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:
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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. 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. 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
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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 +
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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
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
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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
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
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30. 1. GMR; OSEC film thickness dependence Modified spin-valve equation: spin polarization; p 1 p 2 = 0.3
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
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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
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)
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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
38. Spin diffusion length vs. temperature Drew et al , Nature Materials 8 , 109 (2009) Is this the reason for the GMR temperature dependence?
41. Spin-valves of SAM diodes; Isolated conducting molecules Single molecule spin valve with giant TMR of 500% at low temperatures
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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
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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
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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 )
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
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
50. GMR in C 60 OSV; room temperature operation Fujian Wang; 2009 Very stable OSV devices; GMR up to 0.3% at RT
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
52. MR of LSMO is caused by suppression of spin fluctuations MR of the LSMO film Substrate i LSMO H
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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
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
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
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