Session V25: Organic Based Magnetism and Organic Spintronics

Spin Response in Organic Spin-Valves based on LSMO Electrodes

We fabricated spin-valves made of organic semiconductor (OSEC) thin films sandwiched between ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) and cobalt electrodes, using several OSEC small molecules. We measured the temperature (T) and voltage bias (V) dependence of the spin-valve related giant magneto-resistance (GMR) effect. We found a universal GMR decrease with T, where the GMR completely diminishes at $\sim $ 250K regardless of the OSEC layer. We show evidence that the underlying mechanism for the GMR decrease with T is the decrease in the spin injection capability of the LSMO electrode. We also found that the GMR steeply decreases with V, and is asymmetric respect to the applied voltage direction.   

Spin Valve Effects in Hybrid Organic-Inorganic Devices

Magnetoelectronic devices based on organic semiconductors (OSC) hold promise due to the long spin coherence in these materials and the ability to tune relevant properties such as carrier mobility and interface barriers via organic synthesis. We have studied spin valve effects in vertical geometry organic-based devices, using Fe and Co as the bottom and top electrodes. Several different organic semiconductors, including Perylenetetracarboxylic dianhydride (PTCDA) and the previously studied Alq3, have been used as the spin transport layers. At low temperatures, up to 5\% positive hysteretic magnetoresistance (MR) has been observed at low field in devices with semiconductor thickness of 140 nm, which is much larger than the tunneling limit. The MR decreases as the bias voltage or current increases. Possible mechanisms for spin-polarized transport in these devices and prospects for synthesis of materials with improved performance will be discussed.   

Morphology Influenced Properties in Organic Semiconducting Thin Films for Spin-Valves

The physical and electronic properties influenced by the morphology in organic thin films of tris(8-hydroxyquinoline) aluminum (Alq$_3$) are investigated systematically. This material is of interest for spintronics as the tunneling barrier in spin valves and for optoelectronics because it exhibits electroluminescence. In particular, in vacuo vapor deposited thin films are studied by atomic force microscopy, scanning tunneling microscopy and tunneling spectroscopy to determine the spatially resolved correlation of the electronic properties with the morphology. The contributions of various coulombic and thermodynamic parameters to achieving smooth and monolayer thick Alq$_3$ films are also investigated. Additionally, various high Curie temperature ferromagnets, including La$_{1-x}$Sr$_x$MnO$_3$, are explored as the spin-polarized electrode for optimized spin and charge transport properties in the organic/ferromagnetic heterostructures.   

Dipolar-Biased Tunneling of Magnetization in Crystals of Single Molecule Magnets

The molecular cluster Mn12 has attracted much interest as a single-molecule magnet (SMM) and as a multi-redox system. It has a high-spin ground state of $S$=10 and a strong uniaxial magnetic anisotropy, and the combination of the two natures makes an effective potential barrier between the up and down spin states. At low temperatures, the magnetization curve exhibited a hysteresis loop and the quantum tunneling of magnetization (QTM). In the present work, we studied the structure and magnetic properties of the mixed-metal SMM, Mn11Cr, through the analysis of Mn11Cr/Mn12 mixed crystal. High-frequency EPR spectra were well explained by assuming that Mn11Cr was in a ground spin-state of $S$=19/2 with nearly the same EPR parameter set as for Mn12. QTM in Mn11Cr was observed with the same field interval as for Mn12. The magnetization of Mn11Cr and Mn12 in the mixed crystal can be independently manipulated by utilizing the difference between their coercive fields. The resonance fields of QTM in Mn11Cr are significantly affected by the magnetization direction of Mn12, suggesting the effect of dipolar-biased tunneling. Besides SMM, we would also like to report the unusual magnetic properties of spherical hollow nanomagnets, the electrical properties of heterocyclic thiazyl radicals, and their possible applications in spintronics and organic electronics.   

Regioregular polythiophene based spintronic devices: effect of interface

Polymeric spin valves have been fabricated using regio-regular (poly 3-hexylthiophene) (RRP3HT) as the spacer layer sandwiched between La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) and Co electrodes. The devices show high spin valve magnetoresistance (MR) at 5K (80{\%}) which reduces at room temperature to 1.5{\%}. The spin valve behavior is quite similar to a magnetic tunnel junction although the non-magnetic spacer layer ($\sim $100 nm) is much thicker than the tunneling limit. We attribute this behavior to the formation of a thin spin-selective tunneling interface between LSMO and RRP3HT caused by the chemical bonding between RRP3HT and LSMO as observed by x-ray photoelectron spectroscopy measurement. Deliberate destruction of the spin injecting interface by the introduction of a monolayer of organic insulators between LSMO and RRP3HT reduces the spin injection.   

Ferrimagnetic resonance study on photo-induced magnetism in hybrid magnetic semiconductor V(TCNE)$_x$, $x \sim 2$ film

The V(TCNE)$_{x}$, $x\sim2$ is a fully spin-polarized magnetic semiconductor, whose magnetic order exceeds room temperature ($T_ {c}$ $>$ 350 K), and electronic transport follows hopping mechanism through the Coulomb energy split $\pi^{\ast}$ subband. In addition, it was determined that this material has thermally reversible persistent change in both magnetism and conductivity driven by the optical excitation [1]. Here, we report detailed investigation on photo-induced magnetism in V(TCNE)$_x$ by employing ferrimagnetic resonance (PIFMR) study with an in-situ light illumination. Upon optical excitation ($\lambda \sim$ 457.9 nm), the FMR spectra display substantial change in their linewidth and resonance field. Angular dependence analyses of line shift indicate the increase of unixial anisotropy field in the film caused by the light irradiation. The results demonstrated that the change in overall magnetic anisotropy by the illumination plays an important role in inducing photo- induced magnetism in (TCNE) class magnet. \newline \newline [1] J.-W. Yoo, et al. to be published in Phys. Rev. Lett.   

On the Mechanism Causing Large Room-Temperature Magnetoresistance in OLEDs

We report on the experimental study of a recently discovered, large room-temperature magnetoresistance effect in sandwich devices comprised of nonmagnetic electrodes and various organic semiconductor thin films. The effect reaches up to 10{\%} in a magnetic field of 10 mT at room temperature and saturates at fields larger than several tens of milliTeslas. In materials with strong spin-orbit coupling the characteristic magnetic field scale shifts to fields that are 10-100 times larger, consistent with the spin-orbit coupling strength. Our experiments therefore show that the organic magnetorestive effect is caused by spin-dynamics, possibly induced by the hyperfine interaction. We discuss two recently proposed models to explain the organic magnetorestive effect, which are based on spin-dependent exciton formation and spin-dependent hopping, respectively.   

Molecular Beam Epitaxy Growth of Organic Spin Valves

Spin-polarized transport across organic semiconductors has recently been demonstrated in organic spin valves consisting of ferromagnet(FM)/Alq$_3$/FM trilayers [a]. Organic semiconductors are interesting for spintronics due to their low spin-orbit coupling (which could in principle be tuned via chemical synthesis) and opto-electronic coupling to spin. We are utilizing molecular beam epitaxy (MBE) deposition through shadow masks to fabricate Co/Alq$_3$/Fe devices on MgO(001) substrates. Furthermore, we have developed the capability to perform variable temperature magnetotransport measurements without removing the sample from UHV. We find that for Alq$_3$ thickness below 100 nm, the sample exhibits linear I-V curves indicating a short across the Alq$_3$ layer, consistent with previous studies [a]. This is likely due to the interdiffusion of Co as the top electrode is deposited onto the Alq$_3$. By employing cryogenic techniques during top electrode growth, we are able to reduce the effects of Co diffusion and the formation of pinholes. We will report our progress on the in situ magnetotransport measurements to investigate spin-polarized transport in our devices. \linebreak (a) Z. H. Xiong, et.al. Nature 427, 821 (2004).   

Boosting quantum efficiency of single layer organic light emitting device by doping CoFe magnetic nanoparticles.

The effects of doping magnetic CoFe nanoparticles on electroluminance (EL) efficiency vs. current density, and current density vs. applied voltage for single layer organic light emitting devices (OLEDs) have been investigated. The electron trap densities increased with the increase of CoFe dopants, resulting in a high trap-filled limit (TFL) threshold voltage and significant enhancement EL efficiency ($\sim $27{\%}). The EL efficiencies were further improved ($\sim $5-7{\%}) by applied magnetic field. These improvements could be attributed to the enhancements of the ratios of the formation of excitons, and singlets to triplets, respectively and simultaneously. A maximum 32{\%} enhancement combing the two effects in EL efficiency has been achieved.   

Spin dynamics of photoexcited polarons in MEH-PPV: optically detected magnetic resonance studies

We studied the \textit{full dynamics }of the photoluminescence detected magnetic resonance (PLDMR), photoinduced absorption (PA), and PA detected magnetic resonance (PADMR) in MEH-PPV films, as a function of microwave power, P, and modulation frequency, f$_{M}$. We found it critically important to measure both in-phase and quadrature components; otherwise key characteristics of the dynamics are not unraveled. For example, the PLDMR in-phase component \textit{changes sign }at f$_{M }=f_{0}$ of about 30 kHz before decaying at higher frequencies. In contrast the quadrature PLDMR component retains its sign within the same experimental frequency range. We account for these peculiar dynamics by a model in which the polaron recombination is spin dependent (SDR). Specifically by solving the SDR rate equations we found that it correctly explains the PLDMR frequency dependent phase, and reproduce the obtained increase of $f_{0}$ with $P$. Also the SDR model explains equally well the PADMR and PLDMR time resolved measurements.   

Role of triplet polaron pairs in conjugated polymer photophysics

We measure the decay of the long-lived fluorescence of a conjugated oligofluorene at temperatures from 300 K to 20 K. We conclude that nearly all of this emission arises from geminate recombination of photogenerated polaron pairs to reform the singlet exciton, and that charge pair recombination represents a significant contribution to the overall fluorescence quantum yield. The unusual nonmonotonic decay dynamics of the delayed fluorescence can be explained if we assume interconversion between singlet and triplet polaron pairs on the submicrosecond time scale. ($\sim $500 ns.) We are able to model the decay of the delayed fluorescence by assuming activated recombination from a Gaussian energy distribution of singlet polaron pairs centered 0.2 eV below the excited state and having a standard deviation of 0.12 eV. The model is relevant to recent work involving the measurement of singlet-triplet branching ratios and to the yields of electroluminescent devices.   

Efficient plastic scintillators utilizing phosphorescent dopants.

We demonstrate improved light yield from plastic scintillators utilizing a phosphorescent dopant to collect both singlet and triplet excitations created by ionizing radiation. We specifically considered poly(vinyltoluene) and poly(9-vinylcarbazole) doped with an Ir phosphor. We present the spectral, temporal, and integrated yield response as a function of dopant concentration to pulses of 10 keV electrons. Both doped plastics yield a maximum light output $\sim$ 200{\%} of anthracene with decay times $<$ 850 ns. High light yield was obtained for Ir element fractions up to $\sim $ 10 wt{\%} implying that these scintillators may be useful for gamma detection.