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

Magnetoimpedance in as-cast, non-magnetostrictive CoFeBSi amorphous ferromagnetic wires, submitted to AC electric current, i(rms), 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, H-dc, 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 R(s)L(s) arrangement, in series with a parallel L(p)R(p) arm. L(s) and F-p inductor elements are associated with the rotational and domain wall contributions to circumferential permeablity, respectively. R(p) is related to wall damping and R, accounts for all the resistances in the circuit (the wire itself, contacts and so on). The circumferential permeability associated with domain walt movements exhibits a maximum for i(rms) = 5 mA (that is, a circumferential field H-rms(phi) = 12 A m(-1)), similar to the classical behaviour of wall permeability. The increase in bias field has the effect of strongly decreasing the L(p) value; for H-dc = 3500 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, H-dc, and AC circumferential, H-rms(phi), fields) on the inner-core-outer-shell magnetic structure of the wire.