We studied a method of measuring upper critical field (H c2 ) of a superconductor based on the width of ∆H = ∆B region, which appears in the superconductor that volume defects are many and dominant. Here we present the basic concept and details of the method. Although H c2 of a superconductor is fixed according to kind of the superconductor, it is difficult to measure H c2 experimentally, and the results are different depending on the experimental conditions. H c2 was calculated from the theory that pinned fluxes at volume defects are picked out and move into an inside of the superconductor when their arrangement is the same as that of H c2 state of the superconductor. H c2 of MgB 2 obtained by the method was 65.4 Tesla at 0 K. The reason that H c2 obtained by the method is closer to ultimate H c2 is based on that ∆F pinning /∆F pickout is more than 4 when pinned fluxes at volume defects of 163 nm radius are picked out. The method will help to find the ultimate H c2 of volume defect-dominating superconductors.
MgB 2 samples were prepared from a stoichiometry mixture of Mg and B inside stainless steel tubes. The transition temperature of the specimens was 37.5 K with a sharp transition width within 1 K. From the magnetic hysteresis measurements, flux jump was observed up to 15 K. The flux jump is believed to begin at the fluxes pinned at the defects. An over-moving flux situation formed at the places where there were moving fluxes that had been pinned at the defect and steady state ones. The flux jumps depend not on the amount of impurities and second phases, but on their distribution, pinning strength and heat capacity.
We have studied flux-pinning effects of MgB 2 superconductor by doping (Fe, Ti) particles of which radius is 163 nm on average. 5 wt.% (Fe, Ti) doped MgB 2 among the specimens showed the best field dependence of magnetization and 25 wt.% one did the worst at 5 K. The difference of field dependence of magnetization of the two increased as temperature increased. Here we show
We have studied flux-pinning effects of $$\text {MgB}_2$$
MgB
2
superconductor by doping (Fe, Ti) particles of which radius is 163 nm on average. 5 wt.% (Fe, Ti) doped $$\text {MgB}_2$$
MgB
2
among the specimens showed the best field dependence of magnetization and 25 wt.% one did the worst at 5 K. The difference of field dependence of magnetization of the two specimens increased as temperature increased. Here we show experimental results of (Fe, Ti) particle-doped $$\text {MgB}_2$$
MgB
2
specimens according to dopant level and the causes of the behaviors. Flux-pinning effect of volume defects-doped superconductor was modeled in ideal state and relative equations were derived. During the study, we had to divide M-H curve of volume defect-dominating superconductor as three discreet regions for analyzing flux-pinning effects, which are diamagnetic increase region after $$\text {H}_{c1}$$
H
c
1
, $$\Delta \text {H}=\Delta \text {B}$$
Δ
H
=
Δ
B
region, and diamagnetic decrease region. As a result, flux-pinning effects of volume defects decreased as dopant level increased over the optimal dopant level, which was caused by decrease of flux-pinning limit of a volume defect. And similar behaviors are obtained as dopant level decreased below the optimal dopant level, which was caused by the decreased number of volume defects. Comparing the model with experimental results, deviations increased as dopant level increased over the optimal dopant level, whereas the two was well matched on less dopant level. The behavior is considered to be caused by the segregation of the volume defects. On the other hand, the cause that diamagnetic properties of over-doped $$\text {MgB}_2$$
MgB
2
specimens dramatically decreased as temperature increased was the double decreases of flux-pinning limit of a volume defect and the segregation effect, which are caused by over-doping and temperature increase.
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