Magnetoelastic sensor based on GMI of amorphous microwire A.F. Coben Äo a,* , A. Zhukov b,c , J.M. Blanco a , V. Larin c , J. Gonzalez d a Dpto. Electro Ânica y Telecom. and Dpto. Fõ Âsica Aplicada I, EUITI, UPV, Avda. Felipe IV 1B, 20011 San Sebastia Ân, Spain b Donostia International Physics Centre, P.M. De Lardizabal, 4, 20018 San Sebastian, Spain c ``TAMag'' SL, C/ Jose  Abascal 53, Madrid, Spain d Dpto. Fõ Âsica de Materiales, Fac. Quõ Âmica, UPV, P.O. Box 1072, 20080, San Sebastia Ân, Spain Abstract The magnetoelastic sensor based on the stress dependence of the GMI effect in Co 68.5 Mn 6.5 Si 10 B 15 amorphous microwire has been introduced. This amorphous microwire after adequate heat treatment shows the maximum GMI ratio DZ/Z) m up to around 130%. GMI effect of this microwire has been found to be affected by the magnetoelastic anisotropy induced in the sample by the applied tensile stress. This stress dependence of the GMI induces changes on the ac voltage measured between the ends of the sample placed in the magnetic ®eld under applied tensile stress. When the sample is under a load of 3 g such change of the ac voltage across the microwire is about 3.5 V. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Amorphous glass-coated microwires; Giant magneto-impedance effect; Magnetoelastic sensor; Magnetoelastic anisotropy 1. Introduction Amorphous and nanocrystalline glass-coated microwires became recently a new family of soft magnetic materials with many possible technological applications [1,2]. Such applications are related with the magnetic bistability observed in a very short tiny magnetic microwires [1] and with the GMI effect [3]. This GMI effect consists of a large change in the impedance of a magnetic conductor placed in the axial dc magnetic ®eld, H. The large sensitivity of the magneto-impedance on dc magnetic ®elds originates from the dependence of the transverse magnetic permeability upon the dc longitudinal magnetic ®eld and skin effect. It was demonstrated recently that applied tensile stresses modify signi®cantly the magneto-impedance response of conventional amorphous wires [4,5]. The main technologi- cal interest is related to such high sensitivity of the magneto- impedance to an applied magnetic ®eld, which reaches up to 300% relative change of impedance in conventional amor- phous wires. At the meantime, the GMI effect of the microwires is generally lower less than 60%) [3]. Never- theless, it was found quite recently, that excellent magnetic softness could be achieved in glass-coated microwires in nearly Ð zero magnetostrictive compositions after adequate heat treatment and after careful election of their composition [6]. In the present paper, we report on high stress dependent GMI effect in amorphous Co 68.5 Mn 6.5 Si 10 B 15 microwires and its application for the magnetoelastic sensor. 2. Experimental technique and samples Sample composition has been carefully selected among a series of Co 69x Mn 6x Si 10 B 15 0 < x < 1) compositions in order to achieve the best magnetic softness in as-prepared state, as described in [6]. Consequently, the best magnetic softness has been observed for the Co 68.5 Mn 6.5 Si 10 B 15 composition see Fig. 1) with small and negative constant [8]. Accordingly, the sample with x  6:5% Mn has been selected for further processing and experimental studies of the GMI effect. The microwire has a diameter of 16.5 mm 7.5 mm diameter of metallic nucleus and 4.5 mm of glass- coated thickness). Pieces of 100 mm length have been pre- annealed at 1008C for 1 h under axial magnetic ®eld of 14 kA/m in order to enhance their initial magnetic perme- ability [6]. Consequently, enhanced GMI effect is expected after such thermal treatment due to signi®cant enhancement from 2000 to 12 000) of the initial permeability [6]. The electrical impedance of the microwire was evaluated by means of the four-point technique. Microwire sample after mechanical removing of the glass-coating at the ends was soldered into the impedance measurement circuit. Magnetic measurements before and after soldering showed Sensors and Actuators A 91 2001) 95±98 * Corresponding author. 0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0924-424701)00502-7