Effect of AC driving current on magneto-impedance effect

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Abstract

Magneto-impedance (MI) ratio (ΔZ/Z)=Z(H)−Z(Hmax)/Z(Hmax) has been measured in different materials: amorphous (Fe0.94Co0.06)72.5B15Si12.5 wires and Co68.5Mn6.5Si10B15 microwires and nanocrystalline Fe73.5Si13.5B9Nb3Cu1 ribbons, with axial magnetic field H, intensity I, and frequency f, of the AC driving current as the parameters. The common feature of these samples is that they show a peculiar MI effect with a maximum of (ΔZ/Z) at a certain axial magnetic field value, Hm. Dependencies of (ΔZ/Z) and Hm on the AC driving current amplitude I have been observed: Hm decreases with I. The MI ratio can be optimised by means of choosing an adequate value of I. Particularly, in the case of microwires, (ΔZ/Z) has a maximum at I=2.8 mA. The observed effect is explained taking into account the tensor character of magnetic permeability and the circular magnetisation processes.

Introduction

The recently discovered giant magneto-impedance effect, GMI, became a topic of intensive research in the field of applied magnetism during the last few years 1, 2, 3. The main technical and scientific interest is related to the high sensitivity of the impedance in the range of low applied magnetic fields. Relative changes of impedance around 300% for dc fields of the order of tenths of Oe, with a maximum sensitivity up to 100%/Oe for fields less than 1 Oe were found. This research has been done mainly in amorphous wires with vanishing magnetostriction, although some reports are also dealing with amorphous ribbons 4, 5, thin films [6]and quite recently in microwires 7, 8.

The GMI effect has been interpreted in terms of classical electrodynamics by considering the change in the penetration depth of the AC current flowing in a magnetic conductor caused by the DC magnetic applied field. The frequency, f, of the AC current (which is necessary to evaluate the impedance) must be high enough, typically above 100 kHz. The impedance, Z, for a magnetic conductor is given by 1, 2Z=Rdc(kr)Jo(kr)/2J1(kr)with k=(1+j)/δ, where Jo and J1 are the Bessel functions. δ is the penetration depth given byδ=(πσμφf)−1/2where σ is the electrical conductivity, f the frequency of the AC current along the sample, and μφ the circular permeability assumed to be scalar. The DC applied field changes the penetration depth through the modification of μφ which finally results in a change of the impedance 2, 7.

The main experimental and theoretical activity was devoted to study the effect of axial magnetic field and frequency of the AC driving current on the GMI effect. On the other hand, in some cases, the GMI effect depends on the amplitude of AC driving current, I. Therefore, choosing an adequate I, the GMI effect can be significantly enhanced [8]. Accordingly, the aim of this paper is to present and analyse the experimental data concerning the AC driving current dependence of the GMI effect in different amorphous magnetic materials.

Section snippets

Experimental technique

The impedance was evaluated by means of the four point technique when an AC current with frequency ranging from 0.5 to 1.1 MHz flows along the samples. Its amplitude, I, was varied between 1 and 4 mA for microwires, and 1–25 mA for wires and ribbons. This amplitude, I, has been measured by means of an AC current sensing device CT-1. Electrical contacts were carefully prepared with Ag paint. The glass coating was mechanically removed in the case of glass coated microwires. The magneto-impedance

Experimental results and discussion

The dependencies of ΔZ/Z(H) for amorphous microwires Co68.5Mn6.5Si10B15 measured in as-prepared state and after different heat treatments (between 200 and 350°C) are presented in Fig. 1. A drastic change of the ΔZ/Z(H) dependence with annealing temperature as parameter can be seen. Generally, a peculiar shape of the ΔZ/Z(H) dependence first increasing and then decreasing of ΔZ/Z, i.e., with a maximum at certain magnetic field Hm, is typical for as-prepared Co68.5Mn6.5Si10B15 microwire with

Acknowledgements

A. Zhukov gratefully acknowledges the Gobierno Vasco for a fellowship. The work has been supported by the Departamento de Industria del GV under project OD98UN27 and by the GV under project PI-1997-33.

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1

On leave of the `AmoTec', 277038; Kishihev, Moldova.

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