Synthetic Metals (v.161, #7-8)

Editorial (vi).

Organic spin-valves based on fullerene C60 by Ran Lin; Fujian Wang; Markus Wohlgenannt; Chunyong He; Xiaofang Zhai; Yuri Suzuki (553-557).
Recent work suggests that the spin-transport length in organic semiconductors is limited by hyperfine coupling. Therefore, to potentially overcome this limitation, we fabricated spin-valves based on C60 for which the hyperfine coupling is minute. However, our devices do not show a significantly larger spin-diffusion length. This suggests that either a mechanism other than hyperfine coupling causes the loss of spin-polarization, or that in thick devices an increasing conductivity mismatch limits spin-injection.
Keywords: Spintronics; Organic semiconductors; Hyperfine coupling; Fullerene;

Polymeric spin-valves at room temperature by N.A. Morley; D. Dhandapani; A. Rao; H. Al Qahtani; M.R.J. Gibbs; M. Grell; D. Eastwood; B.K. Tanner (558-562).
The organic semiconductor–magnetic electrode interfaces in organic spin-valves are very important for the performance of the device. Here the interfaces between the polymer regio-regular poly(3-hexylthiophene) (RR-P3HT) and the magnetic electrodes Fe50Co50 and Ni81Fe19 in organic spin-valves were investigated. Extra interlayers were placed at the organic–magnetic interface, which included adding self-assembly molecules between the bottom electrode and the polymer, and 1 nm thick Au and Al2O3 insulating layers between the polymer and the top electrode. The magnetoresistance of the spin-valves was measured to determine how the variations in layers affected magnetotransport.
Keywords: Organic spintronics; Polymers; Magnetoresistance; Interface layers;

Charge injection, transport and localization in rubrene by Franklyn Burke; Plamen Stamenov; J.M.D. Coey (563-569).
▶ Ohmic contacts and Schottky contacts with rubrene crystals are studied. ▶ Nonlinear fits of I:V characteristics with Ni80Fe20 contacts give a barrier height of 0.22(1) eV. ▶ Negative magnetoresistance of 4% is observed in 5 T at 70 K for ohmic Ni80Fe20 contacts. ▶ Magnetoresistance is attributed to weak localization, not a spin valve effect. ▶ Spin diffusion length is less than 200 nm.ORubrene crystals bridging a 200–350 nm gap between electrodes made of gold or various ferromagnets (Ni80Fe20, Co90Fe10, Co90Fe10/AlO x ) exhibit distinct current:voltage characteristics according to their contact resistance. Fits to the nonlinear characteristics with Ni80Fe20 contacts give a Schottky barrier height of 0.22(1) eV. Only for ohmic contacts with Ni80Fe20 was magnetoresistance observed; it is essentially negative and linear in field, reaching 4% in 5 T at 70 K. The magnetoresistance is tentatively attributed to weak localization. There is no evidence of spin-valve magnetoresistance in the lateral geometry, where the spin diffusion length should be less than 200 nm.
Keywords: Rubrene; Magnetoresistance; Weak localization; Spin diffusion length; Schottky barrier height;

Investigation of the spin-dependent properties of electron doped cobalt–CuPc interfaces by Sabine Steil; Kathrin Goedel; Andreas Ruffing; Indranil Sarkar; Mirko Cinchetti; Martin Aeschlimann (570-574).
▶ Hybrid metal-organic interfaces as building block for organic spintronic devices. ▶ Influence of caesium doping on electronic spin-properties of a Co-CuPc interface. ▶ Low caesium doping modifies the interface energy level alignment. ▶ High caesium doping induces formation of an unpolarized gap-state. ▶ In both doping regimes, changes in the interface spin-polarization are observed.We have grown metal–organic interfaces by in-situ deposition of ultrathin copper phthalocyanine (CuPc) films on a thin cobalt film on Cu(0 0 1). Evidence for layer-by-layer growth is found. The spin-dependent electronic properties of the Co–CuPc interface and their modification under caesium doping are investigated by spin-resolved photoemission spectroscopy. We observe a doping-induced shift of the highest occupied molecular orbital (HOMO) of CuPc away from the Fermi level (E F), accompanied by the formation of an unpolarised gap-state at 0.7 eV below E F in the high doping regime. Such features are reflected in the behaviour of the detected interfacial spin-polarisation.
Keywords: Organic semiconductors; Alkali-metal doping; Organic spintronic; Metal–organic interfaces;

The origin of interfacial electronic and magnetic degradation for a ferromagnet atop organic conjugated molecules by Yao-Jane Hsu; Ya-Jyuan Hung; Ying-Chang Lin; Yu-Ling Lai; Hsu-Ting Chang; Chia-Hao Wang; Yuet Loy Chan; Ching-Lun Hsia; Meng-Fan Luo; Chih-Hao Lee; D.H. Wei (575-580).
Over the past years, spin and magnetic-field effects in devices operate by means of an organic spin valve using organic conjugated materials. As the interfacial properties in a ferromagnet–organic hybrid structure critically affect the performance of organic spintronic devices, understanding the interplay between interfaces in these hybrid structures becomes important. To study the interfacial electronic and magnetic properties of Co/Pentacene(Pc)/Co/Cr/Si(1 0 0) in a vertical pseudo-spin-valve structure, we employed synchrotron photoelectron spectra (PES), near-edge X-ray absorption fine structure (NEXAFS) and the magneto-optical Kerr effect (MOKE) to examine the interfacial hybrid state for Pc on Co (Co/Pc) and vice versa for Pc/Co bilayers. The evidences from PES, NEXAFS and MOKE suggest an asymmetric electronic and magnetic interface for Co/Pc and Pc/Co. The Co/Pc interface presents a chemically stable surface, but Pc/Co displays a reacted interface with a magnetically dead layer on the surface. Assisted with a calculation based on the density-functional theory, we elucidate the possible origin of the electronic and magnetic degradation for Co atop Pc in the interfacial regime.
Keywords: Organic spin valve; Interface; Photoelectron spectroscopy; Near edge X-ray absorption fine structure; Magneto-optical Kerr effect; Density function theory;

Magnetic disparities at the interfaces of Co–pentacene–Co hybrid structures by D.H. Wei; Yuet-Loy Chan; Ya-Jyuan Hung; Chia-Hao Wang; Ying-Chang Lin; Yu-Ling Lai; Hsu-Ting Chang; Chih-Hao Lee; Y.J. Hsu (581-585).
We investigated two series of ultrathin ferromagnet-organic semiconductor (FM-OSC) hybrid structures fabricated with reversed layer orders in ultra-high vacuum conditions to emulate the interfaces of FM/OSC/FM organic spin-valves. Using X-ray photoemission electron microscopy (PEEM), the magnetic responses of the cobalt (Co) layer in contact with the pentacene (Pc) film were investigated on a microscopic scale. According to the PEEM images acquired on Pc (1.8 nm)/Co bilayers, the Co top layer was found to turn on its ferromagnetic order after its thickness reaches 2.2 nm. Also, upon the onset of magnetic contrast, the element-specific images revealed that the Pc/Co bilayers exhibit a complex domain pattern whose average dimension is at least two orders of magnitude smaller than that of the pattern found in Co (2.4 nm)/Pc bilayers. Further X-ray photoelectron spectroscopy investigations suggest that the magnetic disparities observed between Co/Pc and Pc/Co bilayers are correlated with the different chemistries occurred at their interfaces.
Keywords: Organic spin-valve; Interface; X-ray magnetic circular dichroism; Magnetic domain;

Assembly and patterning of ferromagnetic nanoparticles from solution: A novel low-temperature liquid-phase annealing approach by T. Gang; M. Groen; S. Kinge; W.J.M. Naber; H. Boschker; G. Rijnders; D.N. Reinhoudt; W.G. van der Wiel (586-590).
Ferromagnetic L10-phase FePtAu nanoparticles (5–6 nm) were prepared using a low-temperature, liquid phase annealing approach. The chemically ordered, ferromagnetic L10-phase onset occurs for annealing temperatures as low as 150 °C. Large uniaxial magnetic anisotropy (K u  ∼ 107  erg/cm3) and a large long-range ordering parameter have been obtained. Significantly, the low-temperature liquid-phase annealing preserves the organic functionalization groups of the nanoparticles. Hence, these nanoparticles are flexible for further assembly and patterning. In addition, we present the assembly of FePt nanoparticles on ferromagnetic substrates with a thin alumina capping layer. This enables us to investigate spin dependent transport through the FePt nanoparticles by scanning tunneling microscopy.
Keywords: Magnetic nanoparticles; FePtAu; Solution annealing; Scanning tunneling microscopy; Spintronics;

High-spin and magnetic anisotropy signatures in three-terminal transport through a single molecule by Alexander S. Zyazin; Herre S.J. van der Zant; Maarten R. Wegewijs; Andrea Cornia (591-597).
We have studied three-terminal electron transport through an individual single-molecule magnet Fe4. Several fingerprints of high spin and magnetic anisotropy have been observed: the Kondo effect in adjacent charge states in an intermediate coupling regime and zero-field splitting in the regime of a weaker coupling. We also observed spin transitions at energies close to the ones reported for bulk Fe4 crystals. Finally, we have demonstrated electric-field control on the magnetic properties of a single molecule.
Keywords: Single-molecule magnets; Molecular electronics; Nanomagnetism;

The hyperfine interaction role in the spin response of π-conjugated polymer films and spin valve devices by Tho D. Nguyen; Golda Hukic-Markosian; Fujian Wang; L. Wojcik; Xiao-Guang Li; Eitan Ehrenfreund; Z. Valy Vardeny (598-603).
Although the hyperfine interaction (HFI) has been foreseen to play an important role in organic spin response, clear experimental evidence for it has been scarce. We studied and compared spin dynamics in films and organic spin-valve (OSV) devices based on π-conjugated polymers made of protonated, H-, deuterated, D-hydrogen (having a weaker HFI strength, a HFI), and 13C-rich chains (having stronger a HFI). We demonstrate that HFI indeed plays a crucial role in the spin dynamics of all three polymer isotopes. Films based on the D-polymer show substantial narrower optically detected magnetic resonance of spin ½ polarons; whereas due to the longer spin diffusion, OSV devices based on D-polymers show substantially larger magnetoresistance. We also found that the giant magnetoresistance (GMR) steep temperature dependence in OSV devices is isotope independent, showing that is due to the magnetic response of the ferromagnetic electrodes. In addition, we found that the GMR steep voltage dependence is isotope dependent indicating that is due to a spin injection process at the electrodes, rather than spin transport through the organic interlayer. Finally we report GMR response in OSV devices made of C60 interlayer having very weak HFI. These devices show sharp GMR response, and may therefore be excellent candidates for room temperature operation.
Keywords: Organic spintronics; Organic spin-valves; Spin polarized carrier injection; Giant magnetoresistance; Optically detected magnetic resonance;

Magneto-conductance of π-conjugated polymer based unipolar and bipolar diodes by T.D. Nguyen; B.R. Gautam; E. Ehrenfreund; Z.V. Vardeny (604-607).
▶ The magnetoconductance (MC) of unipolar and bipolar organic diodes. ▶ The MC(B) response is composed of two main regions: |B| < 1–2 mT and |B| > 2 mT. ▶ MC(B) in both field regions is strongly isotope dependent. ▶ The MC(B) response is explained by a model of spin-coupled pairs.The magnetoconductance (MC) of unipolar and bipolar organic diodes based on active layers made of the π-conjugated polymer DOO-PPV is presented in the range of magnetic field strength, B below about 100 mT, and for various polymer exchange isotopes. The MC(B) response is composed of two main regions: (i) a “sign-reversal” region for |B| < 1–2 mT, where MC(B) reverses its sign reaching a maximum absolute value |MC|m at B  =  B m; and (ii) a monotonic region for |B| > 2 mT, where MC(B) monotonically increases (decreases) for bipolar (unipolar) devices with an approximate Lorentzian line shape of width, ΔB. Similar behavior has been observed for the magneto-electroluminescence in bipolar devices. We found that MC(B) in both field regions is strongly isotope dependent; in particular both B m and ΔB are larger for nuclei with larger hyperfine interaction (HFI) constant. The complete MC(B) response, including the novel sign-reversal component is explained by a model of spin-coupled pairs of either same or opposite charge polarons, in which the HFI is explicitly included in the electronic spin Hamiltonian.
Keywords: Polaron pairs; Bi-radicals; Magnetoconductivity; Hyperfine interaction;

The effect of deuteration on organic magnetoresistance by N.J. Rolfe; M. Heeney; P.B. Wyatt; A.J. Drew; T. Kreouzis; W.P. Gillin (608-612).
▶ Three processes affect the efficiency of an OLED with magnetic field. ▶ A hydrogen containing hole transport layer does not affect these processes. ▶ Only the intermixing of polaron pair states is affected by deuteration.The effect of a magnetic field on the current and power conversion efficiency of protiated and fully (97%) deuterated aluminium tris-8(hydroxyquinoline) (AlQ3) organic light emitting diodes, both with and without a hole transport layer, has been studied and fitted using the triplet–polaron interaction model. Three processes with different magnetic saturation fields were found to be occurring; intersystem crossing between polaron pair states, intersystem crossing between exciton states, and triplet–polaron interactions. The differences in magnitude of the processes are responsible for the overall lineshapes of the magnetic field effects on current and efficiency. Comparison of the magnetic field effects on the protiated and deuterated devices revealed that altering the isotopic content of the material only affects intersystem crossing between the polaron pair states, and the inclusion of a hole transport layer has no effect on any of the processes. Therefore, it appears that there are several underlying spin mixing mechanisms that are responsible for organic magnetoresistance.
Keywords: OLED; Organic magnetoresistance; Isotope; Hyperfine; Triplet;

Effect of hyperfine interactions on exciton formation in organic semiconductors by S.P. Kersten; A.J. Schellekens; B. Koopmans; P.A. Bobbert (613-616).
It is still an open question whether the statistical ratio of 1:3 for the formation of singlet vs. triplet excitons in organic semiconductors can be violated. In this work, we use a two-site and a multi-site model to investigate this problem. We show that spin mixing by hyperfine fields leads to a violation of the statistical ratio if singlet and triplet excitons are formed with a different rate. This only occurs if the exciton formation rate is slow enough compared to the hyperfine precession frequency. The violation of the statistical ratio can be suppressed by applying an external magnetic field. In the multi-site model we demonstrate how disorder and Coulomb interactions affect the fraction of singlet excitons formed.
Keywords: Exciton formation; Organic semiconductor; Hyper-fine interaction; Spin mixing; Magnetoresistance;

On the role of minority carriers in the frequency dependence of organic magnetoresistance by P. Janssen; W. Wagemans; W. Verhoeven; E.H.M. van der Heijden; M. Kemerink; B. Koopmans (617-621).
In this work we investigate the frequency dependence of organic magnetoresistance (OMAR) both in small molecule-based (Alq3) and polymer (PPV derivative) materials, and investigate its thickness dependence. For all devices, we observed a strong decrease in magnetoconductance (MC) with increasing frequency of the ac component of the applied magnetic field. Moreover, we observed a strong reduction of the cut-off frequencies for increasing film thickness. By means of admittance spectroscopy and device simulations, we show that the cut-off frequency is related to the inverse transit time of the minority charge carriers. These observations confirm the important role of minority carriers in OMAR, and show that changes in OMAR are not only due to microscopic mechanisms, but also device physics is of significant relevance.
Keywords: Organic semiconductor; Magnetoresistance; Carrier mobility; Minority carriers; Transit time;

Frequency dependence of organic magnetoresistance by Fujian Wang; James Rybicki; Ran Lin; Kent A. Hutchinson; Jia Hou; Markus Wohlgenannt (622-627).
▶ Applications of magnetic sensors often require high frequency operation. ▶ We examine the frequency response of organic magnetoresistive sensors. ▶ We demonstrate that operation up to 100 kHz is possible high current. ▶ High current densities are advantageous, frequency response is limited by carriers with low mobility.Organic magnetoresistive (OMAR) devices show a large enough magnetoresistive response (typically 10%) for potential applications as magnetic field sensors. However, applications often require sensing high frequency magnetic fields, and the examination of the frequency-dependent magnetoresistive response is therefore required. Analysis of time constants that limit the frequency response may also shed light on the mechanism behind the OMAR effect.
Keywords: Spintronics; Organic semiconductors; Organic; Magnetoresistance; Frequency response;

▶ The organic magnetoresistance (OMR) data can be fitted using the triplet polaron interaction (TPI) model. ▶ The TPI model includes two independent processes, namely the exciton trapping and the triplet polaron interaction. ▶ The prefactors for the two processes were found to scale linearly with the triplet population. ▶ The magnitude and shape of the OMR can be predicted.The organic magnetoresistance (OMR) of 50 nm aluminium tris(8-hydroxyquinoline) organic light emitting diodes (OLEDs) has been measured over a range of temperatures and operating voltages. The OMR data for this device, the current change through the device with applied field, have been fitted using the triplet polaron interaction (TPI) model that includes two independent processes, namely the exciton trapping and the triplet polaron interaction. Two Lorentzian functions were used to fit the data, and the prefactors for the two processes were found to scale linearly with the triplet population. Over the whole temperature range the data appears to fall on a single line which covers nearly six orders of magnitude of exciton concentration. This work demonstrates that the magnitude and shape of the OMR can be predicted and will be useful for understanding the fundamental mechanism behind the OMR.
Keywords: Organic magnetoresistance (OMR); Exciton trapping; Triplet polaron interaction (TPI);

The discovery of magnetic field effects (MFEs) in organic light-emitting devices poses a new challenge for the theoretical studies. Based on the spin-boson theory of magnetotransport that we proposed recently, this article considers various contributions to the MFE. The role of triplet exciton, as an irreducible gradient of the environment in magnetotransport, is discussed extensively. Possible effect of the current caused by secondary charge on the MFE is taken into account to discuss the relation between magnetoresistance and magnetoelectroluminescence. Further experimental correspondence is also discussed.
Keywords: Organic semiconductors; Magnetic field effect; Spin-boson theory; Triplet excitons;

Degradation effect on the magnetoresistance in organic light emitting diodes by Tobias D. Schmidt; Andreas Buchschuster; Matthias Holm; Stefan Nowy; Josef A. Weber; Wolfgang Brütting (637-641).
The magnetoresistance in organic light emitting diodes (OLEDs) can be enhanced by electrical stressing of the device. In this study, magnetotransport measurements were performed on pristine and aged organic hetero-layer light emitting diodes based on small molecules with Alq3“(tris(8-hydroxyquinoline)aluminum)” as emitter. Under pristine conditions the maximum organic magnetoresistance (OMR) was of the order of 1% at an applied magnetic field of 100 mT and a voltage of 3.5 V. After electrical stressing at a constant current density we observed an increase of the maximum OMR together with a shift towards higher voltages. The maximum OMR reached almost 6% at an applied magnetic field of 100 mT and a voltage of about 5 V. To verify the correlation between electrical aging and the magnitude of the OMR effect, we investigated OLEDs with different hole injection layers exhibiting significantly different lifetimes.
Keywords: Organic magnetoresistance; Organic light emitting diode; Magnetic field effect; Degradation;

Magnetoresistance phenomena in ferromagnetic/wide band gap polymer system by A.N. Lachinov; Jan Genoe; N.V. Vorob’eva; A.A. Lachinov; F.F. Garifullina; V.M. Kornilov (642-645).
The appearance of huge injection magnetoresistance effect (HMR) in the ferromagnetic/polymer/non-magnetic metal structure has been considered. Poly(3,3′-phthalydiliden-4,4′biphenylen) (PPB), a non-conjugated polymer, is used as the polymer material. The influence of bias voltage polarity on the value of threshold magnetic field needed for conductivity switching has been established. The angular magnetoresistive dependence can be also observed in the same structure. The model of magnetoresistive effects has been proposed. The explanation is based on specific zone structure of the ferromagnet/wide band gap polymer interface.
Keywords: Spin polarization; Huge magnetoresistance; Domain structure; Polymer film;