Flux-Pinning Effects and Mechanism of Water-Quenched 5 wt.% (Fe, Ti) Particle-Doped MgB 2 Superconductor
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ORIGINAL PAPER
Flux-Pinning Effects and Mechanism of Water-Quenched 5 wt.% (Fe, Ti) Particle-Doped MgB2 Superconductor H. B. Lee1
· G. C. Kim1 · Hyoungjeen Jeen1 · Y. C. Kim1
Received: 27 May 2020 / Accepted: 28 July 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract We have studied magnetic behaviors of water-quenched 5 wt.% (Fe, Ti) particle-doped MgB2 compared with those of aircooled one. Generally, grain refinement is achieved by water quenching, which means increases of grain boundaries in a superconductor. We inspected that the increased grain boundary density of a superconductor influenced the kinds of fluxpinning effects on the field dependence of magnetization. As a result, grain boundaries are served as a pinning centers on a higher magnetic field (2.5–6.0 T) whereas they are served as a pathway to facilitate movements of fluxes pinned on volume defects at a lower field (0.2–2.0 T). As inspecting characteristics of grain boundaries, it is understood that they have a fluxpinning effect as well as a flux-penetrating promotion effect. By using TEM, we confirmed that the width of grain boundary in MgB2 is approximately 1 nm, which is not wide enough to pin several flux quanta across the width. As temperature increases, the flux-pinning effect of water-quenched 5 wt.% (Fe, Ti) particle-doped MgB2 decreases significantly when compared with that of air-cooled one. The behavior was because flux-pinning effect of grain boundaries decreases and fluxpenetrating promotion effect of them increases, which are considered to be caused by increased coherence length of the superconductor. Keywords MgB2 · Flux pinning · (Fe · Ti) particles · Grain boundary · Planar defects · TEM · Water quenching
1 Introduction MgB2 has been known a superconductor which has a weak field dependence of magnetization [1–3]. In order to overcome the weakness, many researchers tried to improve field dependence of magnetization by doping method [4–6]. We also have been studying the properties of MgB2 doped with (Fe, Ti) particles of which radius is 163 nm on average [7]. The best performance was obtained on 5 wt.% (Fe, Ti) particle-doped MgB2 among the various specimens. Although the specimen showed considerable results as shown in Fig. 1, we still have a desire that diamagnetic property of the superconductor would increase more on higher field (6.0 T) than that of air-cooled 5 wt.% (Fe,
H. B. Lee
[email protected] 1
Department of Physics, Pusan National University, Busan, 46241, South Korea
Ti) particle-doped MgB2 . On the other hand, we had a question for what kinds of change occur on M-H curve if grain boundaries increased under the condition that volume defects are fixed. To solve the question, we chose water quenching (WQing) method. Generally, if a WQing is carried out for materials, there is no time for grains to grow due to the rapid cooling rate. Thus, grains of the material are become finer, and grain boundary (GB) density of the material increased. And increased GBs mean increasing the numbe
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