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We present calculations of the alternating current (ac) susceptibility in rectangular thin-film type-II superconductors, based on the critical-state model and on the minimization of magnetic energy. First, we simulate a homogeneous superconductor and give an analytical approximate expression for both real and imaginary parts of the ac susceptibility as a function of the amplitude of the ac applied field. Second, ac susceptibility is calculated in a superconductor composed by two parts connected by a region of lower critical-current density. We find two peaks can appear in the imaginary ac susceptibility although, because of the strong demagnetizing fields, these peaks overlap in some cases. Detailed explanations of the imaginary part of the ac susceptibility are given. [ABSTRACT FROM AUTHOR]

The magnetic vortex formation at room temperature and its evolution under in-plane magnetic field is studied in chemically grown self-assembled La0.7Sr0.3MnO3 nanoislands of less than 200 nm in width. We use variable field magnetic force microscopy and numerical simulations to confirm that the vortex state is ubiquitous in these square-base pyramid shape epitaxial La0.7Sr0.3MnO3 nanostructures, and that it requires in-plane magnetic fields below 40 kA/m to be annihilated. [ABSTRACT FROM AUTHOR]

In order to better study the electromagnetic properties of superconducting films, the perpendicular critical-state ac susceptibility χ=χ′-jχ″ of a square superconducting film is numerically calculated using a magnetic energy minimization procedure as a function of the field amplitude Hm. The emphasis is put on the calculation accuracy of the low-Hm limit χ0 and the Hm at χ″=χm″. It is shown that after having accurate χ0, χ(Hm) of the square film can be accurately scaled from the analytical χ(Hm) of a thin disk. [ABSTRACT FROM AUTHOR]

Abstract: A set of analytic expressions for the critical-state magnetization and ac susceptibility curves of rectangular superconducting films of arbitrary aspect ratio in perpendicular field is obtained. They are formulated based on the exact/accurate high/low-field limits for rectangular films together with the exact analytic formulas for thin circular disks. Thus, the accuracy of the expressions is very high in high and low fields. At a characteristic intermediate field amplitude, where takes its maximum, the error is checked by direct numerical calculations to be well below 1%. Thus, the provided expressions can be conveniently and satisfactorily used for practical and theoretical researches of superconducting materials, such as coated conductors. [Copyright &y& Elsevier]