Recently as the storage density of hard-disk increases very fast, the need for highly sensitive reading head using GMR property is rapidly rising. Until now, among the GMR's systems, the most successful structure is the spin-valve system which employs an anti-ferromagnetic layer to fix the magnetization direction of one of two ferromagnetic layers spaced by metallic thin film. The basic structure of the spin valve system consists of ferromagnetic layer/space layer/ ferromagnetic layer/anti-ferromagnetic layer. FeMn shows a good ferromagnetic property if its texture is properly controlled on sputtering. Cu is frequently used for metallic spacer because of its low lattice misfit with the magnetic layer and thereby with the FeMn layer. The ferromagnetic $Ni_80Fe_20$ film shows highly sensitive switching response because of low coercivity force, whereas the Co film displays high MR value due to its high spin-dependant scattering property. The idea in the present study was thus to search for a possibility to combine these two different desirable characteristics in one magnetic film through a ternary alloying between Ni80Fe20 and Co. The MR and magnetic properties of Ta/NiFeCo/Cu/NiFeCo/FeMn magnetic film multi-layer was investigated with the variation of Co content in the NiFeCo ferromagnetic film and its thickness and of the thickness of Cu spacer. The MR of the present spin-valve system was increased by nearly 130% from about 1.78% to 4.07% without much deteriorating of the switching field as the Co content increased from 0% to 29at.% in the $Ni_80Fe_20$ film. This was believed to be due to the strong spin-dependent scattering property of Co. The experiments with the variation of magnetic layer thickness suggested that the Co addition tends to enhance the interface spin-dependent scattering than the bulk spin-dependent scattering in the magnetic layer. The experiments with the variation of the pinned layer thickness indicated that the MR generally decreases with the thickness and that this is rather because of the decrease of exchange field due to its thickness increase than because of the shunt effect of electrical current as commonly cited. The MR generally oscillated with the increase in the thickness of Cu spacer, although its overall value gradually decreased with the thickness. This oscillation was found to be in good coincidence with the oscillation of inter-layer coupling strength with the thickness of Cu spacer. This result thus directly showed that the MR property is greatly influenced by the nature of exchange coupling, i.e, anti-ferromagnetic or ferromagnetic coupling. The experiments with the variation of deposition power showed that the MR increases by 16% from 3.95% to 4.61% with the power despite the depletion of Co in the $Ni_80Fe_20$ film. This is believed to be a direct proof that the increase in the grain size of magnetic films results in a significant increase of the MR property.
