Point and extended defects in superconductors

The effect of point defects created by γ‐irradiation and extended defects created by Pb‐ion irradiation on the behavior of the critical current density, J_c in several superconductors have been investigated. We have found a limitation of the role of both types of defects on enhancing J_c. We report an explanation of the effect of γ‐irradiation and Pb‐ion irradiation doses on J_c in these materials. The explanation is based on combining several competing mechanisms of irradiation which we believe to take place mainly in the regions of the grain boundaries. The influences of these mechanisms were found to vary with the irradiation dose level. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of Pb-ion irradiation on the vortex pinning in melt-textured YBa2Cu3Ox

Melt-textured YBa₂Cu₃O_x quasicrystalline samples were studied for the effects of Pb-ion irradiation on the vortex pinning. The strength of the pinning was monitored by measurements of the saturation remanent magnetization (M_R) as a detailed function of temperature from 4.2 K up to T_c (≈92 K). Our results show that M_R is greatly enhanced by the Pb-ion irradiation at all temperatures below T_c. Specifically, when compared to a similar but nonirradiated sample, the M_R of an optimally irradiated sample is raised by a factor of 3.4 at 4.2 K and, more strikingly, by 12.7 at 77 K. Contrastingly, in an earlier study of a γ-ray irradiated Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ sample, the vortex pinning was seen to be enhanced but mainly near 4.2 K, suggesting that the γ-ray irradiation produces highly localized pinning sites. Hence, our present results imply conversely that the Pb-ion irradiation creates extended structural defects that are effective as vortex pinning sites up to relatively high temperatures, including that of liquid nitrogen.     

Transitions between spin-glass and ferromagnetic (or related) states in disordered Ni-Mn alloys

Magnetic measurements reveal that the alloy Ni₇₈Mn₂₂ undergoes a field-induced spin-glass-to-ferromagnetic transition at a critical field that decreases from ≈ 140 Oe to zero as the temperature is raised from 4.2 to 40 K, its ferromagnetic Curie point being ≈ 270 K. The alloy Ni₇₄Mn₂₆ is found to have a related state of infinite initial susceptibility but no spontaneous magnetization between ≈ 80 and ≈ 155 K. Below 40 and 80 K, respectively, Ni₇₈Mn₂₂ and Ni₇₈Mn₂₂ exhibit the thermoremanent properties of a spin glass.