Excited charge-transfer organics with multiferroicity

Abstract

Multiferroics, showing simultaneous electric and magnetic degree of freedom, has aroused increasing interest due to tailored multiferroic properties and magneto-electric coupling for shaping the development of energy-efficient multifunctional devices. Now, the multiferroics can be classified as two groups:1) inorganic multiferroics, which can be single-phase, multi-phases oxide multiferroic or multiferroic heterojunction and 2) organic counterpart, which is mostly determined by instinct charge-transfer behavior. But it is difficult to find the polarization and the magnetization co-exist in a single-phase oxide multiferroic material, and their coupling range in the multiferroic heterojunction is only several atomic layers, which limits the applications. As a result, more and more different types of organic multiferroics have been studied. Some organic complexes can display dual ferroelectric and ferromagnetic properties at ambient temperature, e.g. thiophene-fullerene donor-acceptor charge-transfer networks. The organic charge-transfer complex is based on electron donor (D+) and acceptor (A-) assembly. D+A- are long-range ordering, the excitons have s lifetime and 1/2 spin, which contributes to the room temperature ferroelectricity and ferromagnetism. The excitons can be excited by external magnetic field, electric field, illumination and stress, and eventually influence the polarization, magnetization and magnetoelectric coupling coefficient. However, there are still many problems to be solved, i.e., searching for new charge-transfer systems and preparing supramolecular co-crystal with ordered molecular chain, further improving magnetoelectric properties; developing the heterojunction technology and epitaxial growth of organic ferroelectric or ferromagnetic systems on excited organic films, which is expected to greatly improve their magnetoelectric coupling effects; inventing more new charge transport organic multiferroic devices to extend the application scope of new multiferroic devices in actual industrial production. Generally speaking, the organic charge-transfer complexes not only greatly enrich the room temperature multiferroics materials, but also provide the technical basis for developing the new multifunctional electronic devices.