Abstract
We have studied flux-pinning effects of text {MgB}_2 superconductor by doping (Fe, Ti) particles of which radius is 163 nm on average. 5 wt.% (Fe, Ti) doped text {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 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}, Delta text {H}=Delta text {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 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.
Highlights
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
Inspecting Region III in M-H curves as shown in the figure, the decrease rate of diamagnetic property along applied magnetic field was lower as a width of H = B region was wider
We studied flux-pinning effects of (Fe, Ti) particle-doped MgB2 specimens according to dopant levels of (Fe, Ti) particles, of which radius is 163 nm on average
Summary
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. Investigating field dependences of magnetization (M-H curves) of the doped MgB2 specimens, 5 wt.% doped specimen showed the best M-H curve and M-H curves of other doped specimen became poorer as dopant level increases or decreases. The behavior means that there was the optimal dopant level for the best M-H curve when MgB2 were doped with the (Fe, Ti) particles. M-H curves of current specimens, which are one of volume defect-dominating superconductors, showed quite different behavior from that of ideal s uperconductor[11,12,13,14,15,16]. The cause of the distinction is that M-H curves of real superconductors are heavily influenced by flux-pinning phenomena of defects and each region has different flux-pinning mechanism
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