We have studied the effect of Au-Cu back layer system ~10 A thick on the properties of a spin valve. The back layers were Cu, Au, co-sputtered CuxAu1-x, laminated [Au/Cu]n and bi-layer [Au/Cu]. When Au was added to the Cu, the resistance of the spin valve abruptly increased most likely due to impurity scattering. The GMR values were not increased significantly for all the structures. In the case of co-sputtered CuxAu1-x, the changes in the resistance, ?R, was increased at a composition of ~Au0.5Cu0.5. This increase in ?R is due to increase in the resistance and not from the enhanced spin-dependent scattering. The structural analyses showed that the orthorhombic Au0.5Cu0.5 was formed in the back layer instead of the face-centered tetragonal Au0.5Cu0.5 as we expected. Thermal annealing over 400℃ may be required to have face-centered tetragonal in the 10 A thick ultra-thin film. In the case of a laminated or bi-layered back layer, the properties of the spin valve were improved, which may be attributed to the increase in the mean free path of conduction electrons.