Magneto-impedance of glass-coated Fe–Ni–Cu microwires

The magneto-impedance (MI) of glass-coated Fe–Ni–Cu microwires was investigated for longitudinal radio-frequency (rf) currents up to a frequency of 200 MHz using a rf lock-in amplifier method. The MI, defined as ΔZ/Z=[Z(H) – Z(H=0.3 T)/Z(H=0.3 T), displays a peak structure (negative MI) at zero field for rf currents with frequencies less than 20 MHz and this crosses over to a sharp dip (positive MI) at higher frequencies. This crossover behavior is ascribed to the skin-depth-limited response primarily governed by the field-dependence of the permeability. Large saturation fields (300 to 600 Oe) and other anomalies indicate the possible influence of giant magneto-resistance (GMR) on the MI.