Ion-Irradiated Laterally Graded Ni/C Multilayers: A Combined X-ray Standing Wave and X-ray Reflectivity Analysis

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An X-ray standing wave (XSW) experimental facility was set up at the Rossendorf Beam Line (ROBL) at the European Synchrotron Radiation Facility (ESRF). Using this facility, the microstructures and ion-beam induced microstructural modifications of a nanoscale laterally graded Ni/C multilayer systems have been studied by the combined X-ray reflectivity (XRR) and XSW technique. The multilayer stack with 15 Ni/C layer pairs was fabricated on a glass substrate by ion beam sputtering. A 2 MeV Cu2+ ion beam was rastered on the samples to obtain uniformly irradiated strips with fluences from $1\times 10^{14}$ to $7\times 10^{14}$ ions/cm2. We have observed that X-ray reflectivity at the first order Bragg peak gradually increases due to increase in multilayer period in the virgin samples. The multilayer period has expanded and interfaces broadened due to ion irradiation. X-ray standing wave analysis indicates that, during deposition, a significant amount of Ni diffuses into C layers. Up to a certain fluence, we have observed that more and more Ni atoms are incorporated into C layers. At higher fluences, Ni is progressively segregated from the C layers. These mixing and demixing phenomena of Ni in C layers as a function of ion fluence have been explained in terms of two competitive processes including ballistic mixing and chemically guided atomic movements.
2011-05-25