Journal of Magnetism and Magnetic Materials 157/158 (1996) 432-433 Iournal Of "."_" magnetism and magnetic materials ELSEVIER The magnetoimpedance effect in rapidly solidified soft magnetic fibers P. Ciureanu a, * p. Rudkowski b G. Rudkowska b, D. Menard a J.F. Currie a 9 9 ~ 9 J.O. StrSm-Olsen b, A. Yelon a a Ecole Polytechnique, Engineering Physics Department, P. O. Box 6079, Station Centre-ville, Montreal, Quebec, H3C 3A7, Canada b McGill University, Physics Department, 3600 University, Montreal, Quebec, H3A 2T8, Canada Abstract Soft magnetic fibers, 20 to 50 /xm in diameter, have been cast by melt extraction. A giant magnetoimpedance effect (GMI) of about 60% was observed in a (NiCo)70FeSiBMn fiber driven by a rf current. The magnetic field and frequency responses of fibers depend on the density and the frequency of the drive current. The GMI effect at saturation decreases from 70% at 3 /~A//xm 2 to 55% at 30 ~A//xm 2. The saturation field increases with an increase in frequency, and is larger for lower current densities. These fibers can be used as sensing dements for a new generation of magnetic field sensors. Keywords: Giant magnetoimpedance effect; Rapid solidification; Soft magnetic fibers Melt extraction is now a well established procedure [1,2]. A rotating wheel extracts fibers from a molten alloy; the fiber solidifies radially with respect to the wheel. The high rate of solidification leaves the fiber in a state of high quenched-in stress. This stress distribution produces a domain structure consisting of principal domains, which are perpendicular to the fiber length, and of longitudinal and circumferential closure domains at the fiber surface [31. Ultra-soft amorphous fibers with near-zero magne- tostriction and a composition of Ni4sCo2sFe6Si9Mn2B13, have been prepared and measured. The average diameter is 30 /xm. The resistivity of the fiber is about 150 /z/2 cm, ten times higher than that of Permalloy, the prototypical anisotropic magnetoresistance effect (AMR) material. No significant AMR effect was observed in amorphous fibers, maybe due to this high resistivity. On the contrary, a giant magnetoimpedance effect of about 60% was measured in NiCo fibers. This effect consists of a significant decrease in the voltage drop across the fiber when a longitudinal dc magnetic field of about 7 kA/m is applied to the fiber, driven by a sinusoidal current of 5 to 20 mAp_p, at frequencies from 0.5 to 100 MHz. The relative change in impedance, A Z/Z, is proportional to the relative change in * Corresponding author. Fax: +514-340-3972 or +514-340- 3706; email: ciureanu@lisa.polymtl.ca. the voltage drop across the fiber. The GMI effect is negative, and defined as AZ AU Uf(Hsat) - Uf(H = O) Z U Uf(H = O) where Hsat is the saturating field and Uf the voltage drop. The variation of the modulus of GMI as a function of the frequency of the driving current for a 28 /zm diameter fiber is shown in Fig. 1. The fiber was driven by a 5 mAp_p sinusoidal current and the current density was low (8 /xA//zm2). The voltage drop, Uf, in zero field, in- creases with increasing frequency, has a maximum of 380 mV at 30 MHz and then slightly decreases. When the fiber is subjected to a saturating longitudinal field, the voltage 80 ---D-- AT-.JZ; 5mA ....O---- Uf(H=0); 5mA ....~---- Uf(Hsat); 5rnA 500 -400 60 ......... ...................... ; ............... . Do i : -300 ~f40 ~ {- Cy~...TC) "-O' ii - 200~' <~ 20 ~ :Z ~=:~:~:.:;,~:~.-.,~&-~,-,~C.-,~e,.~ _ 100 0 , ,,11 ........ iI ...... 0 1 10 100 f (MHz) Fig. 1. The GMI effect and the voltage drop across the fiber in zero and saturating field versus the frequency of the driving current (low current density). 0304-8853/96/$15.00 Copyright © 1996 Elsevier science B.V. All rights reserved. SSDI 0304-8853(95)01044-0