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Several model granular systems are adopted for the imaging of the current percolation, as well as the Ac losses in the artificially granular YBCO thin films. The analysis helps in providing a better understanding of the magnetic behavior of more complex systems like coated conductors.

Theoretically the AC susceptibility of superconducting tapes made of an array of strips, as function of the vertical and horizontal separation between the strips is discussed. A distinction has been made for the cases of interconnected strips and isolated ones.

The force generated when a superconductor is displaced laterally is calculated and reported.

The transverse relaxation time T2 of protons in water suspensions of iron-oxide particles increases with the waiting time tw after the sample is inserted in the gap of the spectrometer magnet. Such a T2 increase becomes significant if the particles are aggregated into large clusters, for which field-induced formation of cluster-chains will occur and T2 should increase with increasing the length of chains. T2 increases with tw even for small particles, for which no chain formation may be induced, and for large clusters when tw is too small to form long enough chains. The T2 increase is accompanied by a significant echo-time dependence. All this is experimentally and theoretically studied. [ABSTRACT FROM AUTHOR]

Hard magnetic materials are usually measured and used in an open magnetic circuit, for which demagnetizing effects are present. The applied field Ha has been conventionally corrected to the volume averaged internal field H by a formula H=Ha-NmM, where M is volume averaged magnetization and Nm is magnetometric demagnetizing factor calculated for the case of uniform magnetization as a function of body shape. Since Nm>0, such a correction always increases the slope of the descending M(Ha) curve without influencing the value of the coercive field with respect to M. However, we show by numerical calculations that if the hard magnetic material is granular and modeled as a rectangular prism, the magnetostatic interactions among the grains will result in a formula H=Ha-NghmM+ΔHc, where the demagnetizing factor for granular hard magnets Nghm can be either positive or negative and the usually positive coercivity increment ΔHc is correlated to collective magnetic reversal of longitudinal grain chains. [ABSTRACT FROM AUTHOR]

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]

In this work we present a model for the calculation of the magnetic properties of superconductors of orthorhombic shape in the perfect shielding state when an external uniform magnetic field is applied in the direction of one of the principal axes of the sample. Our model accounts for demagnetization effects and it is free of fitting parameters and boundary value conditions. We consider planar linear circuits that lie perpendicular to the direction of the applied field. Calculation of the value of the currents is based on magnetic energy minimization. The model is proved to be accurate enough to reproduce experimental results as long as the dimension along the applied field is not much lower than the other dimensions. Calculations of surface currents, as well as measurements and calculations of magnetization and initial susceptibility, are reported. We also present an empirical formula that provides a good fit to the initial susceptibility of a general sample of orthorhombic shape. Demagnetization effects observed on the experimental results are explained in terms of the induced currents in the superconductor. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]