N. Kuroda

Vortex dynamics in Bi2(Sr,La)2CuO6+δ and Bi2Sr2CaCu2O8+δ single crystals with columnar defects

Abstract Vortex dynamics is studied in La-substituted Bi 2 Sr 2 CuO 6+ δ (Bi-2201) and Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) single crystals irradiated with 3.8 GeV 181 Ta 37+ ions through the relation between the frequency and the loss peak temperature in the temperature dependence of the imaginary part of AC magnetic susceptibility χ ″. In unirradiated Bi-2201 and Bi-2212, the behavior of thermally activated flux flow or creep is observed. The Bose-glass behavior is observed at fields below the matching field B Φ in irradiated Bi-2212. On the other hand, the Bose-glass behavior is not observed in irradiated Bi-2201. This difference is due to the extremely weak coupling between pancake vortices along the c -axis in Bi-2201 even under the presence of columnar defects. The vortices in Bi-2201 irradiated with heavy ions are almost decoupled, resulting in the occurrence of thermally activated vortex creep.

Radiation annealing induced by electronic excitation in iron

Abstract The interactions between energetic particles and iron target have been studied. Polycrystalline iron thin films were irradiated at ∼77 K with ∼1 MeV ions, ∼100 MeV heavy ions and GeV heavy ions. Defect accumulation behavior during irradiation was observed by measuring the increment in electrical resistivity of the specimen as a function of ion fluence. The defect annihilation cross-section for each irradiation was derived from the defect accumulation behavior. After irradiation, the defect recovery spectrum for each irradiation was obtained by raising the specimen temperature up to ∼300 K at a constant heating rate (∼2 K/min). Focussing on the radiation annealing, i.e. annihilation of defects during irradiation, the effects of electronic excitation in iron are discussed.

Pinning properties of quenched and melt growth method-YBCO bulk samples

A comparison between two different irradiation effects was accomplished on bulk YBCO samples synthesized with the quenched and melt growth method (QMG-YBCO) to investigate strong pinning properties. High-energy proton-irradiation introduces small defects comparable to unit cell size into the sample, and they act as effective pinning centers for all temperature range in low field around 1 T. No enhancement, however, can be seen in higher field range. The defects introduced with the irradiation reinforce the pinning properties of preexisting pinning centers randomly distributing in the sample. Column-like defects with 3.5 GeV Xe-irradiation were introduced but the pinning properties show no significant enhancement except higher temperature region. This indicates that pre-existing pinning centers are strong enough than the columnar defects. In higher field region, the contribution of columnar defects for pinning becomes relatively large. For both irradiation cases, almost pinning properties are determined by the pre-existing pinning centers.

Effect of heavy ion irradiation on flux pinning properties of Ag-Cu alloy sheathed Bi-2223 tapes

We investigated the co-existing effects of columnar and point defects in Hf-doped Ag-Cu alloy sheathed Bi-2223 tapes. The irreversibility lines are appreciably improved compared to the individual effect by Hf doping or columnar defects.

Effect of damage morphology on the pinning and vortex dynamics in Bi2Sr2CaCu2O8+δ irradiated with GeV heavy ions

Abstract Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) single crystals have been irradiated with swift heavy ions: 0.7 GeV 84 Kr, 3.8 GeV 181 Ta and 3.1 GeV 209 Bi. The critical current density, J c , deduced from the DC magnetization measurement tends to increase with the size of columnar defects. The Bose-glass behavior is observed in vortex dynamics for 3.8 GeV 181 Ta or 3.1 GeV 209 Bi ion irradiation. For Kr irradiation, however, a deviation from the Bose-glass behavior occurs. Effects of damage morphology on the pinning and vortex dynamics are discussed.

Magnetic Properties of Bi-2223 Tapes Irradiated by Xe Ion

We have investigated the magnetic properties of Ag-Cu alloy sheathed Bi-2223 tapes where Hf impurities are included in the pristine alloy We introduced columnar defects in the tapes parallel to the aligned c-axis (normal to the tape surface) by Xe-ion irradiation. We found that the irreversibility line in the heavy-ion irradiated tapes showed the shift toward higher fields. Moreover, the irreversibility curve of the irradiated sample is located in the higher temperature side of the irradiated Ag-sheathed tape (without Hf impurities in the tape) even in the higher magnetic field than the matching field. In addition, the improved irreversibility line is observed in the configuration of the field parallel to the tape (i.e., perpendicular to the columnar defects).

Vortex State of Heavy Ion Irradiated Ni-doped Bi-2212

We have investigated the magnetic properties of Bi2Sr2Ca(Cu2-xNix)20y (Bi-2212) single crystals (x=0, 0.006, 0.01) with columnar defects parallel to the c-axis introduced by Xeion irradiation. We found that the irreversibility line in heavy-ion irradiated Bi-2212 samples showed the shift toward higher fields. In addition, the irreversibility curve of the irradiated sample is slightly affected by Ni-doping in magnetic field below the matching field.

Recoupling Vortices due to Columnar Defects in 3.5 GeV Xe-Irradiated QMG-YBCO

Pinning properties are studied for high-energy heavy ion irradiated quenched melt growth (QMG) YBCO bulk sample. No significant enhancement of critical current densities is observed. Long-term decays of magnetizations, however, show drastic change. The magnetizations become smaller monotonically as the temperatures increase for unirradiated sample in all magnetic field range below 9 tesla. On the other hand, the temperatures recover their absolute values around 50K for the irradiated one. This phenomenon must be originated to columnar defects introduced with the irradiation. This suggests that change of pinning host from inclusions to columnar defects occur suddenly at 50K. Recoupling (or recovery of correlation) along c-axis of vortex occurs with the aid of the columnar defects.

Vortex dynamics in Bi2+xSr2−(x+y)LayCuO6+δ and Bi2Sr2CaCu2Oy irradiated with heavy-ions: correlation between the Bose-glass behavior and the coupling of pancake vortices

Abstract Vortex dynamics is investigated in Bi 2+ x Sr 2−( x + y ) La y CuO 6+ δ (Bi-2201) and Bi 2 Sr 2 CaCu 2 O y (Bi-2212) irradiated with 3.5 GeV 136 Xe 31+ ions through the relation between the frequency and the loss peak temperature in ac susceptibility in order to seek the Bose-glass transition. The Bose-glass behavior is observed at fields below about half of matching filed, B Φ /2 in irradiated Bi-2212. On the contrary to irradiated Bi-2212, the Bose-glass behavior is not observed in irradiated Bi-2201. This is due to the extremely weak coupling of pancake vortices along the c axis in Bi-2201 in spite of the presence of columnar defects. The pancake vortices in Bi-2201 irradiated with heavy-ions are almost decoupled, resulting in the occurrence of thermally activated flux flow or creep.