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Surfactant-Assisted Atomic-Level Engineering of Spin Valves



H D. Chopra, D X. Yang, P J. Chen, William F. Egelhoff Jr.


Surfactant Ag was successfully used to atomically engineer interfaces and nanostructure in NiO-Co-Cu based bottom spin valves. DC magnetron sputtered Co (3.0nm)/Cu (1.5 nm)/Co (2.5 nm)/NiO (50nm) 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, for spin valves deposited in the presence of 3 MLs of surfactant Ag, the net positive coupling was reduced by roughly a factor two or more. As a result, Ag-containing samples exhibit more than an order of magnitude increase in GMR (8.5% in 1ML Ag samples and 13% in 3 MLs Ag samples). Based on extensive TEM studies, a large contribution to net positive coupling in Ag-free samples could be direclty attributed to the presence of numerous pinholes, which bridge the pinned and the free Co layers. In contrast, in-situ x-ray photoelectron spectroscopy (XPS) show that in Ag-containing samples, surfactant Ag floats out to the surface during deposition of successive Co and Cu overlayers, leaving behind smooth surfaces and continuous layers that are less prone to intermixing and pinholes. Results also show that the 3 MLs Ag samples have a slightly higher net positive coupling (7.71 kA/m or 97 Oe) than the 1 ML Ag samples (5.65 kA/m or 71 Oe). HRTEM studies show that Ag-modified expitaxial growth of Co/Cu layers in the 3 MLs Ag samples leads to topographic correlation of Co-Cu interfaces that is donducive to Neel's so-called orange-peel coupling. In contrast, the Co/Cu layers in the 1 ML Ag samples grow in a non-epitaxial manner. Since non-epitaxial growth is known to destroy topographic correlation [H. D. Chopra, B. J. Hockey, P. J. Chen, W. R. Egelhoff, Jr., M. Wutting, and S. Z. Hua, Phys. Rev. B 55, 8390 (1997)], the key requisite to orange-peel coupling is absent in the 1 ML Ag samples. Therefore the 1 ML Ag samples exhibit a lower net positive coupling in comparison to the 3 ML Ag samples. The use of surfactants in the present study illustrates their potential use in atomic engineering of thin films to favorably alter physical properties not only in GMR films, but also in magnetoelectronics devices and other multilayered systems that are currently the focus of intense research activities due to a fundamental interest in their electron transport properties, interesting magnetic behavior, and myriad technological applications.
Physical Review B (Condensed Matter and Materials Physics)
No. 9


giant magnetoresistance, pinholes, spin valves, surfactants


Chopra, H. , Yang, D. , Chen, P. and Egelhoff Jr., W. (2002), Surfactant-Assisted Atomic-Level Engineering of Spin Valves, Physical Review B (Condensed Matter and Materials Physics) (Accessed April 19, 2024)
Created February 28, 2002, Updated October 12, 2021