4  B ( P MW ) =  B (0) [1 + (  /  SL )P MW ] 1/2  SL : spin lattice relaxation rate P MW : MW power
  45. 46. GMR response of H- and D-DOO-PPV OSVs Device thickness of ~25 nm, resistance ~ 200 kOhm Applied voltage ~ 10mV Fitting formula: MR(B)= ½MR max [1- m 1 ( B ) m 2 ( B )]exp[- d f / l s ( B ) ], l s (0)/d f =1 for H-DOOPPV and l s (0)/d f =3 for D-DOOPPV; <ul><li>Bobbert, Wohlgenannt et al., Phys. Rev. Lett. 102 , 156604 (2009). </li></ul>
  46. 47. Nguyen et al. Nature Materials 9, 345 (2010). MR thickness dependence to determine  S MR at 80 mV and 10K Fitting function: MR = MR max exp(-d/ λ S )
  47. 48. c Organic spin-valves using C 60 interlayer 12 C nucleus has spin I =0; abundance 98.8%, no HFI 13 C nucleus has spin I =½; abundance 1.2%; some HFI F. Wang 2009 B I V LSMO C 60 Co/A l
  48. 49. GMR in C 60 OSVs; voltage and temperature dependencies GMR(V) is different at various T; it cannot be due to the FM electrodes Where does the voltage dependence come from? Fujian Wang; 2009
  49. 50. GMR in C 60 OSV; room temperature operation Fujian Wang; 2009 Very stable OSV devices; GMR up to 0.3% at RT
  50. 51. II. HF magnetoresistance; field-dependent carrier injection from the LSMO electrode High-field magnetoresistance is due to magnetic field dependent carrier injection, rather than spin coherent transport One ferromagnet/organic interface PRL 2005 LSMO Alq 3 (NDP, or PFO) Al Alq 3
  51. 52. MR of LSMO is caused by suppression of spin fluctuations MR of the LSMO film Substrate i LSMO H
  52. 53. <ul><li>MR in the organic device is caused by LSMO/organic interface; since R exponentially depends on barrier height </li></ul><ul><li>E F of the e g electrons in LSMO may shift at H > 0: anomalous chemical potential shift in double-exchange ferromagnets </li></ul>N. Furukawa , J. Phy. Soc. Jpn 66 , 2523 (1997) E F up shift  decrease of barrier height  MR Mn Mn Mn Mn Anomalous E F Shift in LSMO E F ( H= 0) La 0.67 Sr 0.33 MnO 3 DOS   E E F ( H >0) LSMO H=0 H=7T Al 
  53. 54. Not seen in regular FM’s Device I-V characteristics at various H Anomalous E F shift in LSMO; effect on device MR 52mV LSMO H=0 H=7T AlQ 3   > 10 meV/T  B H = 0.11 meV
  54. 55. III. Magnetic field dependence of Alq 3 -based OLEDs A. Room temperature, low field Magneto-electroluminescence; not related to spin injection or FM electrodes ( No FM electrodes ) Record 10% at 300K Wohlgenannt; 2006
  55. 56. Magnetic field dependence of Alq 3 -based OLED’s B. Low temperature High field High-field Magneto-EL; not related to spin injection or FM electrodes
  56. 57. Conclusions <ul><li>Status of Organic Spintronics (2011) </li></ul><ul><li>Organic spin-valves have been successfully fabricated using various </li></ul><ul><li>OSECs and FM electrodes; devices with two FM electrodes </li></ul><ul><li>Record obtained value for the spin-valve related GMR of 300% at 11K </li></ul><ul><li>These devices show low-field, spin-valve related GMR response due to </li></ul><ul><li>spin injection and coherent spin transport in the organic layer </li></ul><ul><li>Spin relaxation is governed by hyperfine interaction in polymers; what </li></ul><ul><li>governs spin relaxation in thin films of small molecules is open question </li></ul><ul><li>Possible future applications include magnetic memory devices and </li></ul><ul><li>magnetic-field-controllable s-OLEDs (intensity and colors) </li></ul><ul><li>High field MR in OLEDs due to LSMO electrode (one FM electrode </li></ul><ul><li>device) </li></ul><ul><li>Low and high MR and MEL in OLEDs with no FM electrodes </li></ul>