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It has recently been found that large uniaxial anisotropy fields in excess of 120 kA/m ͑1500 Oe͒ can be created in thin ͑3-5 nm͒ films of Co by obliquely sputtered Ta underlayers. This anisotropy can be used to pin the bottom film of a spin valve while having only a modest effect on the top ''free'' film, separated by a 2.5 nm Cu spacer layer. This article describes measurements of thermal stability in these Ta-pinned spin valves. Using room temperature giant magnetoresistance ͑GMR͒ as a measure, we find that the structure is stable under cumulative 20 min anneals at 25°C intervals up to 300°C; GMR decreases to zero upon further anneals up to 450°C. Measurements taken at elevated temperatures reveal that GMR decreases linearly with temperature, extrapolating to zero at approximately 425°C, while the anisotropy field is much less temperature dependent, remaining nearly constant up to 150°C and gradually decreasing to 50% of its room temperature value at 325°C.

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