Within the framework of the thermally activated process of the flux line or flux line bundles, and by time integration of the 1D equation of motion of the circulating current density J (ρ, t), which is suitable for thin superconducting films (R d, λ), we present numerical calculations of the current profiles, magnetization hysteresis loops and ac susceptibility χ n = χ n + iχ n for n = 1, 3 and 5 of a thin disc immersed in an axial time-dependent external magnetic field B a (t) = B dc + B ac cos(2πνt). Our calculated results are compared with those of the critical state model (CSM) and found to prove the approximate validity of the CSM below the irreversibility field. The differences between our computed results and those of the CSM are also discussed.

Ferromagnetism and accompanying large negative magnetoresistance in Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail. Recent structural analysis of (Bi,Pb)2Sr3Co2O9, which has been believed to be a Co analogue of Bi2Sr2CaCu2O 8+δ , revealed that it has a more complex structure including a CoO2 hexagonal layer [T. Yamamoto et al., Jpn. J. Appl. Phys. 39 (2000) L747]. Pb substitution for Bi not only introduces holes into the conducting CoO2 layers but also creates a certain amount of localized spins. Ferromagnetic transition appears at T = 3.2 K with small spontaneous magnetization along the c axis, and around the transition temperature large and anisotropic negative magnetoresistance was observed. This compound is the first example which shows ferromagnetic long-range order in a two-dimensional metallic hexagnonal CoO2 layer.

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Articles you may be interested inElectro-optical characteristics of omnidirectional liquid crystal domain mode using doughnut-shaped slit electrodes

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