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The dependence of the giant magnetoresistance on Ni81Fe19 soft magnetic layer thickness is investigated experimentally for a simple spin valve with a top-pinned structure of Ta (6 nm)/Ni81Fe19/Co90Fe10 (1 nm)/Cu (1.8 nm)/Co90Fe10 (3.5 nm)/Ir20Mn80 (8 nm)/Ta (6 nm). With Ni81Fe19 thickness increased from 6 nm to 7 nm, the magnetoresistance (MR) ratio decreases sharply from 8.34% to 3.34%, whereas it changes only slightly within the thickness ranges from 2-6 nm and from 7-12 nm, and larger MR ratios are obtained in the range from 2-6 nm. For a spin valve with an optimized thickness of Ir20Mn80 (11 nm) and top Ta (3 nm), the MR dependence is in accordance with the former structure when Ni81Fe19 thickness changes from 3.5 to 5.5 nm, and an optimized spin valve with 4.5-nm-thick Ni81Fe19 is obtained. This spin valve has a large MR ratio (9.15%), low coercive force (0.85 Oe), and high sensitivity, which makes it promising for applications.