Underlayer effect on magnetoresistance of top- and bottom-type spin valves

The effect of underlayers on top- and bottom-type spin valves with PtMn and NiO is investigated. For the top-type spin valves with PtMn, magnetoresistance (MR) ratios are enhanced by using Zr underlayers compared to those for Ta underlayers. The bottom-type spin valves with PtMn showed larger MR ratios than the top-type spin valves as the free layer thickness decreased. The improved giant magnetoresistance results mainly from the increased Δρ due to the smoother interface for the spin valves. Spin valves with NiO underlayers showed smaller resistivity and an almost constant Δρ for a wide range of free layer thicknesses.