Sensors and Actuators A 106 (2003) 329–332 Giant magnetoimpedance effect enhancement by circuit matching V. Raposo a,∗ , M. Vázquez b , A.G. Flores a , M. Zazo a , J.I. Iñiguez a a Dpto. F´ ısica Aplicada, Universidad de Salamanca, Plaza de la Merced s/n, 37008 Salamanca, Spain b Instituto de Ciencia de Materiales (CSIC), Campus de Cantoblanco, 28049 Cantoblanco, Madrid, Spain Abstract GMI response of amorphous wires can be greatly increased by working at resonant conditions in a LC cell. This feature of the circuit can be exploited to increase the sensibility of current GMI-based sensors while selecting the working frequency. © 2003 Elsevier B.V. All rights reserved. Keywords: Amorphus wires; Magnetic field sensor; Giant magnetoimpedance 1. Introduction In the last few years, a number of magnetic sensors have been developed making use of the giant magnetoimpedance (GMI) effect exhibited by ultrasoft magnetic materials [1–8]. GMI effect is caused by the skin effect as a consequence of the changes in the penetration depth induced by the static applied magnetic field through modification of the transverse permeability. When the frequency is high enough the skin effect plays an important role reducing the effective section of the sample and then increasing the impedance. The GMI ratio is defined as GMI(%) = Z(H) - Z(H max ) Z(H max ) × 100 (1) This change of impedance is a consequence of the modifi- cation of the skin effect penetration depth with the applied field, which can be expressed as: δ =  ρ πfµ φ (2) where f is the ac current frequency, ρ the conductivity and µ φ is the circular permeability. Impedance is typically de- termined by measuring the voltage drop at the ends of the heterogeneous wire while keeping constant the amplitude of the ac current of frequency f flowing through it. Nowadays, the maximum GMI rates of up to 800% has been reported for wire shaped amorphous alloys with vanish- ing magnetostriction and diameter of around 100 m. In fact, ∗ Corresponding author. Tel.: +34-92-3294500-1301; fax: +34-92-3294584. E-mail address: victor@usal.es (V. Raposo). even larger GMI response is actually predicted for ideally ultrasoft samples with improved surface homogeneity [9]. The objective of this work has been centered to develop a GMI measurement system with improved GMI response. The system employs very soft amorphous wires with rea- sonably high GMI response to dc magnetic field but modi- fying the circuitry characteristics in such a way that the final response is notably enhanced compared to the intrinsic one corresponding to the wire alone. The chosen sensing magnetic element is Co 68.15 Fe 4.35 Si 12.5 B 15 amorphous wire 125 m in diameter and 15 cm in length. A maximum intrinsic GMI response of up to 800% in optimum measurement conditions has been obtained. An analysis of the experimental system by classical trans- mission line theory and the established standing waves in the line has been carried out. In order to optimize the sys- tem, there are several alternatives for a fixed frequency, such as controlling the length of the line or introducing a cell with a capacitor to work at the resonant frequencies of the circuit. In fact, a computational analysis of the equivalent transmission line has been performed. To achieve such a goal, a new measuring set-up is intro- duced with an extraordinary sensibility and high signal to noise ratio. Special attention has been taken with the electri- cal contacts in the four-probe technique and the impedance of the line to obtain a resonance at the measurement fre- quency. The obtained experimental results correspond up to 30 MHz. GMI rates up to 3000% are here reported as shown in the Fig. 4. This outstanding GMI response, even though it may be considered as artificial and not intrinsic to the magnetic ma- terial, can be very useful to develop a new family of ultra- sensitive magnetic sensors. 0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-4247(03)00195-X