Akiyasu Yamamoto1,2,Shota Ishiwata1,Shinjiro Kikuchi1,Yuta Hasegawa1,Shinnosuke Tokuta1,2,Akimitsu Ishii3,2,Akinori Yamanaka1,2,Yusuke Shimada4,2,Zimeng Guo5,2,Satoshi Hata5,2,Takuto Kojima6,2,Kentaro Kutsukake7,2,Hiroaki Kudo6,2,Noritaka Usami6,2
Tokyo University of Agriculture and Technology1,JST-CREST2,NIMS3,Tohoku University4,Kyushu University5,Nagoya University6,RIKEN7
Akiyasu Yamamoto1,2,Shota Ishiwata1,Shinjiro Kikuchi1,Yuta Hasegawa1,Shinnosuke Tokuta1,2,Akimitsu Ishii3,2,Akinori Yamanaka1,2,Yusuke Shimada4,2,Zimeng Guo5,2,Satoshi Hata5,2,Takuto Kojima6,2,Kentaro Kutsukake7,2,Hiroaki Kudo6,2,Noritaka Usami6,2
Tokyo University of Agriculture and Technology1,JST-CREST2,NIMS3,Tohoku University4,Kyushu University5,Nagoya University6,RIKEN7
122 phase iron-based high temperature superconductors show high upper critical field with small electromagnetic anisotropy<sup>1)</sup> and large critical grain boundary angle<sup>2)</sup>, and therefore is a promising material for applications in polycrystalline form. Foreseeing magnet applications, Weiss et al. have reported demonstration of trapped field of 1 T for K-doped BaFe<sub>2</sub>As<sub>2</sub> (Ba122) polycrystalline bulks synthesized by hot isostatic pressing<sup>3)</sup>. In this study, K-doped Ba122 bulks were synthesized from mechanochemically synthesized precursor which was prepared by high-energy ball-milling of elemental metals with the molar ratios of Ba:K:Fe:As = 0.6:0.4:2:2 in a Ar glove box <sup>4), 5)</sup>. The precursor powder was then spark plasma sintered. Two approaches to optimizing the processing conditions were considered: optimization by researchers’ experience and intuition and by data driven process based on machine learning<sup>6)</sup>. Bayesian optimization was applied to find the best input parameters that maximize the target output property, critical current density (<i>J</i><sub>c</sub>) under magnetic field of 3 Tesla, on the experimentally available range of processing conditions with pre-optimization data by researchers using a software BOXVIA<sup>6)</sup>. High <i>J</i><sub>c</sub> value exceeding 10<sup>5</sup> A/cm<sup>2</sup> was developed by both experiments guided by researchers and Bayesian optimization process. Doubled trapped magnetic field exceeding the previous record by Weiss et al.<sup> 3)</sup> was measured. Detailed trapped magnetic field properties of Ba122 bulk magnets and the key microstructural features revealed by 3-dimensional multiscale analysis<sup>7)</sup>, precession electron diffraction (PED) nano-orientation STEM analysis and the twinning network graph analysis<sup>8)</sup> will be discussed.<br/><br/><b>Acknowledgement: </b><br/>This work was supported by JST CREST (JPMJCR18J4 & JPMJCR17J1).<br/><br/><b>References</b>:<br/>1) H. Hosono et al., Materials Today 21, 278 (2018).<br/>2) T. Katase et al., Nat. Commun. 2, 409 (2011); J. Durrell et al., Rep. Prog. Phys. 74, 124511 (2011).<br/>3) J. D. Weiss et al., Supercond. Sci. Technol. Letter 28, 112001 (2015).<br/>4) S. Tokuta and A. Yamamoto, APL Materials 7, 111107 (2019).<br/>5) S. Tokuta, Y. Shimada, and A. Yamamoto, iScience 25, 103992 (2022).<br/>6) A. Ishii et al., SoftwareX 18, 101019 (2022), J. Alloy Compd. 966, 2239133 (2023).<br/>7) Y. Shimada et al., SuST 32, 084003 (2019); J. Alloy Compd. 923, 166358 (2022).<br/>8) T. Kojima et al., 2021 MRS Fall Meeting & Exhibit, CH04.09.01 (2021).