  Composite wires with a three-layered structure exhibit a large giant
magneto-impedance (GMI) effect, which can be used in sensitive magnetic field
sensors. To further investigate the origin of the GMI effect, composite wires
consisting of a highly conductive copper core, a silicon dioxide layer and an
outer Permalloy shell were prepared by radio frequency (RF) magnetron
sputtering. The GMI ratio was measured at various driving current frequencies
and with different insulating layer thicknesses. A theoretical model by
coupling the Maxwell equations to the Landau-Lifschitz-Gilbert equation was
developed to investigate the composite wire impedance and its dependence on
external magnetic field, current frequency and insulating layer thickness.
Experimental results corroborate the theoretical model.
