Sensors and Actuators A 106 (2003) 240–242 The temperature dependence of the magneto-impedance effect in the Co-based amorphous wires A.A. Rakhmanov a , N. Perov b,∗ , P. Sheverdyaeva b , A. Granovsky b , A.S. Antonov a a Institute of Theoretical and Applied Electrodynamics, IHT RAS, Izhorskaya 19, Moscow 127412, Russia b Faculty of Physics, Moscow State University, Vorob’evy Gory, Moscow 119899, Russia Abstract We used CoFeSiB amorphous wires to investigate the influence of heating and tensile stress on the magneto-impedance value. Mea- surements were performed with as-cast wires without cover of 100 m diameter and 7 mm length. Experiments were carried out at current frequencies 10, 30 and 50 MHz, in temperature range from 23 to 180 ◦ C. © 2003 Elsevier B.V. All rights reserved. 1. Introduction Amorphous materials attract attention during last 15 years due to their outstanding properties. One of the most sig- nificant is the giant magneto-impedance (GMI). Interest in GMI is supported by the possibility to use it for magnetic field sensors [1,2]. These devices may be extremely sensi- tive (up to 10 -6 Oe and more) and have a quick-response to magnetic field. Unfortunately, the problems of producing the amorphous material are not yet solved. The main diffi- culties are that the parameters of the sample vary in the time and the samples are sensitive to the external influence (such as annealing (for example [3]), external stresses [4–6], and heating [7]). In this paper, we investigate the influence of heating and longitudinal stress on the GMI of the Co-based amorphous wire. 2. Experiment We performed experiments using a standard spectrum an- alyzer HP4395A. For temperature measurements a special cell with a heater was constructed made from bifilar coiled nickel–chromium wire and a temperature-sensitive resistor (Fig. 1). This cell allowed the experiment to be carried out at a temperature range from 23 up to 180 ◦ C at current fre- quencies up to 100 MHz. For the experiment with the applied tensile stress, the temperature cell was modified. One end of the wire was free and connected to the cell by elastic wires ∗ Corresponding author. E-mail address: perov@magn.ru (N. Perov). (thin, but consisting of a number of wires). Towards this end of the wire a fiber with a different load was linked. For the investigation, we used (Co 0.94 Fe 0.06 ) 77.5 Si 12.5 B 15 pieces of amorphous wire (presented by Prof. H.A. Davies, Sheffield, UK) with the diameter 100 m and the length 7 mm. It was the as-cast wire without cover. Measurements were performed at the frequencies 10, 30 and 50MHz. We measured a real part (R) and an imaginary part (X) of the impedance (Z) with respect to a longitudinal magnetic field (H). The MI effect was calculated as Z/Z = [Z(H) - Z(H = 0)]/Z(H = 0). The external dc magnetic field changed from -35 to +35 Oe. In order to analyze the MI effect in the sample, we used the maximum magnitude of MI, that is (Z/Z) max 100% (GMI max, Figs. 2–3). The temperature measurements were performed as fol- lows. The sample was heated and GMI was measured. Then the sample was cooled down to room temperature and the GMI was measured again. After that the sample was heated up to the next temperature. To avoid the magnetostriction in- fluence during heating, we fixed the sample on the substrate in a slightly bent position. The experiment with the applied stress was carrying out by the same way. The tensile stress was applied and GMI was measured, then the stress was Fig. 1. Cell for the temperature measurements. (1) Cell; (2) substrate with the sample; (3) heater; (4) ceramic tubes; (5) mica and glue; (6) Ni chrome; (7) thermistor; (8) thermo-conducting paste. 0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-4247(03)00175-4