  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 an RF lock-in amplifier method. The MI, defined as DZ/Z =
[Z(H)-Z(H=0.3T)]/Z(H=0.3T), displays a peak structure (negative MI) at zero
field for RF currents with frequencies less than 20MHz 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.
