Department of Applied Physics, Tokyo University of Agriculture and Technology, Japan1
Department of Material Physics, Nagoya University, Japan2
National High Magnetic Field Laboratory, USA3
The Ultramicroscopy Research Center, Kyushu University, Japan4
Department of Advanced Materials Science and Engineering, Kyushu University, Japan5
JST CREST, Japan6
Iron-based superconductors are one of the most promising materials for high field applications. Extensive researches have been conducted toward AEFe2As2 (AE = alkaline-earth elements, 122 compounds) type compounds due to small anisotropy and lesser problem of weak-link of grain boundaries. Growth of epitaxial thins in 122 compounds was mainly reported on Ba(Fe,Co)2As2 and BaFe2(As,P)2, which can be relatively easily obtained by pulsed laser deposition. Growth of (Ba,K)Fe2As2 epitaxial thin films is essential for both applications and studying physical properties. However, (Ba,K)Fe2As2 epitaxial thin films have not been reported because of difficulty to incorporate high-vapor pressure K to films. In this study, we report the growth and critical current properties of (Ba,K)Fe2As2 thin films on CaF2. Films were grown using custom-designed molecular beam epitaxy (MBE) equipped with various rate monitoring system: electron impact spectrometry for both Fe and Ba and atomic absorption spectrometry for K. As rate was controlled by adjusting cell temperature to ~ 150°C. The growth temperature was set 400°C for optimum doping level. (Ba,K)Fe2As2 thin films were epitaxially grown on CaF2 substrates. Films on CaF2 substrates were strained which might lead to rather low critical temperature of ~36 K. From transmission electron microscope observation, columnar growth with low angle grain boundaries were observed. Critical current density (Jc) reached 14.6 MA/cm2 which is the highest value among the 122 compounds. Jc shows a rather weak dependence on the magnetic field, indicating that the c-axis correlated pinning centers are present in the film. As stated above, grain boundaries lie approximately perpendicular to the substrate surface at a several tens of nanometers interval, which may work as pinning centers.
This work was supported by JST CREST Grant Number JPMJCR18J4. A part of work was also partly supported by Advanced Characterization Platform of the Nanotechnology Platform Japan sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
Keywords: epitaxial, thin film, iron-based superconductor, MBE