An organic approach for nanostructured multiferroics

Room Temperature Single-Component Organic Multiferroics with Large Magnetoelectric Coupling: Proficient Approach for Stray-Magnetic Field Harvesting

Magnetoelectric materials are highly desirable for technological applications due to their ability to produce electricity under a magnetic field. Among the various types of magnetoelectric materials studied, their organic counterparts provide an opportunity to develop solution-processable, flexible, lightweight, and wearable electronic devices. However, there is a rare choice of solution-processable, flexible, lightweight magnetoelectric materials which has tremendous technological interest. A supramolecular scaffold with precisely positioned structure-forming and functional units (electrical dipoles and magnetic spins) is designed so that self-assembly results in functional unit organization. Structure-forming segments allow these scaffolds to self-assemble into hierarchically ordered structures in nonpolar solvents, creating nanofibrous organogel networks. In particular, the xerogel derived from this organogel exhibits the highest magnetoelectric coupling coefficient (αME ≈ 216 mV Oe-1 cm-1) reported to date for organic materials. This is even greater than commonly envisioned composite materials made of piezoelectric polymers and inorganic magnets. This single-component organic multiferroic material displays ferroelectricity (Tc ≈ 46 °C) and paramagnetic behavior at room temperature. With this, it is demonstrated that the possibilities of effectively harvesting stray magnetic fields that are copiously available in the surroundings and wasted otherwise.

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.

Muon-Nitrogen Quadrupolar Level Crossing Resonance in a Charge Transfer Salt

Although muons are primarily regarded as a local spin probe, they can also access the charge state of an atom or molecule via quadrupolar level crossing resonance (QLCR) spectroscopy. We use Li⁺TCNQ^- (TCNQ = 7,7,8,8-tetracyanoquinodimethane), a simple charge transfer salt, to test the potential of this technique in molecular systems by studying the interaction of a positive muon with the TCNQ nitrogen atoms. We show that both a positive muon and muonium are able to add to the nitrogen, leading to a singlet spin state for the addition molecule. This produces a characteristic three line QLCR spectrum, with the observed line positions and intensities determined by the principal values and orientation of the electric field gradient tensor at the nitrogen. Ab initio calculation of this field gradient and the resulting QLCR spectrum give good agreement with the experiment. A nonresonant background contribution to the relaxation rate also provides evidence for spin excitations rapidly diffusing along the TCNQ chains. These reflect mobile unpaired electrons introduced by muonium addition. It is thus shown that a single set of muon measurements can be sensitive to both spin and charge degrees of freedom in the same molecular material.

A theoretical strategy for pressure-driven ferroelectric transition associated with critical behavior and magnetoelectric coupling in organic multiferroics

In organic multiferroics, the charge or spin coupled to the lattice induces lattice symmetry breaking, which is responsible for the ferroelectric (FE) transition. We propose a quantum spin model to describe the ferroelectricity of organic multiferroics, in which the pressure-driven spin-lattice coupling is controlled both by a jump function and a pressure power function. The T-p phase diagram shows different scaling relationships at low and high pressure regions, respectively, which is in accordance with the experimental observation. It is found that the pressure can not only enhance the FE polarization, but also enhance the transition temperature T_c as well as the electrocaloric effect (ECE). The electrocaloric critical scaling laws are proposed to verify the order and universality of the FE transition based on the Banerjee and Franco's criteria. In addition, we propose a temperature mediated mechanism within an isentropic process based on the ECE combined with the pyromagnetic effect, together with multiple physically (magnetic field and pressure jointly) controlled means, to enhance the magnetoelectric coupling around room-temperature, which will provide thermodynamic and quantum controlled means for realizing multi-state logic memory.

Spin-Lattice Coupling Across the Magnetic Quantum-Phase Transition in Copper-Containing Coordination Polymers

We measured the infrared vibrational properties of two copper-containing coordination polymers, [Cu(pyz)₂(2-HOpy)₂](PF₆)₂ and [Cu(pyz)_1.5(4-HOpy)₂](ClO₄)₂, under different external stimuli in order to explore the microscopic aspects of spin-lattice coupling. While the temperature and pressure control hydrogen bonding, an applied field drives these materials from the antiferromagnetic → fully saturated state. Analysis of the pyrazine (pyz)-related vibrational modes across the magnetic quantum-phase transition provides a superb local probe of magnetoelastic coupling because the pyz ligand functions as the primary exchange pathway and is present in both systems. Strikingly, the PF₆^- compound employs several pyz-related distortions in support of the magnetically driven transition, whereas the ClO₄^- system requires only a single out-of-plane pyz bending mode. Bringing these findings together with magnetoinfrared spectra from other copper complexes reveals spin-lattice coupling across the magnetic quantum-phase transition as a function of the structural and magnetic dimensionality. Coupling is maximized in [Cu(pyz)_1.5(4-HOpy)₂](ClO₄)₂ because of its ladderlike character. Although spin-lattice interactions can also be explored under compression, differences in the local structure and dimensionality drive these materials to unique high-pressure phases. Symmetry analysis suggests that the high-pressure phase of the ClO₄^- compound may be ferroelectric.

Quantitative analysis of intermolecular interactions in cocrystals and a pair of polymorphous cocrystal hydrates from 1,4-dihydroquinoxaline-2,3-dione and 1H-benzo[d]imidazol-2(3H)-one with 2,5-dihydroxy-1,4-benzoquinones: a combined X-ray structural and theoretical analysis

Study and quantification of intermolecular interactions in five cocrystals and cocrystals hydrates by PIXEL, DFT, Hirshfeld surface and QTAIM calculations.

Improved uniaxial dielectric properties in aligned diisopropylammonium bromide (DIPAB) doped poly(vinylidene difluoride) (PVDF) nanofibers

Diisopropylammonium bromide (DIPAB) doped poly(vinylidene difluoride) (PVDF) nanofibers (5, 10 and 24 wt% DIPAB doping) with improved and tunable dielectric properties were synthesised via electrospinning. DIPAB nanoparticles were grown in situ during the nanofiber formation. X-Ray diffraction (XRD) patterns and Fourier transform infrared spectroscopy (FTIR) proved that electrospinning of DIPAB doped PVDF solutions led to the formation of a highly electro-active β-phase in the nanofibers. Electrospinning in the presence of DIPAB inside PVDF led to very well aligned nanofibers with preferred (001) orientation that further enhanced the effective dipole moments in the nanofiber structures. The dielectric properties of the composite nanofibers were significantly enhanced due to the improved orientation, ionic and interfacial polarisation upon the applied electrospinning process, ionic nature of DIPAB and the interface between the PVDF nanofibers and equally dispersed DIPAB nanoparticles inside them, respectively. The relative dielectric constant of the PVDF nanofibers was improved from 8.5 to 102.5 when nanofibers were doped with 5% of DIPAB. Incorporating DIPAB in PVDF nanofibers has been shown to be an effective way to improve the structural and dielectric properties of PVDF.

Diisopropylammonium bromide (DIPAB) doped poly(vinylidene difluoride) (PVDF) nanofibers (5, 10 and 24 wt% DIPAB doping) with improved and tunable dielectric properties were synthesised via electrospinning.

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.

Advances in magnetoelectric multiferroics

The manipulation of magnetic properties by an electric field in magnetoelectric multiferroic materials has driven significant research activity, with the goal of realizing their transformative technological potential. Here, we review progress in the fundamental understanding and design of new multiferroic materials, advances in characterization and modelling tools to describe them, and the exploration of devices and applications. Focusing on the translation of the many scientific breakthroughs into technological innovations, we identify the key open questions in the field where targeted research activities could have maximum impact in transitioning scientific discoveries into real applications.

Incommensurate structures of the [CH3NH3][Co(COOH)3] compound

The present article is devoted to the characterization of the structural phase transitions of the [CH3NH3][Co(COOH)3] (1) perovskite-like metal-organic compound through variable-temperature single-crystal neutron diffraction. At room temperature, compound 1 crystallizes in the orthorhombic space group Pnma (phase I). A decrease in temperature gives rise to a first phase transition from the space group Pnma to an incommensurate phase (phase II) at approximately 128 K. At about 96 K, this incommensurate phase evolves into a second phase with a sharp change in the modulation vector (phase III). At lower temperatures (ca 78 K), the crystal structure again becomes commensurate and can be described in the monoclinic space group P21/n (phase IV). Although phases I and IV have been reported previously [Boča et al. (2004). Acta Cryst. C60, m631-m633; Gómez-Aguirre et al. (2016). J. Am. Chem. Soc. 138, 1122-1125; Mazzuca et al. (2018). Chem. Eur. J. 24, 388-399], phases III and IV corresponding to the Pnma(00γ)0s0 space group have not yet been described. These phase transitions involve not only the occurrence of small distortions in the three-dimensional anionic [Co(HCOO)3]- framework, but also the reorganization of the [CH3NH3]+ counter-ions in the cavities of the structure, which gives rise to an alteration of the hydrogen-bonded network, modifying the electrical properties of compound 1.

Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals

Materials showing coupling phenomena between magnetism and (ferro)electricity, i.e., magnetoelectric effects, have attracted a great deal of attention due to their potential applications for future device technologies such as sensors and storage. However, conventional approaches, which usually utilize materials containing magnetic metal ions (or radicals), have a major problem: only a few materials have been found to show the coupling phenomena at room temperature. Recently, we proposed a new approach to achieve room-temperature magnetoelectrics. In contrast to conventional approaches, our alternative proposal focuses on a completely different material, a "liquid crystal", free from magnetic metal ions. In such liquid crystals, a magnetic field can be utilized to control the orientational state of constituent molecules and the corresponding electric polarization through magnetic anisotropy of the molecules; it is an unprecedented mechanism of the magnetoelectric effect. In this context, this paper provides a protocol to measure ferroelectric properties induced by a magnetic field, that is, the direct magnetoelectric effect, in a liquid crystal. With the method detailed here, we successfully detected magnetically-tuned electric polarization in the chiral smectic C phase of a liquid crystal at room temperature. Taken together with the flexibility of constituent molecules, which directly affects the magnetoelectric responses, the introduced method will serve to allow liquid crystal cells to acquire more functions as room-temperature magnetoelectrics and associated optical materials.

A theoretical insight into an isentropic strategy for enhancing magnetoelectric coupling of organic multiferroics

The cross-coupling between magnetic and ferroelectric orders in spin-driven organic multiferroics provides great potential for realizing multi-state logic memory. Creating strong magnetoelectric coupling around room-temperature is the key to eliminate the main roadblock for practical application. Herein, quantum correlation controlled means are employed to tune the transition temperature TC = 300 K, as the optimal operating temperature. After that, based on the magnetocaloric or electrocaloric effect, a temperature mediated mechanism is proposed to enhance magnetoelectric coupling within an isentropic rather than an isothermal process. Furthermore, a moderate magnetic field combined with a relatively weak electric field can jointly control and dramatically enhance the isentropic magnetoelectric coupling around room-temperature.

Bio-field array: a dielectrophoretic electromagnetic toroidal excitation to restore and maintain the golden ratio in human erythrocytes

Erythrocytes must maintain a biconcave discoid shape in order to efficiently deliver oxygen (O2 ) molecules and to recycle carbon dioxide (CO2 ) molecules. The erythrocyte is a small toroidal dielectrophoretic (DEP) electromagnetic field (EMF) driven cell that maintains its zeta potential (ζ) with a dielectric constant (ԑ) between a negatively charged plasma membrane surface and the positively charged adjacent Stern layer. Here, we propose that zeta potential is also driven by both ferroelectric influences (chloride ion) and ferromagnetic influences (serum iron driven). The Golden Ratio, a function of Phi φ, offers a geometrical mathematical measure within the distinct and desired curvature of the red blood cell that is governed by this zeta potential and is required for the efficient recycling of CO2 in our bodies. The Bio-Field Array (BFA) shows potential to both drive/fuel the zeta potential and restore the Golden Ratio in human erythrocytes thereby leading to more efficient recycling of CO2 . Live Blood Analyses and serum CO2 levels from twenty human subjects that participated in immersion therapy sessions with the BFA for 2 weeks (six sessions) were analyzed. Live Blood Analyses (LBA) and serum blood analyses performed before and after the BFA immersion therapy sessions in the BFA pilot study participants showed reversal of erythrocyte rheological alterations (per RBC metric; P = 0.00000075), a morphological return to the Golden Ratio and a significant decrease in serum CO2 (P = 0.017) in these participants. Immersion therapy sessions with the BFA show potential to modulate zeta potential, restore this newly defined Golden Ratio and reduce rheological alterations in human erythrocytes.

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.

A Free-Standing Molecular Spin-Charge Converter for Ubiquitous Magnetic-Energy Harvesting and Sensing

Magnetic-energy harvesting in a centimeter-sized free-standing (BEDT-TTF)C₆₀ charge-transfer single crystal is demonstrated. The crystal shows sensitive magnetic-, thermal-, and mechanical-sensing ability, with an excellent piezoresistance coefficient of -5.1 × 10^-6 Pa^-1 . The self-powered sensing performance, together with its solution processability and flexibility, endow it with the capability of driving a new generation of noncontact magnetic-energy harvesting and sensing technologies.

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.

Electric, magnetic, piezoelectric and magnetoelectric studies of phase pure (BiFeO3–NaNbO3)–(P(VDF-TrFE)) nanocomposite films prepared by spin coating

(BiFeO₃–NaNbO₃)–(P(VDF-TrFE)) co-polymer thin films were fabricated by spin coating technique and their electric, magnetic, electromechanical and magnetoelectric properties were investigated.

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.