Effects of bias field and driving current on the equivalent circuit response of magnetoimpedance in amorphous wires

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Abstract
Magnetoimpedance in as-cast, non-magnetostrictive CoFeBSi amorphous ferromagnetic wires, submitted to AC electric current, irms in the 0.1-20 mA range and frequencies between 100 Hz and 100 kHz, is analysed in terms of equivalent circuits. The effects of the bias longitudinal field, Hdc, up to 3600 A m-1 are also investigated. It is shown that the equivalent circuit representing the wire frequency behaviour can be approximated by a series RsLs arrangement, in series with a parallel LpRp arm. Ls and Lp inductor elements are associated with the rotational and domain wall contributions to circumferential permeability, respectively. Rp is related to wall damping and Rs accounts for all the resistances in the circuit (the wire itself, contacts and so on). The circumferential permeability associated with domain wall movements exhibits a maximum for irms=5 mA (that is, a circumferential field Hrmsphi =12 A m-1), similar to the classical behaviour of wall permeability. The increase in bias field has the effect of strongly decreasing the Lp value; for Hdc=3600 A m-1, the series circuit along accounts for the frequency response of the wire. The association of the circuit elements with basic magnetization processes is discussed. Results are interpreted in terms of the influence of both fields (DC bias, Hdcr,and AC circumferential, Hrmsphi fields) on the inner-core-outer-shell magnetic structure of the wire.