The document summarizes the research of Zeev Valy Vardeny and his collaborators on organic spintronics at the University of Utah. Some of their key findings include: 1) Demonstrating the first organic spin-valve device using LSMO/Alq3/Co layers, which showed a giant magnetoresistance of over 12%. 2) Studying spin dynamics and relaxation processes in organic semiconductors using spin-valve and Hanle effect measurements. 3) Exploring new device concepts like organic spin light-emitting diodes by injecting spin-polarized carriers into organic layers.

1. Organic Spintronics Zeev Valy Vardeny University of Utah; Salt Lake City

2. Collaborators: Physics Department, University of Utah (1) Professors : Jing Shi (UC Riverside); Tho Nguyen; Brian Saam (2) Post doctors : Z. H. Xiong (Prof. at USTC); Burtman (Industry) (3) Graduate Students : F. Wang (UoI), B. Gautam (4) Staff members : L. Woijcik, R. Polson Other Institutions (i) Prof. X. J. Li; USTC, China Prof. Eitan Ehrenfreund; Technion, Israel Presently supported by the NSF-MRSEC program at the UoU (9/2011)

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. Outline:  -conjugated semiconductors; photophysics, OLED devices and spin-physics Spintronics ; an introduction Spin-valve devices and their applications LSMO/Alq 3 /Co organic spin-valve devices; two FM electrodes Spin ½ relaxation process; isotope effect and hyperfine interaction Organic spin valves based on C 60 ; spin-orbit interaction? High field GMR response in organic diodes; one FM electrode Magnetic field effect in OLEDs; no FM electrode

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

8. Advantages of ‘organic electronics’ Physical flexibility; ‘plastic’ Low-cost manufacture: ink printing, roll-to-roll coating large scale production Light weight; portable Low energy consumption; low drive bias voltage high brightness variety of colors; leading to white light emission

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 Singlet energies measured by optical absorption Triplet energies measured by the weak phosphorescence optical absorption X Theory ; Mazumdar, Abe, Bredas Experiment ; Baessler, Friend, Vardeny

10. Spintronics Dictionary Electron spin, hole spin ; these are spin ½ charge carrying excitations. Singlet exciton ; e-h bound pair in s = 0 combined spin state configuration. Triplet exciton ; e-h pair in s = 1 combined spin state. Spin relaxation time ; the time in which the prepared spin sense stays put. Spin injection ; carrier injection with preferred spin sense. Spin diffusion length ; the distance spin carriers diffuse before spin flip occurs. This may be very different from carrier diffusion length; L s  L drift

11. A New possible device: Spin-OLED Regular OLED with non-magnetic electrodes: QE max ~ 25% of SE Triplet excitons: no EL Singlet luminescence OLED with ferromagnet (FM) electrodes : QE from 0 to 50% with H 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. 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. Solutions to the problem of spin injection into SEC 1. Reduce resistance mismatch (reduce  ) 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. 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. Spintronics using inorganic semiconductors Optically injected electron spins can travel coherently over several microns in GaAs ( Awschalom 1997 ) Electrical injection/optical detection was demonstrated in inorganic spin- LEDs (Ohno, 1999; Jonker, 2002) No spin-valves (electrical injection and detection) has been demonstrated so far T c of FM SEC injectors is too low (110K for GaMnAs) Interface quality is hard to control It requires MBE growth Recently 60% spin polarization has been obtained (APS meeting 2010) Does not work for OSEC; Since PL emission is from excitons with weak S-L coupling

16. Possible electrical injection/detection in organic semiconductors Two well-accepted schemes using ferromagnetic (FM) injectors: Hanle effect Polarizer-analyzer effect ( spin-valve effect) 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. The spin-valve device Practical GMR structures: FM/NM/FM for tunneling spin-valves Thickness of few nm < spin diffusion length (~10 nm at RT) Magnetization parallel and anti-parallel configuration Typical GMR ratio ~ 5-10% (record for tunnel junctions: 80%) Typical coercive fields ~ 10 Oe: high-sensitivity for MRAM Used in current-in-plane geometry to get adequate signal 2007 Nobel Prize in Physics awarded to A. Fert and P. Gr ü nberg Parallel: low R Anti-parallel: high R

19. Moodera, Myazaki (1995) Spin-valves with metallic interlayer

20. MRAM applications for spin valves 1MB prototype chip shown by Motorola in June 2002 Write Mode Program initially funded by DARPA in 1996 Commercial chips are available from 2008 2007 Nobel Prize in Physics 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

21. Advantages of Organic Spintronics Long spin relaxation time for injected spin ½ polarons weak spin-orbit coupling; light elements (H SO ~ Z 4 ), and  -electron angular momentum properties (L z =0) weak hyperfine interaction;  -electron orbital properties Variable tunnel barrier height using different electrodes; interface resistance manipulation (Campbell & Smith, 1998; Gillin, 2011) Introduction of carriers by external in-situ doping after fabrication Light emission activity Possibility of all-organic spin devices using organic ferromagnet electrodes for spin-injection (J. Miller/Epstein)

22. Spintronics Debut in Organics Dediu et al. Solid State Com. 122, 181 (2002) ; Magnetoresistance  spin-valve effect Also does not prove spin injection into the OSEC Zero-field High field

23. Vertical spin-valves in our group Bottom electrode : La 0.7 Sr 0.3 MnO 3 (LSMO) on SrTiO 3 substrate High spin-polarization (P  100%) Stable in air (unlike FM metals) Organic spacer : Alq 3 , C 60 , polymers Top electrode : Co, Fe; capped with Al layer Differential coercivity (Hc 1  Hc 2 ) High Curie-temperature Flexibility in deposition SrTiO 3 La 0.7 Sr 0.3 MnO 3 Co, Ni, or Fe Organic No nanolithography is required

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

25. Differential Coercivity Bottom electrode: La 2/3 Sr 1/3 MnO 3 ; top electrode: Co Parallel and anti-parallel magnetization configurations can be controlled Polarizer/analyzer experiment can be performed Hysteresis loops from the two electrodes measured using MOKE

26. First ever organic spin-valve obtained in our research group (April 2004) 12% GMR observed at low fields (to be compared with 5-10% in most metallic spin-valves to date). Inverse GMR that is consistent with the band structure properties of the two FM electrodes; P 1 (LSMO)  1 P 2 (Co)  -0.3 Field (kOe) Xiong et al ., Nature , 2004 GMR of LSMO/Alq 3 /Co at 11K is over 12% Co LSMO

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

29. Alq 3 spin-valve at ‘optimum conditions’ Alq 3 thickness 130 nm Temperature 11K Bias 20 mV 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 .,

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

32. 2. GMR; bias voltage dependence Maximum GMR at V  0 Asymmetry respect to V ‘ Zero-field anomaly’ at V  0 Why GMR decreases with V?

33. 3. GMR; temperature dependence Low- and high-field magnetoresistance Spin-valve GMR at low field Non spin-valve MR effect at high field; due to the LSMO electrode injection (iii) The HFMR shows the superior LSMO films 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?

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)

36. The MR vs. temperature in organic spin-valves The MR value of three different LSMO/OSEC/Co spin valve devices vs. temperature, T normalized at T = 14 K. The MR value decreases with T . It diminishes at T ~ 220 K. Why the ‘quasi-universal’ T- dependence?

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?

39. Molecular Electronics with Self-Assembled Monolayers e H LSMO Cobalt Ralph 2006; Burtman 2007

40. SAM spin-valve Fabrication Approach to molecular spin-valves (Burtman and Ndobe; 2006)

41. Spin-valves of SAM diodes; Isolated conducting molecules Single molecule spin valve with giant TMR of 500% at low temperatures

42. What mechanism determines the spin relaxation rate in organic semiconductors? Small spin orbit coupling (low Z); is it? Small hyperfine interaction ( π -electron wave-function); is it? Spin ½ impurities? Diffusion of magnetic atoms from FM electrodes? This is one of the main questions posed for the NSF/MRSEC program at the UoU (funded on 09/15/2011)

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

44. Nguyen et al. Nature Materials 2010 Photoluminescence and Raman spectra of H- and D-polymers The same PL spectra + PL quantum efficiency (12%)-> similar conjugation length 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. 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”:

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

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; Bobbert, Wohlgenannt et al., Phys. Rev. Lett. 102 , 156604 (2009).

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

53. MR in the organic device is caused by LSMO/organic interface; since R exponentially depends on barrier height E F of the e g electrons in LSMO may shift at H > 0: anomalous chemical potential shift in double-exchange ferromagnets 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 

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

57. Conclusions Status of Organic Spintronics (2011) Organic spin-valves have been successfully fabricated using various OSECs and FM electrodes; devices with two FM electrodes Record obtained value for the spin-valve related GMR of 300% at 11K These devices show low-field, spin-valve related GMR response due to spin injection and coherent spin transport in the organic layer Spin relaxation is governed by hyperfine interaction in polymers; what governs spin relaxation in thin films of small molecules is open question Possible future applications include magnetic memory devices and magnetic-field-controllable s-OLEDs (intensity and colors) High field MR in OLEDs due to LSMO electrode (one FM electrode device) Low and high MR and MEL in OLEDs with no FM electrodes