Propriedades estruturais e magnéticas de filmes finos de óxidos de ferro nanoestruturados sobre superfície vicinal Ag(977)

Abstract

Nanostructures have a high potential for technological application, ranging from magnetic reading and memory devices to spintronics. Associating the dimensions and shapes of these nanostructures with their magnetic behavior is the motivation of this work. One of the most studied magnetic materials is magnetite (Fe3O4), which has ferrimagnetic behavior at room temperature. Several investigations related to the formation of thin films of magnetite grown on various types of substrates can be found in the literature, however, its deposition on metallic vicinal surfaces is uncommon. In this work, we used molecular beam epitaxy (MBE) technique in an ultra-high vacuum (UHV) environment to deposit Fe monolayers followed by oxidation to grow ultra-thin Fe3O4(111) films on Ag(977) vicinal substrate, which has high density of steps. All surface techniques were performed in situ at room temperature. For the analysis and characterization of the surface structure, we used scanning tunneling microscopy (STM), while the crystalographic ordering was verified by Low Energy Electron Diffraction (LEED). The STM images show some structural anisotropy in the formation of nanostructures, with self-ordering parallel to the steps of the vicinal surface for all coverages. For 2ML Fe-O coverage, the STM images and the LEED pattern reveal the Moiré superstructure formation, indicating the predominance of würstite (FeO). For coverages from 3ML, magnetite formation is observed and magnetic measurements using magneto-optical Kerr effect (MOKE) in various applied magnetic field configurations show a superparamagnetic behavior between 4ML and 6ML. For coverages above 12ML, the MOKE measurements reveal an easy axis of magnetization in the sample plane, parallel to the step edges, and a hard axis out of the plane. That uniaxial magnetic anisotropy was verified from the dependence of the coercivity with respect to the in plane angle of the applied field. We have also verified the coverage limit where the transition to a multidomain magnetic state occurs for magnetite.