Magnetismo anômalo em nanoestruturas: um estudo por microscopia eletrônica de transmissão e dicroísmo circular magnético de raios-X

Abstract

In this thesis, we have looked for the understanding of the magnetic behavior of materials at nanometer scale. As an initial guide, we have tried to find a possible anomalous magnetic behavior of Pd. In this way, nanostructured formations of Pd and Pd alloys with ferromagnetic elements (mainly Co), both encapsulated in Carbon Nanotubes (CNTs), were studied. The main experimental techniques involved were X-Ray Magnetic Circular Dichroism (XMCD) and Transmission Electron Microscopy (TEM). Using bimetallic (Pd-Co) films as catalyst, Carbon Nanotubes samples were produced by Plasma Enhanced Chemical Vapour Deposition (PECVD). This process results in a homogeneous sample of CNTs vertically aligned to the used substrate. These CNTs hold on the top encapsulated PdCo nanoparticle with strong shape anisotropy. The TEM analysis of several particles brought morphological, chemical and crystallographical information. The macroscopic magnetic behavior was determined by Vibrating Sample Magnetometry (VSM). The orbital and spin magnetic moments ratio was determined by XMCD for both elements (Pd and Co) in the particles. It could be verified by XMCD the ferromagnetic behavior of Pd atoms. A second part of the thesis involved the investigation of magnetism in Pd and Co nanostructured formations (nanowires or nanoclusters) on Au(110) surfaces. Cobalt coverages deposited on clean Au(110) surfaces were obtained with an electron beam evaporation cell. Low Energy Electron Diffraction (LEED) measurements of the clean surface exhibited the typical Au(110) surface pattern. Diffraction patterns of the surface with Cobalt coverage resulted in a typical diffuse pattern of nanostructures. XMCD measurements showed the ferromagnetism of these nanostructures at room temperature, even if not fully saturated. The XMCD Sum Rules allowed us to determined the giant orbital moment of Co nanoestructures by fixing its intrinsic value of spin moment per atom. A similar experiment with Pd nanoestructures on Au(110) did not resulted in ferromagnetism observation at room temperature. We discuss the possibility of obtaining ferromagnetism in these nanostrucures.