Coupled core–shell model of magnetoimpedance in wires

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Abstract
Magnetoimpedance (MI) has been studied extensively in soft magnetic wires and plates. Although a general theoretical basis has evolved, several details remain poorly understood. In particular, the amplitude of the effect in the low field region has proven impossible to fit within current models which assume a uniform static magnetization within the material. In this article, we present magnetization and MI data on CoFeSiBNb melt-extracted wires and conclude that the behavior of these materials can be analyzed on the basis of a core–shell magnetic structure. This approach introduces a nonuniform magnetization into the MI theory in such wires. We calculate the static magnetic configuration in the presence of an exchange coupling between the two regions and use it to solve for the dynamical magnetization of the outer shell using the Landau–Lifshitz and Maxwell equations to obtain the impedance as a function of the applied field and frequency. The agreement for the MI between theory and experiment is greatly improved from that of previous models.

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