Impedance Spectroscopy in Ferromagnetic Materials
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Impedance Spectroscopy in Ferromagnetic Materials
Raul Valenzuela Institute for Materials Research National University of Mexico P.O. Box 70-360, Mexico, 04510, Mexico ABSTRACT Impedance spectroscopy (IS) is a powerful characterization methodology for a wide range of electric materials such as ceramics, ferroelectrics, ion conductors, piezoelectrics, etc. In this paper, an extension of IS to the case of ferromagnetic materials is presented. In particular, the resolution of the most common magnetization processes (domain wall bulging, domain wall displacement and spin rotation) is analyzed. IS is applied to ferrites and giant magnetoimpedance in amorphous wires. INTRODUCTION Impedance spectroscopy (IS) is a powerful characterization technique for a wide variety of electrical materials [1]: ceramics, ferroelectrics, ion conductors, piezoelectrics, etc. The main advantage of IS is that under certain conditions, it allows the resolution and analysis of the various polarization processes. In polycrystalline materials, for instance, IS leads to the separation of the impedance contribution of grains from that of the grain boundaries. Also, ionic conductivity can be easily distinguished from electronic conductivity. Ferromagnetic materials are in many aspects the magnetic counterpart of ferroelectric materials. It was therefore very tempting to test the aplicability of IS to ferromagnetic materials. The results of this extension is presented here. The developments leading to the resolution of the magnetization processes are exposed in an essentially chronological order. Some case examples are briefly presented: giant magnetoimpedance in amorphous wires and polycrystalline ferrites. EXPERIMENTAL TECHNIQUES Most of the experimental results presented here were obtained with a measuring system that includes an HP 4192 A Impedance Analyzer, controlled by a PC. The software, developed in our laboratory, allows the measurement of 94 points in the 5 Hz-13 MHz frequency range in less than 3 min. In the "longitudinal" geometry measurements, the sample (an amorphous ribbon, or an amorphous wire) is placed inside a solenoid (typically 400 turns) connected to the impedance analyzer. In the "transverse" geometry measurements, the sample is subjected to the AC current produced by the impedance analyzer itself, by insuring the electrical contacts with silver paste. The "magnetoimpedance" measurements were carried out as in the transverse geometry; an additional solenoid (typically 200 turns, powered by a DC current source) is used to apply a DC magnetic field on the sample. Such DC fields were varied between 0 and 64 kA/m (0-80 Oe). R2.1.1
Various types of samples were measured; the first results were obtained on amorphous ribbons of composition Co66Fe4MoB12Si16 (commercially known as "Vitrovac"), kindly provided by Vacuumschmelze, Germany. These materials are produced by rapid solidification techniques (with a thickness about 30 µm, a width of 4-8 mm and typical lengths of 6-10 cm) directly from the melt, to avoid crystallization. Amo
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