In this study, dc magnetron sputtered NiO (50 nm)/Co (2.5 nm)/Cu(1.5 nm)/Co (3.0 nm) bottom spin valves were studied with and without Ag as a surfactant. At Cu spacer thickness of 1.5 nm, a strong positive coupling >13.92 kA/m (>175 Oe) between NiO-pinned and “free” Co layers leads to a negligible giant magnetoresistance (GMR) effect (<0.7%) in Ag-free samples. In contrast, spin valves deposited in the presence of ≈1 monolayer of surfactant Ag have sufficiently reduced coupling, 5.65 kA/m (71 Oe), which results in an order of magnitude increase in GMR (8.5%). Using transmission electron microscopy (TEM), the large positive coupling in Ag-free samples could directly be attributed to the presence of numerous pinholes. In situ x-ray photoelectron spectroscopy shows that, in Ag-containing samples, the large mobile Ag atoms float out to the surface during successive growth of Co and Cu layers. Detailed TEM studies show that surfactant Ag leaves behind smoother interfaces less prone to pinholes. The use of surfactants also illustrates their efficacy in favorably altering the magnetic characteristics of GMR spin valves, and their potential use in other magnetoelectronics devices and multilayer systems.
This study reports the highly deleterious role of a small amount of carbon on the structure and magnetic properties of “giant” magnetoresistance (GMR) NiO–Co–Cu-based spin valves. Controlled incorporation of 1–3 at. % carbon in the Co/Cu layers has been shown to completely eliminate the GMR effect. The presence of carbon gives rise to highly discontinuous Co/Cu layers, resulting in the formation of pinholes, and associated degradation of structure-sensitive magnetic properties. In addition, carbon promotes the formation of a high density of stacking faults in the Co/Cu layers, with carbon nanoprecipitates forming in the vicinity of the stacking faults. Results have implications for other multilayers and magnetoelectronics devices.
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