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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]

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]