A look inside epitaxial cobalt-on-fluorite nanoparticles with three-dimensional reciprocal space mapping using GIXD, RHEED and GISAXS

Abstract

In this work epitaxial growth of cobalt on CaF 2 (111), (110) and (001) surfaces has been extensively studied. It has been shown by atomic force microscopy that at selected growth conditions stand-alone faceted Co nanoparticles are formed on a fluorite surface. Grazing-incidence X-ray diffraction (GIXD) and reflection high-energy electron diffraction (RHEED) studies have revealed that the particles crystallize in the face-centered cubic lattice structure otherwise nonachievable in bulk cobalt under normal conditions. The particles were found to inherit lattice orientation from the underlying CaF 2 layer. Three-dimensional reciprocal space mapping carried out using X-ray and electron diffraction has revealed that there exist long bright h111i streaks passing through the cobalt Bragg reflections. These streaks are attributed to stacking faults formed in the crystal lattice of larger islands upon coalescence of independently nucleated smaller islands. Distinguished from the stacking fault streaks, crystal truncation rods perpendicular to the {111} and {001} particle facets have been observed. Finally, grazing-incidence small-angle X-ray scattering (GISAXS) has been applied to decouple the shape-related scattering from that induced by the crystal lattice defects. Particle faceting has been verified by modeling the GISAXS patterns. The work demonstrates the importance of three-dimensional reciprocal space mapping in the study of epitaxial nanoparticles.

An advanced three-dimensional RHEED mapping approach to the diffraction study of Co/MnF2/CaF2/Si(001) epitaxial heterostructures

“…Formation of Co with a dominating h.c.p. phase is in contrast with the results of Co growth on the CaF 2 (111), CaF 2 (001) and CaF 2 (110) surfaces (Suturin, Fedorov, Korovin et al, 2013), where Co was crystallized in the metastable f.c.c. phase and adopted the orientation of the underlying fluorite layer.…”

“…Also studied were Co layers grown by a two-stage technique which is supposed to increase the Co crystalline quality. The growth recipe has been adopted from our earlier studies of Co/CaF 2 systems (Sokolov et al, 2013;Suturin, Fedorov, Korovin et al, 2013). A seeding layer of 0.1 nm of cobalt was grown on an MnF 2 surface at 373 K, with subsequent deposition of 15 nm of cobalt at 573 K. The important difference from the Co/ CaF 2 (111) case was that the growth temperature at the second stage was chosen to be 573 K instead of 873 K to preserve continuity of the MnF 2 layer, which might become unstable at elevated temperatures.…”

“…Growth of ferromagnetic Co layers and Co island arrays on fluoride surfaces has been recently studied by Suturin, Fedorov, Banshchikov et al (2013) and Suturin, Fedorov, Korovin et al (2013). Magnetic proximity effects in Co/MnF 2 / CaF 2 (111)/Si(111) heterostructures have been reported by Suturin, Fedorov, Banshchikov et al (2013).…”

Epitaxial Ni nanoparticles on CaF2(001), (110) and (111) surfaces studied by three-dimensional RHEED, GIXD and GISAXS reciprocal-space mapping techniques

“…It is particularly stressed that a relationship exists between substrate orientation, stabilization of particular crystal phases and the magnetic properties of the growing metallic film. It has been demonstrated in our previous studies (Sokolov et al, 2013;Suturin, Fedorov, Korovin et al, 2013) that at 773-873 K Co grows on CaF 2 (111) predominantly with an f.c.c. lattice, whereas at room temperature Co crystallizes in a hexagonal close-packed (h.c.p.)…”

“…In particular, grazing-incidence diffraction studies of relaxation and stacking-fault formation in thin cobalt films have been carried out by Baudot et al (2004). It has been demonstrated in our recent studies (Suturin, Fedorov, Korovin et al, 2013) that applying the grazing-incidence small-angle X-ray scattering (GISAXS) method in combination with X-ray diffraction (XRD) is helpful in sorting out the diffraction-pattern streaking related to the stacking faults from streaking related to the island shape. Being able to understand and control the internal crystal structure, size and shape of ferromagnetic islands is deemed highly important for tuning the magnetic properties of the resulting heterostructures.…”