WP 3

Ion-beam induced magnetic nanopatterning


M3.1. The map of optimal dose-energy condition for hydrogen and helium ions

Comparative study of magnetic properties of highly oriented pyrolytic graphite irradiated with different particles suggests that ions (H+, He+, C+) with the energy in the range of hundreds keV yield larger values of induced magnetization compared to the protons with the energy of several MeV. These values increase with ion fluence but abruptly decrease when the concentration of interstitial defects becomes so large that the graphite stacking sequence is collapsed.

We have performed a comparative study of samples of graphite irradiated with hydrogen H+ and helium He+ ions at medium energy: 225 keV. In our experiments we were able to increase magnetism of graphite by three orders of magnitude, magnetization in saturation reaching 3 Am2kg-1 for hydrogen bombardment and 6 Am2kg-1 for helium bombardment. Proton induced X-ray emission (PIXE) verified the absence of metallic impurities in the samples; impurity tests were made after the completion of magnetic measurements in order to reveal any unintentional contaminations.

We have developed a general computational approach for the inclusion of the dynamics of atomic rearrangements under the imaging electron beam into transmission electron microscopy image simulations. The effect of the electron beam on atomic structure is described by the event driven molecular dynamics, in which events of the electron collision with a sample are represented by the corresponding displacement cross sections specific to the energy of the electron beam and the type of atom interacting with the imaging electron.

The proposed approach adds another dimension to the existing image simulation concept as it includes robustness of a sample as one of the input parameters. Thus, the HRTEM imaging conditions can be evaluated and optimized not only with respect to instrumental resolution and contrast, but also with respect to the electron dose. This approach will be especially beneficial for the-beam sensitive samples, where resolution is defined by the signal-to-noise ratio rather than by the quality of the instrument.

M 3.2 Elaboration of graphite-based magnetic nanostructures

Graphene obtained from thermal exfoliation of graphite oxide was chemically functionalized with nickel nanoparticles (NPs) without exposing the system to oxidizing agents. Its structural, physical and chemical properties have been studied by means of TEM, X-ray photoelectron and Raman spectroscopies, and SQuID magnetometry. The formation of 17 nm super-paramagnetic (SPM) monodispersed Ni NPs was observed. Nitrogen sorption experiments at 77 K yield a Brunauer-Emmet-Teller specific surface area (BET-SSA) of 505 m(2) g(-1) and helium adsorption at room temperature gives a skeletal density of 2.1 g cm(-3). The interaction with atomic hydrogen was investigated by means of Muon Spin Relaxation (mu SR) showing a considerable fraction of captured muonium (similar to 38%), indicative of strong hydrogen-graphene interactions. Hydrogen adsorption has been measured via pressure concentration isotherms demonstrating a maximum of 1.1 mass% of adsorbed hydrogen at 77 K and thus a 51% increased hydrogen adsorption compared to other common carbon based materials.