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Chapter 12:
Magnetoimpedance in Multilayered Films for Miniature Magnetic Sensors
The discovery of the magnetoimpedance (MI) effect in 1994 had a strong impact on the development of micromagnetic sensors operating in the nano-tesla range. In certain soft magnetic materials, such as composites of amorphous thin wires, the impedance change (MI ratio) is in the range of 50 - 100% in the megahertz frequency band for external magnetic fields of a few oersted. However, when decreasing the size of the sensor element, the maintenance of such high sensitivity becomes a major concern. Special thin-film structures have been proposed and fabricated to improve the MI performance in miniature elements. This chapter concerns the principal advantages of MI in magnetic∕metallic multilayered materials and their applications to magnetic sensing technology. Physical concepts, theoretical analysis based on the field-dependent surface impedance matrix, experimental results, and sensor designs are discussed. These encompass multifold enhancement of the MI ratio, a considerable extension of the operational frequency range, and the field symmetry of the MI behavior. Layered systems allow special types of magnetic anisotropy (either transverse or crossed) to be established in order to realize antisymmetric and asymmetric MI effects. This property is of a particular interest for magnetic-sensor applications.
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