Inverse giant magnetoresistance in Fe/Cu/Gd1-xCox spin-valves

For bottom spin-valves of NiO/Fe/Cu/Gd1-xCox, giant magnetoresistance has been measured as a function of thickness and composition of GdCo layers as well as temperature. For all spin-valves involved here, the giant magnetoresistance has been attributed to contributions of spin-dependent scattering at interfaces and in bulk. The interfacial contribution produces positive giant magnetoresistance ratio for various compositions of GdCo alloys. However, the bulk contribution produces negative one for the Co contents from 50 at. % to a critical value x(C)(R) (between 69 at. % and 77 at. %) and positive one for higher Co contents. It is suggested that the interfacial asymmetric factor of spin-dependent scattering is larger than 1.0 at the Cu/GdCo for various alloy compositions of GdCo and that the bulk asymmetric factor in GdCo layer is smaller and larger than 1.0 for Co contents below and above x(C)(R), respectively. For spin valves with Co contents below and/or above x(C)(R), the giant magnetoresistance ratio varies nonmonotonically and/or monotonically as a function of the GdCo layer thickness (temperature) for a specific temperature (a specific GdCo layer thickness), respectively. The change of the bulk asymmetric factor in the GdCo layer with the alloy composition can be attributed to the variation of either the spin alignment of Co and Gd atoms or the spin polarization of the GdCo layer.