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The intrinsic pinning properties of FeSe 0.5 Te 0.5 , which is the superconductor with T c of about 14 K, were studied by the analysis of magnetization curves by the extended critical state model. In the magnetization measurements by SQUID magnetometer, the external magnetic fields were applied parallel and perpendicular to c-axis of the sample.The critical current density J c 's under the perpendicular field of 1 T were estimated by using the Kimishima model as about 1.6 × 10 4 , 8.8 × 10 3 , 4.1 × 10 3 , and 1.5 × 10 3 A/cm 2 at 5, 7, 9, and 11 K, respectively, and the temperature dependence of J c could be 2 fitted with the exponential law of J c (0)×exp(-αT /T c ) up to 9 K and power law of J c (0)×(1-T / T c ) n near T c .

It is known that the peak effect in Y-123 superconductors is caused by oxygen deficient regions with lower T c . In order to study the flux pinning mechanism of these defects, the effect of the addition of 211 particles on the peak effect was measured. It was found that the addition of the 211 phase deteriorated the pronounced peak effect at medium magnetic fields. This suggests that the pinning mechanism of the oxygen deficient regions is repulsive and based on the kinetic energy interaction under the proximity effect. The peak effect itself is considered to be caused by a transitional variation in the elastic property of flux lines. In the low-and high-field regions, the dominant pinning centres are 211 particles. The observed results on the critical current density and the irreversibility field were compared with the theoretical predictions of the flux creep-flow model, assuming pinning by the 211 particles.

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