Recent development of iron-based superconducting films

The discovery of F-doped LaFeAsO in 2008 brought a worldwide research surge of iron-based superconductors. As a new type of high-temperature superconductors, iron-based superconductors have attracted much attention due to their unique properties and significant differences from copper oxide and traditional superconductors. Iron-based superconductors have small anisotropy, high superconducting transition temperature and upper critical field, which are preferred in film preparation and applications. At the same time, the features of iron-based superconductor films, such as higher superconducting transition temperature in epitaxial films and the ability to stabilize metastable equivalents, are interesting to the researchers. The studies on films are very useful for the basic research on superconducting physics and the search of new superconducting materials. The iron-based superconductors discovered so far can be divided into four systems: (1) 1111 system: LnFeAs (O, F) (Ln= lanthanides); (2) 122 system: AEFe(2)As(2) (AE = alkaline earth metals); (3) 111 system: LiFeAs and NaFeAs; (4) 11 system: FeCh (Ch=chalcogenides). 1111 system is the first reported iron-based superconductor. It has a high critical transition temperature up to 58 K as well as a higher upper critical field. The disadvantage of 1111 superconductors is the lager anisotropy and the broadened superconducting transition under the magnetic field. The critical transition temperature of 122 system is lower than 1111 system, but it can still reach 38 K, and its anisotropy and elemental stability is better than 1111 system. The structure of 11 system is the simplest in iron-based superconductor family. It contains no toxic element As, but the transition temperature is low (8 K). 111 system is unstable in the air, limiting its application prospects. In addition, the critical current density is very important for the application of superconductors, which has been obtained in 122-system superconducting tapes. Meanwhile, the 11-system superconducting film stands out because of its easy growth, simple structure and high transport current density, which can exceed 105 A/cm(2) even at 30 T. During the past years, researchers around the world have made great progress in the fabrication and synthesis of ironbased superconducting single crystals, bulk materials, tapes and thin films. Among them, the development of thin films cannot be ignored. The fabrication of high-quality thin films has three purposes: Basic research, tape application and electronic application. Basic and application-oriented research is developing quickly. Increasing the critical current density is an important strategy for realizing commercialized and cost-effective high-temperature superconductor technology, which can be improved by microstructure engineering. Naturally grown pinning centers and artificial pinning centers are able to pin on magnetic vortices in magnetic fields, which can effectively prevent the dissipation of energy and raise critical current density. Based on the results achieved in 1111 system, doped 122 system, and 11 system, we analyzed the possible connections between the growth technique, microstructure and superconducting properties of iron-based superconducting films.