1. GARS Directorate Seminar

    "Ion-Irradiation-Induced Atomic Redistribution in Pt(Fe)/C(Fe) Multilayers"

    Presented by Sambhanuth Bera, Northern Illinois University

    Wednesday, April 3, 2013, 11 am
    Building 130, Modular Conference Room

    Hosted by: Dr. Simerjeet Gill and Dr. Lynne Ecker

    I will present ion-irradiation-induced microstructural modifications, atomic migration, and nanoparticle formation in nanoscale periodic Pt/C multilayer systems containing 15 at. % Fe which have been analyzed by x-ray reflectivity, x-ray standing wave (XSW), transmission electron microscopy, and grazing incidence x-ray diffraction studies. The multilayer stack with 15 Pt(Fe)/C(Fe) layer pairs (period 4.2 nm) was irradiated with a 2 MeV Au2+ ion beam, which was rastered on the sample to obtain uniformly irradiated strips with different fluences. Up to a certain threshold ion fluence we have observed mixing between Pt and C atoms. At higher fluences demixing effect is pronounced. This phenomenon has been explained in terms of two competitive processes including ballistic and chemically guided atomic movements. As revealed by XSW measurements, ion irradiation causes preferential migration of Fe toward Pt layers from C layers. Consequently Fe concentration in the Pt layers increases with a corresponding decrease in the C layers. From the gradual change in shape of the Fe fluorescence yield curve, over the first order Bragg peak region, due to ion irradiation we can infer that Fe atoms migrate from C to Pt layers in the outward direction. This outward migration of Fe has been explained in terms of surface segregation of Fe in Pt under the irradiation condition. Cross-sectional transmission electron microscopy has revealed the gradual change in the microstructure, considerable atomic redistribution, and cluster formation for irradiation at the highest ion fluence, where FePt nanoparticle formation is observed in GIXRD experiments. The results and analysis is also provide an understanding of the evolution of ion-beam-induced ferromagnetism and the increasing coercive field with ion fluence in our Pt(Fe)/C(Fe) multilayer system. Publications: Physical Review Letters 98 (2007) 196103, Journal of Applied Physics 102, 014308 (2007), Journal of Applied Physic