Critical current density
Jc and its temperature dependence in multifilamentary NbTi wires were measured. Further their pinning behavior was examined. Used samples which have been fabricated by a double stacking method are divided into two groups. One group was heat-treated in the fabrication process in order to produce the precipitates of α-Ti and the filament diameter
df of NbTi wires ranges from 0.39 to 1.53μm. For the other group, such a heat-treatment was not done and the
df of some samples was reduced to the level of below 0.1μm. In this paper, the experimental results on the former group, namely heat-treated (HT) samples with Cu-CuNi mixed matrix or CuNi matrix, are given. The
Jc of samples is affected by
df. In case of 4.2K, these samples show a slight difference in
Jc at high fields. As the temperature rises, the effect of
df on
Jc becomes remarkable and the
Jc-
B curves are very different among samples. Normally, samples with small
df show low
Jc values and reveal a rapid decrease in
Jc with increasing magnetic field. An anomalous change in
Jc at low fields of below 0.1T was seen at temperatures close to
Tc for samples with sub-micron sized filaments. The pinning force
Fp vs. reduced field
b(=
B/
Bc2) curves at 4.2K for different samples coincide with each other except for the sample with
df=0.39μm. The reduced pinning curves (
Fp/
Fp, max vs.
b) obtained at various temperatures have similar shapes for each sample. The
bm where the maximum of
Fp takes place is around 0.35 except for the sample with
df=0.39μm. The pinning properties of these samples are discussed in terms of their upper critical field
Bc2 and transition temperature
Tc.
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