Microstructural evolution, atomic migration, and FePt nanoparticle formation in ion-irradiated Pt(Fe)/C(Fe) multilayers

Abstract

Ion-irradiation-induced microstructural modifications, atomic migration, and nanoparticle formation in nanoscale periodic Pt/C multilayer systems containing about 15 at. % Fe have been analyzed by x-ray reflectivity, x-ray standing wave (XSW), transmission electron microscopy, and grazing incidence x-ray diffraction (GIXRD) 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 fluences (ϕ) from 1×1014 to 1×1015 ions/cm2. Up to a certain threshold ion fluence (ϕt) we have observed mixing between Pt and C atoms. At higher fluences (ϕ>ϕt) 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. The decrease in Fe concentration in the C layers has an exponential dependence on the ion fluence. 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 (1×1015 ions/cm2), where FePt nanoparticle formation is observed in GIXRD experiments. The results and analysis presented in this work provide an understanding of the evolution of ion-beam-induced ferromagnetism and the increasing coercive field with ion fluence in the Pt(Fe)/C(Fe) multilayer system [B. N. Dev et al., Microelectronic Eng. 83, 1721 (2006)].