Novel broadband measurement technique on PCB cells for the field- and stress-dependent impedance in ferromagnetic wires

Abstract

The lack of broadband impedance measurements has restricted the potential capabilities of ferromagnetic wire composites, which have been mainly evaluated through theoretical impedance calculated from existing models without considering wire domain microstructure. Here, we have developed an accurate method for measuring the impedance of ferromagnetic wires in a wide frequency range, from 100 kHz up to 15 GHz, under magnetic and stress stimuli. The broadband measurements were ensured by a more accurate microwave calibration on the PCB cell, extending the reference planes up to the wire sample ends and by compensating the sample waveguide properties using the phase unwrapping technique proposed in this work. In addition, a specially designed dogbone PCB measurement cell allows both field- and stress-dependent impedance measurements, efficiently transferring the tensile stress from the cell to the wire sample fixed onto it. The measured wire impedance can be then recalculated into the surface impedance, which constitutes the boundary condition in the antenna equation, describing the microwave scattering properties of a wire. The developed approach will make it possible to build a practically predictive scattering theory in composite materials with embedded wire inclusions and thus their implementation in real-world applications, such as remote stress monitoring.