Abstract

Experimental and theoretical studies on the influence of ac magnetic field frequency on the axial diagonal (zeta(zz)) and off-diagonal (zeta(phi z)) components of the magnetoimpedance (MI) tensor in (Co0.94Fe0.06)(72.5)Si12.5B15 amorphous wires have been performed. The frequency (f) of an ac current flowing along the wire was varied from 1 to 20MHz with the current amplitude less than 15 mA. In order to enhance the zeta(phi z) component, the amorphous wire was submitted to torsion annealing for developing and preserving a helical magnetic anisotropy in the surface of the wire. The experimental measurements show that the value of the impedance is proportional to the square-root of the ac current frequency, root f, in the vicinity of H-ex H-k and this increase is due to the contribution of the resistance (real part of the impedance). The measurements also indicate that the peaks of the MI curve shift slightly towards higher field values with increasing f. In a theoretical study the magnetoimpedance expressions,, and have been deduced using the Faraday law in combination with the solutions of the Maxwell and Landau-Lifshitz-Gilbert (LLG) equations. By analysing quantitatively the spectra of zeta(zz), and zeta(phi z), the phenomenon of the shift in the peaks of the MI curve with f has been considered as a characteristic of the helical anisotropy in the domain structure of the wire surface.