Spin-dependent transport in molecular tunnel junctions

Efficient Spin-Selective Electron Transport at Low Voltages of Thia-Bridged Triarylamine Hetero[4]helicenes Chemisorbed Monolayer

The Chirality Induced Spin Selectivity (CISS) effect describes the capability of chiral molecules to act as spin filters discriminating flowing electrons according to their spin state. Within molecular spintronics, efforts are focused on developing chiral-molecule-based technologies to control the injection and coherence of spin-polarized currents. Herein, for this purpose, we study spin selectivity properties of a monolayer of a thioalkyl derivative of a thia-bridged triarylamine hetero[4]helicene chemisorbed on a gold surface. A stacked device assembled by embedding a monolayer of these molecules between ferromagnetic and diamagnetic electrodes exhibits asymmetric magnetoresistance with inversion of the signal according to the handedness of molecules, in line with the presence of the CISS effect. In addition, magnetically conductive atomic force microscopy reveals efficient electron spin filtering even at unusually low potentials. Our results demonstrate that thia[4]heterohelicenes represent key candidates for the development of chiral spintronic devices.

Rashba effect on finite temperature magnetotransport in a dissipative quantum dot transistor with electronic and polaronic interactions

The Rashba spin-orbit coupling induced quantum transport through a quantum dot embedded in a two-arm quantum loop of a quantum dot transistor is studied at finite temperature in the presence of electron-phonon and Hubbard interactions, an external magnetic field and quantum dissipation. The Anderson-Holstein-Caldeira-Leggett-Rashba model is used to describe the system and several unitary transformations are employed to decouple some of the interactions and the transport properties are calculated using the Keldysh technique. It is shown that the Rashba coupling alone separates the spin-up and spin-down currents causing zero-field spin-polarization. The gap between the up and down-spin currents and conductances can be changed by tuning the Rashba strength. In the absence of a field, the spin-up and spin-down currents show an opposite behaviour with respect to spin-orbit interaction phase. The spin-polarization increases with increasing electron-phonon interaction at zero magnetic field. In the presence of a magnetic field, the tunneling conductance and spin-polarization change differently with the polaronic interaction, spin-orbit interaction and dissipation in different temperature regimes. This study predicts that for a given Rashba strength and magnetic field, the maximum spin-polarization in a quantum dot based device occurs at zero temperature.

Nanoscale molecular rectifiers

The use of molecules bridged between two electrodes as a stable rectifier is an important goal in molecular electronics. Until recently, however, and despite extensive experimental and theoretical work, many aspects of our fundamental understanding and practical challenges have remained unresolved and prevented the realization of such devices. Recent advances in custom-designed molecular systems with rectification ratios exceeding 10⁵ have now made these systems potentially competitive with existing silicon-based devices. Here, we provide an overview and critical analysis of recent progress in molecular rectification within single molecules, self-assembled monolayers, molecular multilayers, heterostructures, and metal-organic frameworks and coordination polymers. Examples of conceptually important and best-performing systems are discussed, alongside their rectification mechanisms. We present an outlook for the field, as well as prospects for the commercialization of molecular rectifiers.

Combined spin filtering actions in hybrid magnetic junctions based on organic chains covalently attached to graphene

We present a bias-controlled spin-filtering mechanism in spin-valves including a hybrid organic chain/graphene interface. Wet growth conditions of oligomeric molecular chains would usually lead, during standard CMOS-compatible fabrication processes, to the quenching of spintronics properties of metallic spin sources due to oxidation. We demonstrate by X-ray photoelectron spectroscopy that the use of a protective graphene layer fully preserves the metallic character of the ferromagnetic surface and thus its capability to deliver spin polarized currents. We focus here on a small aromatic chain of controllable lengths, formed by nitrobenzene monomers and derived from the commercial 4-nitrobenzene diazonium tetrafluoroborate, covalently attached to the graphene passivated spin sources thanks to electroreduction. A unique bias dependent switch of the spin signal is then observed in complete spin valve devices, from minority to majority spin carriers filtering. First-principles calculations are used to highlight the key role played by the spin-dependent hybridization of electronic states present at the different interfaces. Our work is a first step towards the exploration of spin transport using different functional molecular chains. It opens the perspective of atomic tailoring of magnetic junction devices towards spin and quantum transport control, thanks to the flexibility of ambient electrochemical surface functionalization processes.

Magnetoresistive effect in a quantum heterostructure with helical spacer: interplay between helicity and external electric field

Giant magnetoresistive effect in a multi-layered structure not only depends on the properties of magnetic systems, it also strongly depends on the type of non-magnetic spacer that is clamped between magnetic layers. In this work, we critically investigate the role of a helical spacer in presence of a transverse electric field. Two kinds of helical geometries, possessing short-range (SRH) and long-range hopping (LRH) of electrons, are taken into account mimicking single-stranded DNA and protein molecules respectively. Sandwiching the magnetic-non-magnetic-magnetic quantum heterostructure between source and drain contact electrodes, we investigate the properties of giant magnetoresistance (GMR) following the Green's function formalism within a tight-binding framework. The interplay between SRHs and LRHs of electrons provides several nontrivial signatures in GMR, especially in the presence of transverse electric field, as it makes the system a deterministic disordered one, similar to the well-known Aubry-Andre-Harper from. The famous gapped nature of energy band structure in presence of cosine modulation leads to high degree of magnetoresistance at multiple Fermi energies, compared to the traditional spacers. The magnetoresistive effect can be monitored selectively by adjusting the electric field strength and its direction. Comparing the results between the SRH and LRH cases, we find that the later one is more superior. Finally, to make the system more realistic we include the effect of dephasing. Our analysis may provide some fundamental aspects of designing electronic and spintronic devices based on magnetoresistive effect.

Construction and physical properties of low-dimensional structures for nanoscale electronic devices

Over the past decades, construction of nanoscale electronic devices with novel functionalities based on low-dimensional structures, such as single molecules and two-dimensional (2D) materials, has been rapidly developed. To investigate their intrinsic properties for versatile functionalities of nanoscale electronic devices, it is crucial to precisely control the structures and understand the physical properties of low-dimensional structures at the single atomic level. In this review, we provide a comprehensive overview of the construction of nanoelectronic devices based on single molecules and 2D materials and the investigation of their physical properties. For single molecules, we focus on the construction of single-molecule devices, such as molecular motors and molecular switches, by precisely controlling their self-assembled structures on metal substrates and charge transport properties. For 2D materials, we emphasize their spin-related electrical transport properties for spintronic device applications and the role that interfaces among 2D semiconductors, contact electrodes, and dielectric substrates play in the electrical performance of electronic, optoelectronic, and memory devices. Finally, we discuss the future research direction in this field, where we can expect a scientific breakthrough.

Easy axis anisotropy creating high contrast magnetic zones on magnetic tunnel junctions based molecular spintronics devices (MTJMSD)

Magnetic tunnel junction-based molecular spintronics device (MTJMSD) may enable novel magnetic metamaterials by chemically bonding magnetic molecules and ferromagnets (FM) with a vast range of magnetic anisotropy. MTJMSD have experimentally shown intriguing microscopic phenomenon such as the development of highly contrasting magnetic phases on a ferromagnetic electrode at room temperature. This paper focuses on Monte Carlo Simulations (MCS) on MTJMSD to understand the potential mechanism and explore fundamental knowledge about the impact of magnetic anisotropy. The selection of MCS is based on our prior study showing the potential of MCS in explaining experimental results (Tyagi et al. in Nanotechnology 26:305602, 2015). In this paper, MCS is carried out on the 3D Heisenberg model of cross-junction-shaped MTJMSDs. Our research represents the experimentally studied cross-junction-shaped MTJMSD where paramagnetic molecules are covalently bonded between two FM electrodes along the exposed side edges of the magnetic tunnel junction (MTJ). We have studied atomistic MTJMSDs properties by simulating a wide range of easy-axis anisotropy for the case of experimentally observed predominant molecule-induced strong antiferromagnetic coupling. Our study focused on understanding the effect of anisotropy of the FM electrodes on the overall MTJMSDs at various temperatures. This study shows that the multiple domains of opposite spins start to appear on an FM electrode as the easy-axis anisotropy increases. Interestingly, MCS results resembled the experimentally observed highly contrasted magnetic zones on the ferromagnetic electrodes of MTJMSD. The magnetic phases with starkly different spins were observed around the molecular junction on the FM electrode with high anisotropy.

Conductance switching of azobenzene-based self-assembled monolayers on cobalt probed by UHV conductive-AFM

We report the formation of self-assembled monolayers of a molecular photoswitch (azobenzene-bithiophene derivative, AzBT) on cobalt via a thiol covalent bond. We study the electrical properties of the molecular junctions formed with the tip of a conductive atomic force microscope under ultra-high vacuum. The statistical analysis of the current-voltage curves shows two distinct states of the molecule conductance, suggesting the coexistence of both the trans and cis azobenzene isomers on the surface. The cis isomer population (trans isomer) increases (decreases) upon UV light irradiation. The situation is reversed under blue light irradiation. The experiments are confronted to first-principle calculations performed on the molecular junctions with the Non-Equilibrium Green's Function formalism combined with Density Functional Theory (NEGF/DFT). The theoretical results consider two different molecular orientations for each isomer. Whereas the orientation does not affect the conductance of the trans isomer, it significantly modulates the conductance of the cis isomer and the resulting conductance ON/OFF ratio of the molecular junction. This helps identifying the molecular orientation at the origin of the observed current differences between the trans and cis forms. The ON state is associated to the trans isomer irrespective of its orientation in the junction, while the OFF state is identified as a cis isomer with its azobenzene moiety folded upward with respect to the bithiophene core. The experimental and calculated ON/OFF conductance ratios have a similar order of magnitude. This conductance ratio seems reasonable to make these Co-AzBT molecular junctions a good test-bed to further explore the relationship between the spin-polarized charge transport, the molecule conformation and the molecule-Co spinterface.

Theoretical Investigation of the Interfaces and Mechanisms of Induced Spin Polarization of 1D Narrow Zigzag Graphene- and h-BN Nanoribbons on a SrO-Terminated LSMO(001) Surface

The structure of the interfaces and the mechanisms of induced spin polarization of 1D infinite and finite narrow graphene- and h-BN zigzag nanoribbons placed on a SrO-terminated La_1-xSr_xMnO₃ (LSMO) (001) surface were studied using density functional theory (DFT) electronic structure calculations. It was found that the π-conjugated nanofragments are bonded to the LSMO(001) surface by weak disperse interactions. The types of coordination of the fragments, the strength of bonding, and the rate of spin polarization depend upon the nature of the fragments. Infinite and finite graphene narrow zigzag nanoribbons are characterized by the lift of the spin degeneracy and strong spin polarization caused by interface-induced structural asymmetry and oxygen-mediated indirect exchange interactions with Mn ions of LSMO support. Spin polarization changes the semiconducting nature of infinite graphene nanoribbons to half-metallic state with visible spin-up density of states at the Fermi level. The h-BN nanoribbon binding energy is weaker than graphene nanoribbon ones with noticeably shorter interlayer distance. The asymmetry effect and indirect exchange interactions cause spin polarization of h-BN nanoribbon as well with formation of embedded states inside the band gap. The results show a possibility to use one-atom thick nanofragments to design LSMO-based heterostructures for spintronic nanodevices with h-BN as an inert spacer to develop different potential barriers.

Large Magnetoresistance at Room Temperature in Organic Molecular Tunnel Junctions with Nonmagnetic Electrodes

We report room-temperature resistance changes of up to 30% under weak magnetic fields (0.1 T) for molecular tunnel junctions composed of oligophenylene thiol molecules, 1-2 nm in length, sandwiched between gold contacts. The magnetoresistance (MR) is independent of field orientation and the length of the molecule; it appears to be an interface effect. Theoretical analysis suggests that the source of the MR is a two-carrier (two-hole) interaction at the interface, resulting in spin coupling between the tunneling hole and a localized hole at the Au/molecule contact. Such coupling leads to significantly different singlet and triplet transmission barriers at the interface. Even weak magnetic fields impede spin relaxation processes and thus modify the ratio of holes tunneling via the singlet state versus the triplet state, which leads to the large MR. Overall, the experiments and analysis suggest significant opportunities to explore large MR effects in molecular tunnel junctions based on widely available molecules.

Self-assembled monolayers based spintronics: from ferromagnetic surface functionalization to spin-dependent transport

Chemically functionalized surfaces are studied for a wide range of applications going from medicine to electronics. Whereas non-magnetic surfaces have been widely studied, functionalization of magnetic surfaces is much less common and has almost never been used for spintronics applications. In this article we present the functionalization of La2/3Sr1/3MnO3, a ferromagnetic oxide, with self-assembled monolayers for spintronics. La2/3Sr1/3MnO3 is the prototypical half-metallic manganite used in spintronics studies. First, we show that La2/3Sr1/3MnO3 can be functionalized by alkylphosphonic acid molecules. We then emphasize the use of these functionalized surfaces in spintronics devices such as magnetic tunnel junctions fabricated using a nano-indentation based lithography technique. The observed exponential increase of tunnel resistance as a function of alkyl chain length is a direct proof of the successful connection of molecules to ferromagnetic electrodes. For all alkyl chains studied we obtain stable and robust tunnel magnetoresistance, with effects ranging from a few tens to 10 000%. These results show that functionalized electrodes can be integrated in spintronics devices and open the door to a molecular engineering of spintronics.

Toward interfacing organic semiconductors with ferromagnetic transition metal substrates: enhanced stability via carboxylate anchoring

The controlled growth of chemically and structurally well-defined as well as thermally stable carboxylate-anchored self-assembled monolayers on ferromagnetic 3d transition metal substrates provides promising model systems for advanced studies of spin-dependent electron transport.

Aromatic molecules as spintronic devices

In this paper, we study the spin-dependent electron transport through aromatic molecular chains attached to two semi-infinite leads. We model this system taking into account different geometrical configurations which are all characterized by a tight binding Hamiltonian. Based on the Green's function approach with a Landauer formalism, we find spin-dependent transport in short aromatic molecules by applying external magnetic fields. Additionally, we find that the magnetoresistance of aromatic molecules can reach different values, which are dependent on the variations in the applied magnetic field, length of the molecules, and the interactions between the contacts and the aromatic molecule.

The first decade of organic spintronics research

The significant milestones in organic spintronics achieved during the first decade of research are reviewed.

Spin-dependent transport properties of a chromium porphyrin-based molecular embedded between two graphene nanoribbon electrodes

By using the nonequilibrium Green's function formalism combined with the density-functional theory, we present a theoretical study of the spin-dependent electron transport of a chromium porphyrin-based molecule device.

Interface effects on tunneling magnetoresistance in organic spintronics with flexible amine-Au links

Organic spintronics is a promising emerging field, but the sign of the tunneling magnetoresistance (TMR) is highly sensitive to interface effects, a crucial hindrance to applications. A key breakthrough in molecular electronics was the discovery of amine-Au link groups that give a reproducible conductance. Using first-principles calculations, we predict that amine-Au links give improved reproducibility in organic spintronics junctions with Au-covered Fe leads. The Au layers allow only states with sp character to tunnel into the molecule, and the flexibility of amine-Au links results in a narrow range of TMR for a fixed number of Au layers. Even as the Au thickness changes, the TMR remains positive as long as the number of Au layers is the same on both sides of the junction. Since the number of Au layers on Fe surfaces or Fe nanoparticles can now be experimentally controlled, amine-Au links provide a route towards robust TMR in organic spintronics.

Mechanically controllable break junctions for molecular electronics

A mechanically controllable break junction (MCBJ) represents a fundamental technique for the investigation of molecular electronic junctions, especially for the study of the electronic properties of single molecules. With unique advantages, the MCBJ technique has provided substantial insight into charge transport processes in molecules. In this review, the techniques for sample fabrication, operation and the various applications of MCBJs are introduced and the history, challenges and future of MCBJs are discussed.

Unveiling self-assembled monolayers' potential for molecular spintronics: spin transport at high voltage

Molecular magnetic tunnel junctions using self-assembled monolayers (SAMs) as tunnel barriers show stable and efficient spin transport properties. Large tunnel magnetoresistance with a flat bias voltage dependence of the magnetoresistance is observed in La(2/3) Sr(1/3) MnO(3) /dodecylphosphonic acid SAM/Co nanocontacts. This opens the door to spintronic tailoring though SAM engineering and could also lead to new venues for spin injection in organic devices.

Self-assembled monolayer-functionalized half-metallic manganite for molecular spintronics

(La,Sr)MnO(3) manganite (LSMO) has emerged as the standard ferromagnetic electrode in organic spintronic devices due to its highly spin-polarized character and air stability. Whereas organic semiconductors and polymers have been mainly envisaged to propagate spin information, self-assembled monolayers (SAMs) have been overlooked and should be considered as promising materials for molecular engineering of spintronic devices. Surprisingly, up to now the first key step of SAM grafting protocols over LSMO surface thin films is still missing. We report the grafting of dodecyl (C12P) and octadecyl (C18P) phosphonic acids over the LSMO half-metallic oxide. Alkylphosphonic acids form ordered self-assembled monolayers, with the phosphonic group coordinated to the surface and alkyl chains tilted from the surface vertical by 43° (C12P) and 27° (C18P). We have electrically characterized these SAMs in nanodevices and found that they act as tunnel barriers, opening the door toward the integration of alkylphosphonic acid//LSMO SAMs into future molecular/organic spintronic devices such as spin OLEDs.

Giant resistance change across the phase transition in spin-crossover molecules

The electronic origin of a large resistance change in nanoscale junctions incorporating spin-crossover molecules is demonstrated theoretically by using a combination of density functional theory and the nonequilibrium Green's function method for quantum transport. At the spin-crossover phase transition, there is a drastic change in the electronic gap between the frontier molecular orbitals. As a consequence, when the molecule is incorporated in a two-terminal device, the current increases by up to 4 orders of magnitude in response to the spin change. This is equivalent to a magnetoresistance effect in excess of 3000%. Since the typical phase transition critical temperature for spin-crossover compounds can be extended to well above room temperature, spin-crossover molecules appear as the ideal candidate for implementing spin devices at the molecular level.

Room-temperature tunnel magnetoresistance in self-assembled chemically synthesized metallic iron nanoparticles

We report on room temperature magnetoresistance in networks of chemically synthesized metallic Fe nanoparticles surrounded by two types of organic barriers. Electrical properties, featuring Coulomb blockade, and magnetotransport measurements show that this magnetoresistance arises from spin-dependent tunnelling, so the organic ligands stabilizing the nanoparticles are efficient spin-conservative tunnel barrier. These results demonstrate the feasibility of an all-chemistry approach for room temperature spintronics.

Organic spintronics

Organic semiconductors are emerging materials in the field of spintronics. Successful achievements include their use as a tunnel barrier in magnetoresistive tunnelling devices and as a medium for spin-polarized current in transport devices. In this paper, we give an overview of the basic concepts of spin transport in organic semiconductors and present the results obtained in the field, highlighting the open questions that have to be addressed in order to improve devices performance and reproducibility. The most challenging perspectives will be discussed and a possible evolution of organic spin devices featuring multi-functional operation is presented.

Spin polarized electron tunneling and magnetoresistance in molecular junctions

This chapter reviews tunneling of spin-polarized electrons through molecules positioned between ferromagnetic electrodes, which gives rise to tunneling magnetoresistance. Such measurements yield important insight into the factors governing spin-polarized electron injection into organic semiconductors, thereby offering the possibility to manipulate the quantum-mechanical spin degrees of freedom for charge carriers in optical/electrical devices. In the first section of the chapter a brief description of the Jullière model of spin-dependent electron tunneling is reviewed. Next, a brief description of device fabrication and characterization is presented. The bulk of the review highlights experimental studies on spin-polarized electron tunneling and magnetoresistance in molecular junctions. In addition, some experiments describing spin-polarized scanning tunneling microscopy/spectroscopy on single molecules are mentioned. Finally, some general conclusions and prospectus on the impact of spin-polarized tunneling in molecular junctions are offered.

A core-shell strategy for constructing a single-molecule junction

Understanding the effects of intermolecular interactions on the charge-transport properties of metal/molecule/metal junctions is an important step towards using individual molecules as building blocks for electronic devices. This work reports a systematic electron-transport investigation on a series of "core-shell"-structured oligo(phenylene ethynylene) (Gn-OPE) molecular wires. By using dendrimers of different generations as insulating "shells", the intermolecular π-π interactions between the OPE "cores" can be precisely controlled in single-component monolayers. Three techniques are used to evaluate the electron-transport properties of the Au/Gn-OPE/Au molecular junctions, including crossed-wire junction, scanning tunneling spectroscopy (STS), and scanning tunneling microscope (STM) break-junction techniques. The STM break-junction measurement reveals that the electron-transport pathways are strongly affected by the size of the side groups. When the side groups are small, electron transport could occur through three pathways, including through single-molecule junctions, double-molecule junctions, and molecular bridges between adjacent molecules formed by aromatic π-π coupling. The dendrimer shells effectively prohibit the π-π coupling effect, but at the same time, very large dendrimer side groups may hinder the formation of Au-S bonds. A first-generation dendrimer acts as an optimal shell that only allows electron transport through the single-molecule junction pathway, and forbids the other undesired pathways. It is demonstrated that the dendrimer-based core-shell strategy allows the single-molecule conductance to be probed in a homogenous monolayer without the influence of intermolecular π-π interactions.

Tunneling magnetoresistance with sign inversion in junctions based on iron oxide nanocrystal superlattices

Magnetic tunnel junctions sandwiching a superlattice thin film of iron oxide nanocrystals (NCs) have been investigated. The transport was found to be controlled by Coulomb blockade and single-electron tunneling, already at room temperature. A good correlation was identified to hold between the tunnel magnetoresistance (TMR), the expected magnetic properties of the NC arrays, the charging energies evaluated from current-voltage curves, and the temperature dependence of the junction resistance. Notably, for the first time, a switching from negative to positive TMR was observed across the Verwey transition, with a strong enhancement of TMR at low temperatures.

Molecular photovoltaics in nanoscale dimension

This review focuses on the intrinsic charge transport in organic photovoltaic (PVC) devices and field-effect transistors (SAM-OFETs) fabricated by vapor phase molecular self-assembly (VP-SAM) method. The dynamics of charge transport are determined and used to clarify a transport mechanism. The 1,4,5,8-naphthalene-tetracarboxylic diphenylimide (NTCDI) SAM devices provide a useful tool to study the fundamentals of polaronic transport at organic surfaces and to discuss the performance of organic photovoltaic devices in nanoscale. Time-resolved photovoltaic studies allow us to separate the charge annihilation kinetics in the conductive NTCDI channel from the overall charge kinetic in a SAM-OFET device. It has been demonstrated that tuning of the type of conductivity in NTCDI SAM-OFET devices is possible by changing Si substrate doping. Our study of the polaron charge transfer in organic materials proposes that a cation-radical exchange (redox) mechanism is the major transport mechanism in the studied SAM-PVC devices. The role and contribution of the transport through delocalized states of redox active surface molecular aggregates of NTCDI are exposed and investigated. This example of technological development is used to highlight the significance of future technological development of nanotechnologies and to appreciate a structure-property paradigm in organic nanostructures.

Noise characteristics of charge tunneling via localized states in metal--molecule--metal junctions

We report the noise characteristics of charge transport through an alkyl-based metal--molecule--metal junction. Measurements of the 1/f noise, random telegraph noise, and shot noise demonstrated the existence of localized traps in the molecular junctions. These three noise measurements exhibited results consistent with trap-mediated tunneling activated over approximately 0.4 V by trapping and detrapping processes via localized states (or defects). The noise characterizations will be useful in evaluating the influences of localized states on charge transport in molecular or other electronic junctions.

Impact on interface spin polarization of molecular bonding to metallic surfaces

We have studied the repercussion of the molecular adsorption mechanism on the electronic properties of the interface between model nonmagnetic or magnetic metallic surfaces and metallo-organic phthalocyanines molecules (Pcs). Our intertwined x-ray absorption spectroscopy experiments and computational studies reveal that manganese Pc (MnPc) is physisorbed onto a Cu(001) surface and retains the electronic properties of a free molecule. On the other hand, MnPc is chemisorbed onto Co(001), leading to a dominant direct exchange interaction between the Mn molecular site and the Co substrate. By promoting an interfacial spin-polarized conduction state on the molecule, these interactions reveal an important lever to tailor the spintronic properties of hybrid organic-metallic interfaces.

Transverse electric field modulated tunneling magnetoresistance in a DNA molecular device

Quantum spin-dependent transport in a ferromagnetic(FM)/DNA/ferromagnetic(FM) device is theoretically investigated based on the lattice Green function method and the Landauer-Büttiker theory. The effect of a transverse electric field on magnetoresistance (MR) of the device is investigated. It is predicted that either the direction or strength of the transverse electric field can change the MR of the device. We suggest a possible application of modulating MR of the FM/DNA/FM device by a transverse electric field.

Spin routes in organic semiconductors

Organic semiconductors are characterized by a very low spin-orbit interaction, which, together with their chemical flexibility and relatively low production costs, makes them an ideal materials system for spintronics applications. The first experiments on spin injection and transport occurred only a few years ago, and since then considerable progress has been made in improving performance as well as in understanding the mechanisms affecting spin-related phenomena. Nevertheless, several challenges remain in both device performance and fundamental understanding before organic semiconductors can compete with inorganic semiconductors or metals in the development of realistic spintronics applications. In this article we summarize the main experimental results and their connections with devices such as light-emitting diodes and electronic memory devices, and we outline the scientific and technological issues that make organic spintronics a young but exciting field.

Theory for spin diffusion in disordered organic semiconductors

We present a theory for spin diffusion in disordered organic semiconductors, based on incoherent hopping of a charge carrier and coherent precession of its spin in an effective magnetic field, composed of the random hyperfine field of hydrogen nuclei and an applied magnetic field. From Monte Carlo simulations and an analysis of the waiting-time distribution of the carrier we predict a surprisingly weak temperature dependence, but a considerable magnetic-field dependence of the spin-diffusion length. We show that both predictions are in agreement with experiments on organic spin valves.