Magnetoelectric effect in organic molecular solids

Perspective on room temperature and low-field-induced magnetoelectric coupling in molecular complexes

Magnetoelectric (ME) coupling refers to the interaction between electric and magnetic orders in materials. Based on ME coupling, the phenomenon that an external magnetic field induces electric polarization and an external electric field induces change in mangetization can be observed and is referred to as the ME effect. Examples of the ME effect include magnetodielectric (MD), magnetoferroelectric (MF), magnetoresistence (MR) and electrically controlled magnetism effects. In recent years, the ME effect has attracted increasing attention due to the wide range of potential applications in fields such as information storage, sensors, and spintronics. The ME effect can be observed in both single-phase and composite systems but obtaining ME coupling in pure inorganic materials is extremely challenging. For example, in multiferroics with magnetism and electricity, the material must exhibit a magnetic ordered phase (ferromagnets or ferrimagnets), which coexists with the ferroelectric phase in the same temperature range. However, the materials containing both ordering phases within a single species are exceedingly rare, and those capable of coupling the two are even scarcer. MD materials are relatively easy to obtain because they are not constrained by polar point groups in their structure. With advancements in science and technology, new materials with potential ME coupling are increasingly being identified, particularly in the field of molecular materials. Molecular materials, due to their ease of design and synthesis, can not only achieve the regulation of magnetic field on polarization but also complete the control of electric field on magnetism. This paper briefly reviews recent research progress on the ME effect in molecular materials, focusing on three aspects: magnetodielectrics, magnetoferroelectrics, and electronically controlled magnetism. Typical complexes exhibiting the ME effects in these three categories are analyzed and summarized.

Direct observation of electric field-induced magnetism in a molecular magnet

We report the direct observation of an electrically-induced magnetic susceptibility in the molecular nano- magnet [Fe₃O(O₂CPh)₆(py)₃]ClO₄·py, an Fe₃ trimer. This magnetoelectric effect results from the breaking of spatial inversion symmetry due to the spin configurations of the antiferromagnetic trimer. Both static and very low frequency electric fields were used. Fractional changes of the magnetic susceptibility of 11 ppb[Formula: see text] per kVm^-1 for the temperature range 8.5 < T < 13.5 K were observed for applied electric fields up to 62 kV m^-1. The changes in susceptibility were measured using a tunnel diode oscillator operating at liquid helium temperatures while the sample is held at a higher regulated temperature.

A Database for Crystalline Organic Conductors and Superconductors

We present a prototype database for quasi two-dimensional crystalline organic conductors and superconductors based on molecules related to bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, ET). The database includes crystal structures, calculated electronic structures, and experimentally measured properties such as the superconducting transition temperature and critical magnetic fields. We obtained crystal structures from the Cambridge Structural Database and created a crystal structure analysis algorithm to identify cation molecules and execute tight binding electronic structure calculations. We used manual data entry to encode experimentally measured properties reported in publications. Crystalline organic conductors and superconductors exhibit a wide variety of electronic ground states, particularly those with correlations. We hope that this database will ultimately lead to a better understanding of the fundamental mechanisms of such states.

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.

Room temperature magnetoelectric coupling in a molecular ferroelectric ytterbium(III) complex

Magnetoelectric (ME) materials combine magnetic and electric polarizabilities in the same phase, offering a basis for developing high-density data storage and spintronic or low-consumption devices owing to the possibility of triggering one property with the other. Such applications require strong interaction between the constitutive properties, a criterion that is rarely met in classical inorganic ME materials at room temperature. We provide evidence of a strong ME coupling in a paramagnetic ferroelectric lanthanide coordination complex with magnetostrictive phenomenon. The properties of this molecular material suggest that it may be competitive with inorganic magnetoelectrics.

A Lieb-like lattice in a covalent-organic framework and its Stoner ferromagnetism

Lieb lattice has been extensively studied to realize ferromagnetism due to its exotic flat band. However, its material realization has remained elusive; so far only artificial Lieb lattices have been made experimentally. Here, based on first-principles and tight-binding calculations, we discover that a recently synthesized two-dimensional sp² carbon-conjugated covalent-organic framework (sp²c-COF) represents a material realization of a Lieb-like lattice. The observed ferromagnetism upon doping arises from a Dirac (valence) band in a non-ideal Lieb lattice with strong electronic inhomogeneity (EI) rather than the topological flat band in an ideal Lieb lattice. The EI, as characterized with a large on-site energy difference and a strong dimerization interaction between the corner and edge-center ligands, quenches the kinetic energy of the usual dispersive Dirac band, subjecting to an instability against spin polarization. We predict an even higher spin density for monolayer sp²c-COF to accommodate a higher doping concentration with reduced interlayer interaction.

While ferromagnetism has been observed in an sp² covalent-organic framework, its origin remains unclear. Here, by first-principle and tight-binding calculations, the authors identify the Lieb-lattice-like feature of the two-dimensional covalent-organic material and the Stoner mechanism responsible for its magnetic behavior.

Evidence for a quantum dipole liquid state in an organic quasi-two-dimensional material

Mott insulators are commonly pictured with electrons localized on lattice sites, with their low-energy degrees of freedom involving spins only. Here, we observe emergent charge degrees of freedom in a molecule-based Mott insulator κ-(BEDT-TTF)₂Hg(SCN)₂Br, resulting in a quantum dipole liquid state. Electrons localized on molecular dimer lattice sites form electric dipoles that do not order at low temperatures and fluctuate with frequency detected experimentally in our Raman spectroscopy experiments. The heat capacity and Raman scattering response are consistent with a scenario in which the composite spin and electric dipole degrees of freedom remain fluctuating down to the lowest measured temperatures.

Evidence for Electronically Driven Ferroelectricity in a Strongly Correlated Dimerized BEDT-TTF Molecular Conductor

By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor κ-(BEDT-TTF)_{2}Hg(SCN)_{2}Cl, we provide evidence for order-disorder type electronic ferroelectricity that is driven by the charge order within the (BEDT-TTF)_{2} dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and nondimerized or weakly dimerized systems at 1/4 filling, exhibiting a charge order. Our results indicate that intradimer charge degrees of freedom are of particular importance in correlated κ-(BEDT-TTF)_{2}X salts and can create novel states, such as electronically driven multiferroicity or charge-order-induced quasi-one-dimensional spin liquids.