Flux-Pinning Effects and Mechanism of Water-Quenched 5 wt.% (Fe, Ti) Particle-Doped MgB2 Superconductor

We have studied magnetic behaviors of water-quenched 5 wt.% (Fe, Ti) particle-doped MgB2 compared with those of air-cooled 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 flux-pinning 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 flux-pinning 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 flux-penetrating promotion effect of them increases, which are considered to be caused by increased coherence length of the superconductor.