Session Y14: Focus Session: Spins in Carbon - Organic Spintronics

A new twist on organic spintronics: Controlling transport in organic sandwich devices using fringe fields from ferromagnetic films

Organic spintronics studies the physics of spin-injection and magnetic-field dependent transport phenomena in organic semiconductors, possibly leading to devices with added functionality. So far, studies have focused on spin-valve architectures as well as entirely non-magnetic devices that nevertheless show large room-temperature magnetoresistance through the so-called organic magnetoresistive effect. We demonstrate a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially-varying magnetic fringe fields of a magnetically-unsaturated ferromagnet. A large variation from hopping site to hopping site of the nuclear hyperfine field is known to dramatically affect electronic transport in organics, whose resistances are very sensitive to small applied magnetic fields, so the ferromagnet's fringe fields might act as a substitute either for the applied magnetic field or for the inhomogeneous hyperfine field. The size of the effect, magnetic field dependence, and hysteretic properties rule out a model where the fringe fields from the ferromagnet provide a local magnetic field that changes the electronic transport properties through the hyperfine field, and show our effects originate from electrical transport through the inhomogeneous fringe fields coming from the ferromagnet. Surprisingly these inhomogeneous fringe fields vary over length scales roughly two orders of magnitude larger than the hopping length in the organic materials, challenging the fundamental models of magnetoresistance in organic layers which require the correlation length of the inhomogeneous field to correspond roughly to the hopping length. Our devices, which do not rely on spin injection, tunneling anisotropic magnetoresistance or spin-valve behavior, may provide a simple approach to integrating magnetic metals and organics for hybrid spintronic devices. These devices may find application as high-voltage readouts of the magnetic state of low-impedance ferromagnetic films.   

Magnetic Field Effects: Triplet-Charge Annihilation versus Triplet-Triplet Annihilation in Organic Semiconductors

Triplet-charge reaction and triplet-triplet annihilation are two important processes in generating magnetic field effects in organic semiconductors. This presentation reports experimental studies on triplet-charge annihilation (TCA) versus triplet-triplet annihilation (TTA) in organic semiconductors. Specifically, we separately adjust the triplet-charge and triplet-triplet interactions towards the generation of TCA and TTA by changing triplet density, charge confinement, and charge/exciton ratio based on organic light-emitting diodes. We then use magnetic field effects of electroluminescence (MFE$_{EL})$ to study the TCA and TTA through spin interactions. We observe that the electroluminescence can clearly show negative response to applied magnetic field when triplets and charges are simultaneously confined within close proximity. On contrast, the electroluminescence only exhibits positive MFE$_{EL}$ when triplets are confined within close proximity. Therefore, it can be concluded that the TCA is a major process to annihilate triplets through Coulomb interaction in organic semiconductors.   

Electron Spin Relaxation: The Role of Spin-Orbit Coupling in Organic Semiconductors

Rapid development of organic materials has lead to their availability in commercial products. Until now, the spin degree of freedom has not generally been used in organic materials. As well as engineering difficulties, there are fundamental questions with respect to the electron spin relaxation (eSR) mechanisms in organic molecules. Muons used as a microscopic spin probe, localized to a single molecule, can access information needed to identify the relevant model for eSR. In this presentation I will introduce the ALC-MuSR technique describing how eSR can be extracted and the expected effects. I will show how the technique has been applied to small organic molecules such as the group III Quinolate series and functionalized molecules with a pentacene-like backbone. Lastly I will present the Z-number and temperature dependence in these organic molecules and show strong evidence for a spin-orbit based eSR mechanism.   

Using quantum spin-phase to elucidate weakly coupled pair hopping rates in organic semiconductors

We have carried out electrically detected spin echo experiments on recombining electron-hole pairs (polaron pairs)\footnote{McCamey, D.~R. \textit{et al}., \textit{Nat Mater} \textbf{7}, 728 (2008).} in MEH-PPV organic light emitting diodes at different temperatures and device currents. We find long spin-phase relaxation times [$T_{2}$ = 324(18) ns] at room temperature and less than twice this value [$T_{2}$ = 611(39) ns] at $T$ = 10K. Next to this very weak temperature dependency, we also observe nearly no dependency of $T_{2}$ on the free carrier density in the material. We attribute this coherence decay behavior to charge carrier hopping transitions between localization sites which are exposed to differing hyperfine fields. Although the nuclear spin relaxation times in our material are much longer than the time scale of our experiments, the stochastic movement through this random environment leads to a time dependent fluctuating field causing an irreversible phase loss. We have simulated the echo experiments with this hopping process incorporated, and find good agreement with experimental data. The results of our study contrast free polaron transport processes which show an Arrhenius-type activation in time-of-flight experiments.   

Spin-flip induced magnetoresistance in positionally disordered organic solids

A theory for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up `spin-blocked' pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field-dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory.   

Magnetoresistance in organic spin-valve devices with low work-function metal

Organic thin-films have recently generated interest for studies related to spin-based phenomena. Long spin transport through organic semiconductors has been predicted due to their weak spin-orbit and small hyperfine interactions. This property gives them great potential for spintronic applications. While magnetoresistance (MR) has been reported by multiple research groups, controversy still remains over whether the basic mechanism is the transport of spin-polarized carriers through the organic semiconducting thin films or transport via unintended tunneling paths through the devices. We investigate the MR of organic thin-film spin valve devices fabricated from tris[8-hydroxyquinoline]aluminium (Alq3), high Tc transition ferromagnetic metals, and low work-function metals. The devices are fabricated without the exposure to the air to minimize the oxidation. We observe the MR as large as 3 {\%} at 4.5K in devices containing Alq3 as thick as 150 nm.   

Origin of magnetoresistance in organic spin-injection structures

Spin injection into organic materials is usually inferred from the device resistance difference between antiparallel and parallel magnetic configurations (magnetoresistance, or MR) in an organic spin valve (OSV). The common features of the observed MR in OSVs include: 1) the MR is pronounced only at a low bias, where the device I-V characteristic is essentially linear; 2) the MR quickly decreases with bias and temperature while the decrease in device resistance is insignificant; 3) the MR is usually negative for OSVs with thick organic films, particularly when the two ferromagnets are similar in electronic structure. Despite superficial resemblance between OSVs and inorganic spin-injection structures, the MR in the former cannot be explained by theories developed for the latter. Here we show that the resistance of an OSV is controlled by the carrier density deep inside the organic and the MR is due to the difference in the carrier density for the two magnetic configurations. The sign of MR is determined by the electron spin polarization at a finite energy above the Fermi level in the electrodes. This picture explains common features of MR and suggests new strategies for probing and manipulating spin in organic spintronic structures.   

Magnetoresistance in an all-organic-based spin valve

We demonstrate spin injection and detection in an all-organic-based magnetic tunnel junction using two layers of organic-based magnet V[TCNE]$_x$ (x$\sim$2, TCNE: tetracyanoethylene) as the magnetic contacts and organic semiconductor rubrene (C$_{42}$H$_{28}$) as the spacer. For the V[TCNE]$_x$ film growth, we exploited two different growth techniques, chemical vapor deposition and molecular layer deposition, which result in different coercivities of V[TCNE]$_x$ films. The spin valve devices show negative magnetoresistance (MR), the sign of which does not change with temperature and bias. To explain the unusual negative MR, we propose a simple phenomenological bias-enhanced selective tunneling (BEST) model based on the different spin polarizations of the molecular energy levels of V[TCNE]$_x$. Our results show the significance of bias induced energy level shift in organic spintronic devices due to relatively narrow spin polarized bandwidths. This work was supported in part by AFOSR Grant No. FA9550-06-1-0175, DOE Grant Nos DE-FG02-01ER45931, DE-FG02-86ER45271, NSF Grant No. DMR-0805220, the Center for Emergent Materials (an NSF-MRSEC; Award Number DMR-0820414) at The Ohio State University and the Institute for Materials Research at The Ohio State University.   

Manganese Phthalocyanines on manganese surfaces: An ab-initio approach to the tunneling magnetoresistance

We have performed computational studies of spin polarized transport based on Density Functional Theory (DFT) using a non-equilibrium Green's functions formalism implemented in our ANT code [1] to compute tunneling magnetoresistance (TMR) on different manganese based systems. Specifically we present results for STM tips on a clean manganese surface, which is well known to show an antiferromagnetic coupling between layers when grown on an iron surface. In this system we have studied the dependence of the TMR on the tip-surface distance, the position of the tip, and on the bias voltage. Also we present a comparison between the Landauer formalism and the Tersoff-Hamman approach, usually used in this context. Finally we analyse the interaction between Manganese Phthalocyanine (MnPc) molecules on the manganese surface, focusing on the TMR signature and its dependence on the different adsorption sites.\\[4pt] [1] J.J. Palacios et. al. Ab-initio Quantum Transport (ANT). alacant.dfa.ua.es   

Spin injection in cobalt and copper phthalocyanines

An organic light emitting diode (OLED) is a heterojunction comprising two different organic semiconductors (OS): one for hole and another one for electron transport. Electrons and holes recombine at the interface between them. Depending on their total spin the recombination may (singlet state) or may not (triplet state) produce visible light. The efficiency of OLEDs could be doubled by injecting a spin-polarized current (Spin-OLED).\footnote{I. Bergenti et al. Organic Electronics, 2004, 5, 3} One obstacle to attain a spin-OLED is the lack of studies of spin diffusion lengths ($\lambda_{s}$) and injection efficiencies on hole-transport OS. We investigate spin injection in phthalocyanines (Pcs). The Pcs are ideal candidates to reach the above goal because: 1) of their high hole mobility; 2) Cu-Pc in particular is currently used in OLEDs; 3) their high thermal and chemical stability; 4) these molecules contain a single metal atom making them a unique system to study its effect on the spin injection. To determine $\lambda_{s}$ we investigate multilayer structures having transition metal/ Cu-Pc or Co-Pc interfaces including magnetic tunnel junctions with OS as barriers. Further characterization of these interfaces has been performed by soft X-Ray measurements at the NSLS-BNL facility.   

The role of thin MgO(100) epilayer for polarized charge injection into top-emitting OLED

A new top-emitting OLED (TOLED) structure, which is formed on an Si(100) substrate and an epitaxial MgO(100)/Fe(100)/MgO(100) bottom electrode, was investigated. Our TOLED design included a semi-transparent cathode Al, a stack of conventional organic electroluminescent layers ($\alpha $-NPD/Alq$_{3}$/LiF) and a thin Cu-Phthalocyanine (CuPc) film to enhance the hole injection into the luminescent layers. At room temperature (RT), magnetoluminescence of $\sim $5 {\%} was observed in low magnetic field up to 1 Tesla , which is obviously larger than that of the OLEDs with epitaxial and polycrystalline Fe anodes without MgO(100) covering layer. Our results indicate that the magnetic field effect on the electroluminescence could be strongly related to the magnetic properties of bottom electrode, more precisely the interfacial properties between CuPc layer and the anode. Therefore, we focused on understanding interface electronic states and energy alignment by using x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. Our results showed that the use of appropriate oxide layers could represent a new interface engineering technique for improving reliability and functionality in organic semiconductor devices.   

Spin Polarized Scanning Tunneling Microscopy of Alq3 on Cr(001)

The field of organic spitronics has been strongly motivated in recent years by the observation of giant magnetoresistive effects in tris-(8-hydroxyquinoline) aluminum (Alq3) films and nanostructures. It is crucial to understand the spin- dependent electronic structure at metal-Alq3 interfaces. We have carried out spin polarized scanning tunneling microscopy to measure the local density of electronic states for submonolayer films of Alq3 grown on the layered anitferromagnetic Cr(001) surface. We report an energy-dependent tunneling conductance asymmetry for single molecules adsorbed on differently magnetized (001) terraces and discuss its connection with metal-molecule hybridization and magnetoresistive effects in Alq3 spintronic devices.   

Ferroelectric control of the spin polarization in an organic spin valve

Recently engineering the spin propagation in organic spin valves has shown increasingly interesting properties. In this work we demonstrate novel ferroelectric control of the spin polarization in an organic spin valve. By inserting a thin ferroelectric buffer layer between a bottom La$_{0.67}$Sr$_{0.33}$MnO$_{3 }$(LSMO) electrode and the organic Alq$_{3}$ layer, a controlled spin polarization through the ferroelectric interface is achieved. The spin valve exhibits both positive and negative magnetoresistance depending on the applied bias. We conclude that this results from the energy level shift by the ferroelectric dipoles between Alq$_{3}$ and LSMO (Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy).