Magnetoimpedance (MI) in amorphous wires: new materials and applications

Abstract The discovery of the magnetoimpedance (MI) effect in 1994 had a strong impact on the development of magnetic sensors. Along with traditional areas of sensing applications (data storage, bio‐medical electronics, robotics and security), the MI elements have a high potential for applications in smart sensory systems (self‐sensing composites) operating at microwave frequencies owing to still very large MI ratios of 50–100% in Co‐rich amorphous wires at GHz frequencies. Here we introduce two types of MI wire composites: 2D‐arrays and mixtures of wire pieces. In such materials the effective permittivity has strong dispersion in a frequency band determined by a plasma frequency or a dipole resonance, respectively. If MI wires are used as constituent elements, this dispersion may be very sensitive to the magnetic properties of wires since the wire impedance determines the relaxation parameter of the effective permittivity. For example, increasing the wire impedance by establishing the magnetisation along the axis with an external magnetic field broadens the resonance band, decreases the reflection amplitude and may open a bandpass. Depending on the magnetic anisotropy in wires, tuning can be realized with both magnetic field and stress. It is foreseen that MI‐wire composite materials could be suitable for large scale applications, in particular, for free space filters in secure wireless systems and for microwave nondestructive testing and control in civil engineering. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)