Nanostructured Materials under Ion and Microwave Radiation

Abstract

This thesis discusses how ion radiation and microwaves interact with nanoscale-structured materials. In the case of ion radiation, the experiments show that ion processing, either with low-energy ions in reactive ion etching or with higher energy ions in focused ion beams, produces inelastic strain in polycrystalline thin metallic films. This results in the bending of thin strips of metallic films, which cannot be explained by elastic models. The concept of ion-induced plastic strain implies the insertion of adatoms into grain boundaries within the metal matrix. In ion etching processes, thin strips of metallic films with different widths were released from the substrate at different times. Therefore, the rate of atomic flow into grain boundaries is different for different strips. The larger curvatures in narrower strips are the result of a faster rate of adatom insertion into the grain boundaries. With a high-energy focused ion beam, plastic strain can be created locally, allowing the fabrication of non-trivial three-dimensional structures at nanometer scales. In the case of microwave radiation, the materials studied include cobalt nanoparticles and carbon nanotubes. The magnetic resonance and absorption in cobalt nanoparticles are observed in various magnetizing fields at frequencies between 0.5 and 18 GHz, by using a wideband method. The obtained experimental results show that the energy absorption is associated with the ferromagnetic resonance of cobalt nanoparticles. The results include measurements of blocking temperature and saturation magnetization with SQUID magnetometry. The absorption spectra are analyzed theoretically by combining Kittel's theory for uniaxial spherical particles, the Landau-Lifshitz-Gilbert equation and effective medium models. At zero magnetizing field, the observed resonance occurs at higher frequencies compared to the non-interacting particle model. The shift of resonance is suggested to be caused by the clustering of particles. Transmission electron microscopic images demonstrate that indeed particles aggregate in the forms of clusters, superlattices, and chains. The absorption properties of yarns of carbon nanotubes are also presented in the thesis.