Multiferroicity of carbon-based charge-transfer magnets

Deterministic Magnetization Reversal in Synthetic Antiferromagnets using Natural Light

Traditional current-driven spintronics is limited by localized heating issues and large energy consumption, restricting their data storage density and operation speed. Meanwhile, voltage-driven spintronics with much lower energy dissipation also suffers from charge-induced interfacial corrosion. Thereby finding a novel way of tuning ferromagnetism is crucial for spintronics with energy-saving and good reliability. Here, a visible light tuning of interfacial exchange interaction via photoelectron doping into synthetic antiferromagnetic heterostructure of CoFeB/Cu/CoFeB/PN Si substrate is demonstrated. Then, a complete, reversible magnetism switching between antiferromagnetic (AFM) and ferromagnetic (FM) states with visible light on and off is realized. Moreover, a visible light control of 180° deterministic magnetization switching with a tiny magnetic bias field is achieved. The magnetic optical Kerr effect results further reveal the magnetic domain switching pathway between AFM and FM domains. The first-principle calculations conclude that the photoelectrons fill in the unoccupied band and raise the Fermi energy, which increases the exchange interaction. Lastly, a prototype device with visible light control of two states switching with a 0.35% giant magnetoresistance ratio change (maximal 0.4%), paving the way toward fast, compact, and energy-efficient solar-driven memories is fabricated.

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.

Organic Cocrystals: Recent Advances and Perspectives for Electronic and Magnetic Applications

Cocrystal engineering is an advanced supramolecular strategy that has attracted a lot of research interest. Many studies on cocrystals in various application fields have been reported, with a particular focus on the optoelectronics field. However, few articles have combined and summarized the electronic and magnetic properties of cocrystals. In this review, we first introduce the growth methods that serve as the basis for realizing the different properties of cocrystals. Thereafter, we present an overview of cocrystal applications in electronic and magnetic fields. Some functional devices based on cocrystals are also introduced. We hope that this review will provide researchers with a more comprehensive understanding of the latest progress and prospects of cocrystals in electronic and magnetic fields.

Spin Injection and Transport in Organic Materials

This review introduces some important spin phenomena of organic molecules and solids and their devices: Organic spin injection and transport, organic spin valves, organic magnetic field effects, organic excited ferromagnetism, organic spin currents, etc. We summarize the experimental and theoretical progress of organic spintronics in recent years and give prospects.

Magnetic field-directed hybrid anisotropic nanocomposites

A facile bottom-up approach is developed to grow magnetic metallic Cu/FeCo (core/shell) nanowires, where their distribution and orientation can be controlled by magnetic field. The nanocomposites consisting of a ferroelectric polymer matrix and magnetic nanowire arrays exhibit the orientation-controlled anisotropy and interfacial magnetoelectric coupling effect.

Tunable two-dimensional interfacial coupling in molecular heterostructures

Two-dimensional van der Waals heterostructures are of considerable interest for the next generation nanoelectronics because of their unique interlayer coupling and optoelectronic properties. Here, we report a modified Langmuir-Blodgett method to organize two-dimensional molecular charge transfer crystals into arbitrarily and vertically stacked heterostructures, consisting of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)/C₆₀ and poly(3-dodecylthiophene-2,5-diyl) (P3DDT)/C₆₀ nanosheets. A strong and anisotropic interfacial coupling between the charge transfer pairs is demonstrated. The van der Waals heterostructures exhibit pressure dependent sensitivity with a high piezoresistance coefficient of -4.4 × 10^-6 Pa^-1, and conductance and capacitance tunable by external stimuli (ferroelectric field and magnetic field). Density functional theory calculations confirm charge transfer between the n-orbitals of the S atoms in BEDT-TTF of the BEDT-TTF/C₆₀ layer and the π* orbitals of C atoms in C₆₀ of the P3DDT/C₆₀ layer contribute to the inter-complex CT. The two-dimensional molecular van der Waals heterostructures with tunable optical-electronic-magnetic coupling properties are promising for flexible electronic applications.Two-dimensional van der Waals heterostructures are of interest due to their unique interlayer coupling and optoelectronic properties. Here authors develop a Langmuir-Blodgett method to organize charge transfer molecular heterostructures with externally tunable conductance and capacitance and broadband photoresponse.

Ferromagnetic mechanism in organic photovoltaic cells with closed-shell structures

We construct a model to reveal the spin polarization or ferromagnetism observed in organic composite nw-P3HT/C₆₀ with closed-shell structures. Different from the organic ferromagnets with open-shell structures, the ferromagnetism of nw-P3HT/C₆₀ comes from the charge transfers from the polymer to the small molecules. The transferred electrons become spin polarized and they are coupled together through the holes in the polymer. Finally, a ferromagnetic order appears in the pure organic composite. The magnetic moment of the system is mainly provided by the spin polarized small molecules. The magnetization is dependent upon the density of the transferred charges, which is consistent to our experimental observations. Our investigation also shows that some new spin phenomena may appear in excited states for organic semiconductors which is absent in the ground states.

Bimodal supramolecular functionalization of carbon nanotubes triggered by covalent bond formation

Many applications of carbon nanotubes require their chemical functionalization. Both covalent and supramolecular approaches have been extensively investigated. A less trodden path is the combination of both covalent and noncovalent chemistries, where the formation of covalent bonds triggers a particularly stable noncovalent interaction with the nanotubes. We describe a series of naphthalene diimide (NDI) bisalkene molecules that, upon mixing with single-walled carbon nanotubes (SWNTs) and Grubbs' catalyst, undergo two different reaction pathways. On one hand, they ring-close around the SWNTs to form rotaxane-like mechanically interlocked derivatives of SWNTs (MINTs). Alternatively, they oligomerize and then wrap around the SWNTs. The balance of MINTs to oligomer-wrapped SWNTs depends on the affinity of the NDI molecules for the SWNTs and the kinetics of the metathesis reactions, which can be controlled by varying the solvent. Thorough characterization of the products (TGA, TEM, AFM, Raman, UV-vis-NIR, PLE, XPS and UPS) confirms their structure and shows that each type of functionalization affects the electronic properties of the SWNTs differently.

Multiferroic Nanopatterned Hybrid Material with Room-Temperature Magnetic Switching of the Electric Polarization

A nanopatterned hybrid layer is designed, wherein the electric polarization can be flipped at room temperature by a magnetic field aided by an electrical field. This is achieved by embedding ferromagnetic nanopillars in a continuous organic ferroelectric layer, and amplifying the magnetostriction-generated stress gradients by scaling down the supracrystalline cell of the material.

Integrated Charge Transfer in Organic Ferroelectrics for Flexible Multisensing Materials

The ultimate or end point of functional materials development is the realization of strong coupling between all energy regimes (optical, electronic, magnetic, and elastic), enabling the same material to be utilized for multifunctionalities. However, the integration of multifunctionalities in soft materials with the existence of various coupling is still in its early stage. Here, the coupling between ferroelectricity and charge transfer by combining bis(ethylenedithio)tetrathiafulvalene-C60 charge-transfer crystals with ferroelectric polyvinylidene fluoride polymer matrix is reported, which enables external stimuli-controlled polarization, optoelectronic and magnetic field sensing properties. Such flexible composite films also display a superior strain-dependent capacitance and resistance change with a giant piezoresistance coefficient of 7.89 × 10(-6) Pa(-1) . This mutual coupled material with the realization of enhanced couplings across these energy domains opens up the potential for multisensing applications.

Multifunctional Charge-Transfer Single Crystals through Supramolecular Assembly

Centimeter-sized segregated stacking TTF-C60 single crystals are crystallized by a mass-transport approach combined with solvent-vapor evaporation for the first time. The intermolecular charge-transfer interaction in the long-range ordered superstructure enables the crystals to demonstrate external stimuli-controlled multifunctionalities and angle/electrical-potential-dependent luminescence.

Spin polarization of excitons in organic multiferroic composites

Recently, the discovery of room temperature magnetoelectricity in organic charge transfer complexes has reignited interest in the multiferroic field. The solution processed, large-area and low cost organic semiconductor materials offer new possibilities for the functional all organic multiferroic devices. Here we report the spin polarization of excitons and charge transfer states in organic charge transfer composites by using extended Su-Schrieffer-Heeger model including Coulomb interaction and spin-flip effect. With the consideration of spin polarization, we suggest a possible mechanism for the origin of excited ferromagnetism.

Magnetic dipolar interaction induced cobalt nanowires

The dipolar interaction of magnetic nanoparticles is of intense interest to engineer material self-assembly for anisotropic functional nanostructures. Here we report the solution synthesis of cobalt nanowires, where the one-dimensional nanowire formation is ultimately dependent on the magnetic dipolar interaction to realize in situ assembly of cobalt nanoparticles. The morphology transition of cobalt nanostructures is well controlled via the ligand-free synthesis and thermal decomposition of zero-valent cobalt precursor. This study provides a self-assembly approach to the development of anisotropic cobalt nanostructures and a better understanding of nucleation parameters, which are demonstrated to correlate strongly with the size and morphology of final cobalt nanowires. This approach may be extended to other magnetic materials for the control of their nanostructure and magnetic performance.

All-polymeric control of nanoferronics

In the search for light and flexible nanoferronics, significant research effort is geared toward discovering the coexisting magnetic and electric orders in crystalline charge-transfer complexes. We report the first example of multiferroicity in centimeter-sized crystalline polymeric charge-transfer superstructures that grow at the liquid-air interface and are controlled by the regioregularity of the polymeric chain. The charge order-driven ferroic mechanism reveals spontaneous and hysteretic polarization and magnetization at the donor-acceptor interface. The charge transfer and ordering in the ferroic assemblies depend critically on the self-organizing and molecular packing of electron donors and acceptors. The invention described here not only represents a new coupling mechanism of magnetic and electric ordering but also creates a new class of emerging all-organic nanoferronics.

Room Temperature Multiferroicity of Charge Transfer Crystals

Room temperature multiferroics has been a frontier research field by manipulating spin-driven ferroelectricity or charge-order-driven magnetism. Charge-transfer crystals based on electron donor and acceptor assembly, exhibiting simultaneous spin ordering, are drawing significant interests for the development of all-organic magnetoelectric multiferroics. Here, we report that a remarkable anisotropic magnetization and room temperature multiferroicity can be achieved through assembly of thiophene donor and fullerene acceptor. The crystal motif directs the dimensional and compositional control of charge-transfer networks that could switch magnetization under external stimuli, thereby opening up an attractive class of all-organic nanoferronics.

An organic approach for nanostructured multiferroics

Multiferroics are materials that simultaneously exhibit more than one ferroic order parameters, such as ferroelectricity, ferroelasticity and ferromagnetism. Recently, multiferroicity has received a significant revival of interest due to the colossal magnetoelectric coupling effect for the development of nano-ferronics. In this mini-review, we focus on a recent study of ferroelectricity, magnetism and magnetoelectric coupling within the newly discovered organic charge-transfer complexes. A systemic understanding of the origin of organic ferroelectricity and magnetism is provided. Furthermore, based on the recent mechanism of the magnetoelectric coupling effect: spin-ordering-induced electric polarization and ferroelectricity-induced spin alignment, we further present the recent progress in organic charge-transfer multiferroics and metal-organic framework multiferroics. The coexistence of polarization and magnetism at room temperature of organic charge-transfer complexes will be critical for the development of all-organic multiferroics.