Magnetoimpedance-Microstructures

Abstract

This thesis concerns the development and investigation of giant magnetoimpedance (GMI) FeCoB amorphous thin films for magnetic field micro-sensing applications in cooperation with Siemens AG Erlangen. The thin films were produced by sputter deposition in the form of a trilayer microstructure with an inner Cu lead. The effects of field annealing were investigated by measurements performed before and after annealing.<br />The frequency dependence of the magnetic permeability of the material was investigated with the help of a permeameter. The hysteresis measurements and the domain imaging were performed using the magneto-optical Kerr effect. The thin films exhibit good magnetic properties, which are essential for magnetic thin film microsensors with high sensitivity and accuracy. The dependence of the films impedance on frequency and on an externally applied magnetic field was measured by means of a network analyzer, a pair of Helmholtz coils, a current supply and a computer. The thin films show promising GMI response.<br />A magnetic field measurement system using the GMI sensor as a phase shift element has been designed and tested. The GMI sensor is integrated in an oscillator circuit. The measurement system detects the phase change caused by the magnetic field to be measured yielding an effective field sensor system of high sensitivity.<br />The prediction of the overall magnetic properties of the performance of the GMI thin film sensor, before and after field annealing, was performed applying an energetic model. The model is based on considerations of energy balance and statistical domain behaviour. The parameters are related to macroscopic hysteresis features such as coercivity, initial susceptibility, saturation and remanence. The calculated M (H) curves perpendicular to the longitudinal axis of the strip (which should be the hard axis direction), before and after annealing, are in good agreement with the measurements allowing several predictions of parameters as sensitivity, hysteresis and misalignment of the easy axis.<br />