Emerging Spintronic Materials and Functionalities

The explosive growth of the information era has put forward urgent requirements for ultrahigh-speed and extremely efficient computations. In direct contrary to charge-based computations, spintronics aims to use spins as information carriers for data storage, transmission, and decoding, to help fully realize electronic device miniaturization and high integration for next-generation computing technologies. Currently, many novel spintronic materials have been developed with unique properties and multifunctionalities, including organic semiconductors (OSCs), organic-inorganic hybrid perovskites (OIHPs), and 2D materials (2DMs). These materials are useful to fulfill the demand for developing diverse and advanced spintronic devices. Herein, these promising materials are systematically reviewed for advanced spintronic applications. Due to the distinct chemical and physical structures of OSCs, OIHPs, and 2DMs, their spintronic properties (spin transport and spin manipulation) are discussed separately. In addition, some multifunctionalities due to photoelectric and chiral-induced spin selectivity (CISS) are overviewed, including the spin-filter effect, spin-photovoltaics, spin-light emitting devices, and spin-transistor functions. Subsequently, challenges and future perspectives of using these multifunctional materials for the development of advanced spintronics are presented.

Structural Isomeric Effect on Spin Transport in Molecular Semiconductors

Molecular semiconductor (MSC) is a promising candidate for spintronic applications benefiting from its long spin lifetime caused by light elemental-composition essence and thus weak spin-orbit coupling (SOC). According to current knowledge, the SOC effect, normally dominated by the elemental composition, is the main spin-relaxation causation in MSCs, and thus the molecular structure-induced SOC change is one of the most concerned issues. In theoretical study, molecular isomerism, a most prototype phenomenon, has long been considered to possess little difference on spin transport previously, since elemental compositions of isomers are totally the same. However, here in this study, quite different spin-transport performances are demonstrated in ITIC and its structural isomers BDTIC experimentally, for the first time, though the charge transport and molecular stacking of the two films are very similar. By further experiments of electron-paramagnetic resonance and density-functional-theory calculations, it is revealed that noncovalent-conformational locks (NCLs) formed in BDTIC can lead to enhancement of SOC and thus decrease the spin lifetime. Hence, this study suggests the influences from the structural-isomeric effect must be considered for developing highly efficient spin-transport MSCs, which also provides a reliable theoretical basis for solving the great challenge of quantificational measurement of NCLs in films in the future.

Strategies and applications of generating spin polarization in organic semiconductors

The advent of spintronics has undoubtedly revolutionized data storage, processing, and sensing applications. Organic semiconductors (OSCs), characterized by long spin relaxation times (>μs) and abundant spin-dependent properties, have emerged as promising materials for advanced spintronic applications. To successfully implement spin-related functions in organic spintronic devices, the four fundamental processes of spin generation, transport, manipulation, and detection form the main building blocks and are commonly in demand. Thereinto, the effective generation of spin polarization in OSCs is a precondition, but in practice, this has not been an easy task. In this context, considerable efforts have been made on this topic, covering novel materials systems, spin-dependent theories, and device fabrication technologies. In this review, we underline recent advances in external spin injection and organic property-induced spin polarization, according to the distinction between the sources of spin polarization. We focused mainly on summarizing and discussing both the physical mechanism and representative research on spin generation in OSCs, especially for various spin injection methods, organic magnetic materials, the chiral-induced spin selectivity effect, and the spinterface effect. Finally, the challenges and prospects that allow this topic to continue to be dynamic were outlined.

Dynamical Behavior of Pure Spin Current in Organic Materials

Growing concentration on the novel information processing technology and low-cost, flexible materials make the spintronics and organic materials appealing for the future interdisciplinary investigations. Organic spintronics, in this context, has arisen and witnessed great advances during the past two decades owing to the continuous innovative exploitation of the charge-contained spin polarized current. Albeit with such inspiring facts, charge-absent spin angular momentum flow, namely pure spin currents (PSCs) are less probed in organic functional solids. In this review, the past exploring journey of PSC phenomenon in organic materials are retrospected, including non-magnetic semiconductors and molecular magnets. Starting with the basic concepts and the generation mechanism for PSC, the representative experimental observations of PSC in the organic-based networks are subsequently demonstrated and summarized, by accompanying explicit discussion over the propagating mechanism of net spin itself in the organic media. Finally, future perspectives on PSC in organic materials are illustrated mainly from the material point of view, including single molecule magnets, complexes for the organic ligands framework as well as the lanthanide metal complexes, organic radicals, and the emerging 2D organic magnets.

Degradation Analysis of Organic Light-Emitting Diodes through Dispersive Magneto-Electroluminescence Response

Understanding the stability and degradation of organic light-emitting diodes (OLEDs) under working conditions is a significant area of research for developing more effective OLEDs and further improving their performance. However, studies of degradation processes by in situ noninvasive methods have not been adequately developed. In this work, tris-(8-hydroxyquinolino) aluminum (Alq3)-based OLED degradation processes have been analyzed through the investigation of the device dispersive magneto-electroluminescence (MEL(B)) response measured at room temperature. By studying the change in the MEL(B) response during the device degradation under different external stimuli, such as exposing the device to the atmosphere and prolonged illumination by a strong visible light source, we have gained insight into the microscopic spin-dependent phenomena that control the recombination of e-h polaron pairs in the device. We found that the device degradation leads to a shorter e-h polaron lifetime, smaller dispersive parameter, and broader lifetime distribution function that shows increased disorder in the active layer. This study could offer a potential tool that may be beneficial for assessing the degradation of OLED devices based on various active layers.

Multimodal Reactivity of N-H Bonds in Triazanes and Isolation of a Triazinyl Radical

The employment of nitrogen Lewis acids based on nitrenium cations has been increasingly featured in the fields of main group chemistry and catalysis. A formally reduced form of nitrenium D─cyclic triazanes E─are intriguing chemical compounds, the chemistry of which is completely unexplored. In this work, we reveal that N-H-triazanes exhibit unusual N-H bond properties; namely, they can serve as protons, hydrides, or hydrogen atom donors. This unique multimodal reactivity provides an N-cation, N-anion, or N-radical from the same species. It allowed us to isolate, for the first time, a stable naphto[1,2,3]triazinyl radical, which was fully characterized both computationally and experimentally, including its monomeric X-ray structure. Moreover, this radical can be prepared directly from the nitrenium cation by a single electron reduction (E = -0.46 V), and this process is reversible. We envision versatile uses of this radical in synthetic and materials chemistry.

Room-temperature magnetoresistance in Ni78Fe22/C8-BTBT/Ni78Fe22 nanojunctions fabricated from magnetic thin-film edges using a novel technique

Molecular spintronic devices are gaining popularity because the organic semiconductors with long spin relaxation times are expected to have long spin diffusion lengths. A typical molecular spintronic device consists of organic molecules sandwiched between two magnetic layers, which exhibits magnetoresistance (MR) effect. Nanosized devices are also expected to have a high spin polarization, leading to a large MR effect owing to effective orbital hybridization. However, most studies on nanosized molecular spintronic devices have investigated the MR effect at low temperatures because of the difficulty in observing the MR effect at room temperature. Here we focus on high-mobility molecules expected to show long spin diffusion lengths, which lead to the observation of the MR effect in nanoscale junctions at room temperature. In this study, we fabricate magnetic nanojunctions consisting of high-mobility molecules, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), sandwiched between two Ni₇₈Fe₂₂ thin films with crossed edges. Transmission electron microscopy (TEM) images reveal that C8-BTBT molecular layers with smooth and clear interfaces can be deposited on the Ni₇₈Fe₂₂ thin-film edges. Consequently, we observe a clear positive MR effect, that is, R _P < R _AP, where R _P and R _AP are the resistances in the parallel (P) and antiparallel (AP) configurations, respectively, of two magnetic electrodes in the Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions at room temperature. The obtained results indicate that the spin signal through the C8-BTBT molecules can be successfully observed. The study presented herein provides a novel nanofabrication technique and opens up new opportunities for research in high-mobility molecular nano-spintronics.

Enhanced Electron Transfer and Spin Flip through Spin-Orbital Couplings in Organic/Inorganic Heterojunctions: A Nonadiabatic Surface Hopping Simulation

The circumstances of transferred electrons across organic/inorganic interfaces have attracted intensive interest because of the distinctive electronic structure properties of those two components. Leveraging ab initio nonadiabatic molecular dynamics methods in conjunction with spin dynamics induced by spin-orbital couplings (SOCs), this study reports two competitive channels during photoinduced dynamical processes in the prototypical ZnPc/monolayer MoS₂ heterojunction. Interestingly, the electron-transfer and relaxation processes occur simultaneously because of the enhancement of electron-phonon couplings and expansion of dynamical pathways by SOCs, suggesting that the electron-transfer rate and relaxation processes can be tuned by SOCs, hence yielding the performance promotion of photovoltaic and photocatalytic devices. Additionally, approximately half of the transferred electrons flip their spin within 1.6 ps because of strong SOCs in MoS₂, achieving great agreement with experimental measurements. This investigation provides instructive perspectives for designing novel devices and applications based on organic/inorganic heterojunctions, demonstrating the importance of spin dynamics simulations in exploring sophisticated photoinduced processes in materials.

Enhanced Magnetic Order and Reversed Magnetization Induced by Strong Antiferromagnetic Coupling at Hybrid Ferromagnetic-Organic Heterojunctions

Organic-molecular magnets based on a metal-organic framework with chemically tuned electronic and magnetic properties have been attracting tremendous attention due to their promising applications in molecular magnetic sensors, magnetic particle medicines, molecular spintronics, etc. Here, we investigated the magnetic behavior of a heterojunction comprising a ferromagnetic nickel (Ni) film and an organic semiconductor (OSC) 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) layer. Through the magneto-optical Kerr effect (MOKE), a photoemission electron microscopy (PEEM), X-ray magnetic circular dichroism (XMCD), and X-ray photoelectron spectroscopy (XPS), we found that the adsorption of F4-TCNQ on Cu(100)/Ni not only reverses the in-plane magnetization direction originally exhibited by the Ni layer but also results in enhanced magnetic ordering. Furthermore, the cyano group (CN) in adsorbed F4-TCNQ was found spin-polarized along with conspicuous charge transfer with Ni. The density functional theory (DFT) calculations suggest that the experimentally found spin polarization originates from hybridization between the CN group's π orbitals and Ni's d band. These findings signify that the hybrid states at the organic-ferromagnet interface play a key role in tailoring the magnetic behavior of interfaces. For the case of the F4-TCNQ and Ni heterojunction reported here, interface coupling is an antiferromagnetic one.

Resolving the Spatial Variation and Correlation of Hyperfine Spin Properties in Organic Light-Emitting Diodes

Devices that exploit the quantum properties of materials are widespread, with quantum information processors and quantum sensors showing significant progress. Organic materials offer interesting opportunities for quantum technologies owing to their engineerable spin properties, with spintronic operation and spin resonance magnetic-field sensing demonstrated in research grade devices, as well as proven compatibility with large-scale fabrication techniques. Yet several important challenges remain as moving toward scaling these proof-of-principle quantum devices to larger integrated logic systems or spatially smaller sensing elements, particularly those associated with the variation of quantum properties both within and between devices. Here, spatially resolved magnetoluminescence is used to provide the first 2D map of a hyperfine spin property-the Overhauser field-in traditional organic light-emitting diodes (OLEDs). Intra-device variabilities are found to exceed ≈30% while spatially correlated behavior is exhibited on lengths beyond 7 µm, similar in size to pixels in state-of-the-art active-matrix OLED arrays, which has implications for the reproducibility and integration of organic quantum devices.

Metal-containing organic compounds for memory and data storage applications

With the upcoming trend of Big Data era, some new types of memory technologies have emerged as substitutes for the traditional Si-based semiconductor memory devices, which are encountering severe scaling down technical obstacles. In particular, the resistance random access memory (RRAM) and magnetic random access memory (MRAM) hold great promise for the in-memory computing, which are regarded as the optimal strategy and pathway to solve the von Neumann bottleneck by high-throughput in situ data processing. As far as the active materials in RRAM and MRAM are concerned, organic semiconducting materials have shown increasing application perspectives in memory devices due to their rich structural diversity and solution processability. With the introduction of metal elements into the backbone of molecules, some new properties and phenomena will emerge accordingly. Consequently, the RRAM and MRAM devices based on metal-containing organic compounds (including the small molecular metal complexes, metallopolymers, metal-organic frameworks (MOFs) and organic-inorganic-hybrid perovskites (OIHPs)) have been widely explored and attracted intense attention. In this review, we highlight the fundamentals of RRAM and MRAM, as well as the research progress of the applications of metal-containing organic compounds in both RRAM and MRAM. Finally, we discuss the challenges and future directions for the research of organic RRAM and MRAM.

Metal Organic Spin Transistor

Organic molecules and specifically bio-organic systems are attractive for applications due to their low cost, variability, environmental friendliness, and facile manufacturing in a bottom-up fashion. However, due to their relatively low conductivity, their actual application is very limited. Chiral metallo-bio-organic crystals, on the other hand, have improved conduction and in addition interesting magnetic properties. We developed a spin transistor using these crystals and based on the chiral-induced spin selectivity effect. This device features a memristor type behavior, which depend on trapping both charges and spins. The spin properties are monitored by Hall signal and by an external magnetic field. The spin transistor exhibits nonlinear drain-source currents, with multilevel controlled states generated by the magnetization of the source. Varying the source magnetization enables a six-level readout for the two-terminal device. The simplicity of the device paves the way for its technological application in organic electronics and bioelectronics.

Probing an Isolated Conjugated Polymer Molecule with Radiation-Generated Spin-Correlated Polaron Pairs

An understanding of the interplay between the spin and electronic degrees of freedom of polarons migrating along conjugated polymer molecules is required to further the development of organic electronics and spintronics. In this study, a novel experimental approach is proposed for studying spin-correlated polaron pairs (PPs) on an isolated molecule of a conjugated polymer. The polymer molecule of interest is immobilized in a nonluminescent poly(vinyl chloride) matrix, which is irradiated with X-rays to rapidly form secondary PPs on the conjugated polymer. The migration, recombination, and evolution of the spin state of the PPs can be monitored at nanosecond resolution by observing the recombination fluorescence under different magnetic fields. Examples supporting this concept are presented.

Rubrene Thin Films with Viably Enhanced Charge Transport Fabricated by Cryo-Matrix-Assisted Laser Evaporation

Among organic semiconductors, rubrene (RB; C₄₂H₂₈) is of rapidly growing interest for the development of organic and hybrid electronics due to exceptionally long spin diffusion length and carrier mobility up to 20 cm²V⁻¹s⁻¹ in single crystals. However, the fabrication of RB thin films resembling properties of the bulk remains challenging, mainly because of the RB molecule’s twisted conformation. This hinders the formation of orthorhombic crystals with strong π–π interactions that support the band transport. In this work, RB films with a high crystalline content were fabricated by matrix-assisted laser evaporation and the associated structure, composition, and transport properties are investigated. Enhanced charge transport is ascribed to the crystalline content of the film. Spherulitic structures are observed on top of an amorphous RB layer formed in the initial deposition stage. In spherulites, orthorhombic crystals dominate, as confirmed by X-ray diffraction and the absorption and Raman spectra. Surprisingly, nanowires several microns in length are also detected. The desorption/ionization mass and X-ray photoelectron spectra consistently show minimal material decomposition and absence of RB peroxides. The observed carrier mobility up to 0.13 cm²V⁻¹s⁻¹, is close to the technologically accepted level, making these rubrene films attractive for spintronic and optoelectronic applications.

Ferromagnetism above Room Temperature in a Ni-Doped Organic-Based Magnetic Semiconductor

Ferromagnetic semiconductors with structural flexibility are an indispensable feature for future flexible spin-electronic applications. In this case, we introduce magnetic ingredients into an organic semiconductor, namely, pentacene, to form a ferromagnetic organic semiconductor (FOS). The first observation for ferromagnetic Ni-doped pentacene semiconductors at room temperature in the field of semiconductor spintronics is reported in this article. To date, the mechanism of FOSs with ferromagnetism is not understood yet, especially when their Curie temperature is enhanced above room temperature. Here, we demonstrate dopants of Ni atoms and the modulation of the growth temperature in the FOS films to achieve room-temperature ferromagnetic properties in a series of FOS films, one of which has a maximum coercivity of 257.6 Oe. The spin-exchange interaction between a Ni atom and a pentacene molecule is detected through the magnetic hysteresis obtained using a superconducting quantum interference device magnetometer. We verify the effectiveness of this spin coupling through magnetic force microscopy, Raman spectroscopy, scanning Kelvin probe microscopy, and theoretical simulation. A model for the indirect spin coupling between Ni atoms is proposed for the mechanism of room-temperature ferromagnetic ordering of spins due to the exchange force indirectly. We believe that the π-electrons of pentacene molecules at the triple state for this model can support the spin coupling of electrons of Ni atoms. Our findings facilitate the development of brand-new spintronic devices with structural flexibility and room-temperature ferromagnetism.

Magnetism at the interface of non-magnetic Cu and C60

The signature of magnetism without a ferromagnet in a non-magnetic heterostructure is novel as well as fascinating from a fundamental research point of view. It has been shown by Al'Mari et al. that magnetism can be induced at the interface of Cu/C₆₀ due to a change in the density of states. However, the quantification of such an interfacial magnetic moment has not been performed yet. In order to quantify the induced magnetic moment in Cu, we have performed X-ray magnetic circular dichroism (XMCD) measurements on Cu/C₆₀ multilayers. We have observed room temperature ferromagnetism in the Cu/C₆₀ stack. Further XMCD measurements show that a ∼0.01 μ_B per atom magnetic moment has been induced in Cu at the Cu/C₆₀ interface.

Spin relaxation in radical pairs from the stochastic Schrödinger equation

We show that the stochastic Schrödinger equation (SSE) provides an ideal way to simulate the quantum mechanical spin dynamics of radical pairs. Electron spin relaxation effects arising from fluctuations in the spin Hamiltonian are straightforward to include in this approach, and their treatment can be combined with a highly efficient stochastic evaluation of the trace over nuclear spin states that is required to compute experimental observables. These features are illustrated in example applications to a flavin-tryptophan radical pair of interest in avian magnetoreception and to a problem involving spin-selective radical pair recombination along a molecular wire. In the first of these examples, the SSE is shown to be both more efficient and more widely applicable than a recent stochastic implementation of the Lindblad equation, which only provides a valid treatment of relaxation in the extreme-narrowing limit. In the second, the exact SSE results are used to assess the accuracy of a recently proposed combination of Nakajima-Zwanzig theory for the spin relaxation and Schulten-Wolynes theory for the spin dynamics, which is applicable to radical pairs with many more nuclear spins. We also analyze the efficiency of trace sampling in some detail, highlighting the particular advantages of sampling with SU(N) coherent states.

Chirality-Induced Spin Coherence in Electron Transfer Reactions

Recently, there has been much interest in the chirality-induced spin selectivity effect, whereby electron spin polarization, which is dependent on molecular chirality, is produced in electrode-molecule electron transfer processes. Naturally, one might consider if a similar effect can be observed in simple molecular charge transfer reactions, for example, in light-induced electron transfer from an electron donor to an electron acceptor. In this work, I explore the effect of electron transfer on spins in chiral single radicals and chiral radical pairs using Nakajima-Zwanzig theory. In these cases, chirality, in conjuction with spin-orbit coupling, does not lead to spin polarization, but instead, the electron transfer generates quantum coherence between spins states. In principle, this chirality-induced spin coherence could manifest in a range of experiments, and in particular, I demonstrate that the out of phase electron spin echo envelope modulation pulse electron paramagnetic resonance experiment would be able to detect this effect in oriented radical pairs.

Slow magnetic relaxation and luminescence properties in neodymium(iii)-4,4,4-trifluoro-1-(2-naphthyl)butane-1,3-dionato complexes incorporating bipyridyl ligands

A new series of eight-coordinated Nd(iii) complexes derived from 4,4,4-trifluoro-1-(naphthalen-2-yl)butane-1,3-dionate and bi-pyridyl ligands revealed multifunctional molecular materials as photoluminescent single-molecule magnets (SMMs).

Two-dimensional conjugated metal–organic frameworks (2D c-MOFs): chemistry and function for MOFtronics

Two-dimensional conjugated MOFs are emerging for multifunctional electronic devices that brings us “MOFtronics”, such as (opto)electronics, spintronics, energy devices.

The Application of Organic Semiconductor Materials in Spintronics

π-Conjugated semiconductors, primarily composed of elements with low atomic number, are regarded as promising spin-transport materials due to the weak spin-orbit coupling interaction and hence long spin relaxation time. Moreover, a large number of additional functions of organic semiconductors (OSCs), such as the abundant photo-electric properties, flexibility, and tailorability, endow the organic spintronic devices more unique properties and functionalities. Particularly, the integration of the photo-electric functionality and excellent spin transport property of OSCs in a single spintronic device has even shown great potential for the realization of spin manipulation in OSCs. In this review, the application of OSCs in spintronic study will be succinctly discussed. As the most important and extensive application, the long-distance spin transport property of OSCs will be discussed first. Subsequently, several multifunctional spintronic devices based on OSCs will be summarized. After that, the organic-based magnets used for the electrodes of spintronic devices will be introduced. Finally, according to the latest progress, spin manipulation in OSCs via novel spintronic devices together with other prospects and challenges will be outlined.

Spin-Orbit Interactions in Ultrafast Molecular Processes

We investigate spin-orbit interactions in ultrafast molecular processes employing the exact factorization of the electron-nuclear wave function. We revisit the original derivation by including spin-orbit coupling, and show how the dynamics driven by the time-dependent potential energy surface alleviates inconsistencies arising from different electronic representations. We propose a novel trajectory-based scheme to simulate spin-forbidden non-radiative processes, and we show its performance in the treatment of excited-state dynamics where spin-orbit effects couple different spin multiplets.

Interface hybridization and spin filter effect in metal-free phthalocyanine spin valves

Spin–orbit coupling has been regarded as the core interaction to determine the efficiency of spin conserved transport in semiconductor spintronics. Here, we show the spin filter effect should be responsible for the magnetoresistance of H₂Pc device.

Recent Advances in Molecular Spintronics: Multifunctional Spintronic Devices

The field of spintronics has triggered an enormous revolution in information storage since the first observation of giant magnetoresistance (GMR). Molecular semiconductors are characterized by having very long spin relaxation times up to milliseconds, and are thus widely considered to hold immense potential for spintronic applications. Along with the development of molecular spintronics, it is clear that the study of multipurpose spintronic devices has gradually grown into a new research and development direction. The abundant photoelectric properties of molecular semiconductors and the intriguing functionality of the spinterface, together with novel designs of device structures, have promoted the integration of multiple functions and different mechanisms into discrete spintronic devices. Here, according to the different relationships between the integrated mechanisms, multifunctional molecular spintronic devices containing parallel and interactive types are highlighted. This is followed by the introduction of pure-spin-current-type molecular spintronic devices that have already demonstrated great potential for multifunction exploration. Finally, the challenges and outlook that make this field young and energetic are outlined.

Suppressing Photoinduced Charge Recombination via the Lorentz Force in a Photocatalytic System

Suppressing the recombination of photogenerated charges is one of the most important routes for enhancing the catalytic performance of semiconductor photocatalysts. In addition to the built-in field produced by semiconductor heterostructures and the photo-electrocatalysis realized by applying an external electrical potential to photocatalysts assembled on electrodes, other strategies are waiting to be scientifically explored and understood. In this work, a Lorentz force-assisted charge carrier separation enhancement strategy is reported to improve the photocatalytic efficiency by applying a magnetic field to a photocatalytic system. The photocatalytic efficiency can be improved by 26% just by placing a permanent magnet beneath the normal photocatalytic system without any additional power supply. The mechanism by which the Lorentz force acts oppositely on the photogenerated electrons and holes is introduced, resulting in the suppression of the photoinduced charge recombination. This work provides insights into the specific role of the Lorentz force in suppressing the recombination of electron-hole pairs in their initial photogenerated states. This suppression would increase the population of charge carriers that would subsequently be transported in the semiconductor. It is believed that this strategy based on magnetic effects will initiate a new way of thinking about photoinduced charge separation.

Photo-induced spin switching in a modified anthraquinone modulated by DNA binding

An anthraquinone modified with a nitroxide radical and able to intercalate into DNA has been synthesized to obtain a molecule the spin state of which can be manipulated by visible light and DNA binding. The doublet ground state of the molecule can be photo-switched to either a strongly coupled spin state (quartet + doublet), when isolated, or to an uncoupled spin state (triplet and doublet), when bound to DNA. The different spin state that is obtained upon photoexcitation depends on the intercalation of the quinonic core into double-stranded DNA which changes the conformation of the molecule, thereby altering the exchange interaction between the excited state localized on the quinonic core and the nitroxide radical. The spin state of the system has been investigated using both continuous-wave and time-resolved EPR spectroscopy.

Tuning Charge Carrier and Spin Transport Properties via Structural Modification of Polymer Semiconductors

Targeted design of organic semiconductors in organic spintronics is relatively limited. Therefore, four conjugated polymers with analogous structures based on isoindigo (IID) units were designed and synthesized to investigate the structure-property relationships in spin and charge carrier transport. Structural design strategies include introduction of pyridinic nitrogen atoms into IID units to change electronic structures and alteration of different branching points of alkyl chains to adjust the aggregation structure. By fabricating polymer field-effect transistors (PFETs) and organic spin valves (OSVs), all of the polymers exhibited good ambipolar field-effect properties (all of the mobilities exceeding 0.3 cm² V^-1 s^-1) and relatively high magnetoresistance (MR) values (maximum up to 25%). Most importantly, it is found that the introduction of pyridinic nitrogen into the IID units can improve MR values of OSVs and electron mobilities of PFETs, whereas the extension of alkyl chain branching points can reduce MR values of the conjugated polymers. This work is the first attempt to thoroughly study the structure-property relationship in the OSVs, combined with molecular design of the conjugated polymers, which provides a guideline for molecular engineering, especially for organic spintronics.

Ultrafast long-range spin-funneling in solution-processed Ruddlesden-Popper halide perovskites

Room-temperature spin-based electronics is the vision of spintronics. Presently, there are few suitable material systems. Herein, we reveal that solution-processed mixed-phase Ruddlesden-Popper perovskite thin-films transcend the challenges of phonon momentum-scattering that limits spin-transfer in conventional semiconductors. This highly disordered system exhibits a remarkable efficient ultrafast funneling of photoexcited spin-polarized excitons from two-dimensional (2D) to three-dimensional (3D) phases at room temperature. We attribute this efficient exciton relaxation pathway towards the lower energy states to originate from the energy transfer mediated by intermediate states. This process bypasses the omnipresent phonon momentum-scattering in typical semiconductors with stringent band dispersion, which causes the loss of spin information during thermalization. Film engineering using graded 2D/3D perovskites allows unidirectional out-of-plane spin-funneling over a thickness of ~600 nm. Our findings reveal an intriguing family of solution-processed perovskites with extraordinary spin-preserving energy transport properties that could reinvigorate the concepts of spin-information transfer.

Spin Transport in Organic Molecules

Because of the considerable advantages of functional molecules as well as supramolecules, such as the low cost, light weight, flexibility, and large area preparation via the solution method, molecular electronics has grown into an active and rapidly developing research field over the past few decades. Beyond those well-known advantages, a very long spin relaxation time of π-conjugated molecules, due to the weak spin-orbit coupling, facilitates a pioneering but fast-growing research field, known as molecular spintronics. Recently, a series of sustained progresses have been achieved with various π-conjugated molecular matrixes where spin transport is undoubtedly an important point for the spin physical process and multifunctional applications. Currently, most studies on spin transport are carried out with a molecule-based spin valve, which shows a typical geometry with a thin-film molecular layer sandwiched between two ferromagnetic electrodes. In such a device, the spin transport process has been demonstrated to have a close correlation with spin relaxation time and charge carrier mobility of π-conjugated molecules. In this review, the recent advances of spin transport in these two aspects have been systematically summarized. Particularly, spin transport in π-conjugated molecular materials, considered as promising for spintronics development, have also been highlighted, including molecular single crystal, cocrystal, solid solution as well as other highly ordered supramolecular structures.

Electric Field-Controlled Magnetization in GaAs/AlGaAs Heterostructures-Chiral Organic Molecules Hybrids

We study GaAs/AlGaAs devices hosting a two-dimensional electron gas and coated with a monolayer of chiral organic molecules. We observe clear signatures of room-temperature magnetism, which is induced in these systems by applying a gate voltage. We explain this phenomenon as a consequence of the spin-polarized charges that are injected into the semiconductor through the chiral molecules. The orientation of the magnetic moment can be manipulated by low gate voltages, with a switching rate in the megahertz range. Thus, our devices implement an efficient, electric field-controlled magnetization, which has long been desired for their technical prospects.

Enhancing Light Emission in Interface Engineered Spin-OLEDs through Spin-Polarized Injection at High Voltages

The quest for a spin-polarized organic light-emitting diode (spin-OLED) is a common goal in the emerging fields of molecular electronics and spintronics. In this device, two ferromagnetic (FM) electrodes are used to enhance the electroluminescence intensity of the OLED through a magnetic control of the spin polarization of the injected carriers. The major difficulty is that the driving voltage of an OLED device exceeds a few volts, while spin injection in organic materials is only efficient at low voltages. The fabrication of a spin-OLED that uses a conjugated polymer as bipolar spin collector layer and ferromagnetic electrodes is reported here. Through a careful engineering of the organic/inorganic interfaces, it is succeeded in obtaining a light-emitting device showing spin-valve effects at high voltages (up to 14 V). This allows the detection of a magneto-electroluminescence (MEL) enhancement on the order of a 2.4% at 9 V for the antiparallel (AP) configuration of the magnetic electrodes. This observation provides evidence for the long-standing fundamental issue of injecting spins from magnetic electrodes into the frontier levels of a molecular semiconductor. The finding opens the way for the design of multifunctional devices coupling the light and the spin degrees of freedom.

Electronic and magnetic properties of a ferromagnetic cobalt surface by adsorbing ultrathin films of tetracyanoethylene

Ultrathin films of tetracyanoethylene (TCNE) on Co(100) were investigated by means of spin-integrated and spin-resolved photoemission spectroscopy ((sp-)UPS), X-ray photoemission spectroscopy (XPS), near edge X-ray absorption fine-structure spectroscopy (NEXAFS), and X-ray magnetic circular dichroism (XMCD). We found a coverage-dependent modulation of the interface dipole and a switching between a metallic and a resistive spin filtering at the interface triggered by two distinct adsorption geometries of TCNE. The strongest hybridization and spin structure modifications are found at low coverage with a face-on adsorption geometry indicating changes in the distance between the surface Co atoms beneath. TCNE has the potential to manipulate the magnetic moments in the Co surface itself, including the possibility of magnetic hardening effects. In summary, the system TCNE/Co offers an experimentally rather easy and controllable way to build up a stable molecular platform stabilizing the reactive ferromagnetic Co surface and customizing the electronic and magnetic properties of the resulting spinterface simultaneously. This makes this system very attractive for spintronic applications as an alternative, less reactive but highly spin polarized foundation beside graphene-based systems.

Reliable Spin Valves of Conjugated Polymer Based on Mechanically Transferrable Top Electrodes

Organic spintronic devices present one of the most appealing technologies for future spintronic devices by taking advantage of the spin degree of freedom. Conjugated polymers are attractive for the exemplified device of organic spin valves (OSVs) due to their weak spin-orbit coupling, solution-processability, low production cost, and mechanical flexibility. However, the performance of polymer SVs is a matter of debate, as the evaporated top ferromagnetic (FM) electrode will penetrate into the organic layer during a typical fabrication process, especially in the device with an organic layer thickness of nanometers. It will cause a severe problem in controllable and reproducible spin manipulations, not to mention the clarification of the spin-dependent transport mechanism. Here, a universal, simple, and low-cost method based on a transferred electrode is developed for a polymer spin valve with stable and reliable state operation. It is demonstrated in an OSV device with a vertical structure of La_2/3Sr_1/3MnO₃ (LSMO)/P3HT/AlO _x/Co/Au that this approach not only builds a damage-free interface between magnetic electrodes and an organic spacer layer but also can be generalized for other devices with delicate active layers. Furthermore, a multistate writing and reading prototype is achieved on the premise of robust and quick magnetic response. The results reveal the importance of a spinterface and effective thickness of the organic layer in fundamental spintronic research and may lead to a strong potential in future flexible, large-area, and robust organic multifunctional circuits.

Current crowding issues on nanoscale planar organic transistors for spintronic applications

The predominance of interface resistance makes current crowding ubiquitous in short channel organic electronics devices but its impact on spin transport has never been considered. We investigate electrochemically doped nanoscale PBTTT short channel devices and observe the smallest reported values of crowding lengths, found for sub-100 nm electrodes separation. These observed values are nevertheless exceeding the spin diffusion lengths reported in the literature. We discuss here how current crowding can be taken into account in the framework of the Fert-Jaffrès model of spin current propagation in heterostructures, and predict that the anticipated resulting values of magnetoresistance can be significantly reduced. Current crowding therefore impacts spin transport applications and interpretation of the results on spin valve devices.

Magnetoresistance Effect and the Applications for Organic Spin Valves Using Molecular Spacers

Organic spin devices utilizing the properties of both spin and charge inherent in electrons have attracted extensive research interest in the field of future electronic device development. In the last decade, magnetoresistance effects, including giant magetoresistance and tunneling magnetoresistance, have been observed in organic spintronics. Significant progress has been made in understanding spin-dependent transport phenomena, such as spin injection or tunneling, manipulation, and detection in organic spintronics. However, to date, materials that are effective for preparing organic spin devices for commercial applications are still lacking. In this report, we introduce basic knowledge of the fabrication and evaluation of organic spin devices, and review some remarkable applications for organic spin valves using molecular spacers. The current bottlenecks that hinder further enhancement for the performance of organic spin devices is also discussed. This report presents some research ideas for designing organic spin devices operated at room temperature.

Effect of magnetic fullerene on magnetization reversal created at the Fe/C60 interface

Probing the hybridized magnetic interface between organic semiconductor (OSC) and ferromagnetic (FM) layers has drawn significant attention in recent years because of their potential in spintronic applications. Recent studies demonstrate various aspects of organic spintronics such as magnetoresistance, induced interface moment etc. However, not much work has been performed to investigate the implications of such OSC/FM interfaces on the magnetization reversal and domain structure which are the utmost requirements for any applications. Here, we show that non-magnetic Fullerene can obtain non-negligible magnetic moment at the interface of Fe(15 nm)/C60(40 nm) bilayer. This leads to substantial effect on both the magnetic domain structure as well as the magnetization reversal when compared to a single layer of Fe(15 nm). This is corroborated by the polarized neutron reflectivity (PNR) data which indicates presence of hybridization at the interface by the reduction of magnetic moment in Fe. Afterwards, upto 1.9 nm of C60 near the interface exhibits magnetic moment. From the PNR measurements it was found that the magnetic C60 layer prefers to be aligned anti-parallel with the Fe layer at the remanant state. The later observation has been confirmed by domain imaging via magneto-optic Kerr microscopy.

Bacteriorhodopsin based non-magnetic spin filters for biomolecular spintronics

We discuss spin injection and spin valves, which are based on organic and biomolecules, that offer the possibility to overcome some of the limitations of solid-state devices, which are based on ferromagnetic metal electrodes.

Enhanced Magnetic Hybridization of a Spinterface through Insertion of a Two-Dimensional Magnetic Oxide Layer

Interfaces between organic semiconductors and ferromagnetic metals offer intriguing opportunities in the rapidly developing field of organic spintronics. Understanding and controlling the spin-polarized electronic states at the interface is the key toward a reliable exploitation of this kind of systems. Here we propose an approach consisting in the insertion of a two-dimensional magnetic oxide layer at the interface with the aim of both increasing the reproducibility of the interface preparation and offering a way for a further fine control over the electronic and magnetic properties. We have inserted a two-dimensional Cr₄O₅ layer at the C₆₀/Fe(001) interface and have characterized the corresponding morphological, electronic, and magnetic properties. Scanning tunneling microscopy and electron diffraction show that the film grows well-ordered both in the monolayer and multilayer regimes. Electron spectroscopies confirm that hybridization of the electronic states occurs at the interface. Finally, magnetic dichroism in X-ray absorption shows an unprecedented spin-polarization of the hybridized fullerene states. The latter result is discussed also in light of an ab initio theoretical analysis.