Estudo in-situ de filmes ultrafinos de Fe3O4/Pd(100), Fe3O4/Ag(100) e -Fe2O3/Pd(111) por dicroísmo circular magnético de raios-X

Abstract

The spin magnetic moment (ms) for ultrathin magnetite films (Fe3O4) as determined experimentally by various groups presents much lower values than the moment for a single crystal (bulk ), which is very intriguing. Another open question is: what is the smallest thickness required for a magnetite film to maintain the bulk ferrimagnetic order at room temperature? Motivated by these questions, we have performed a systematic study on the evolution of the magnetic spin moment with increasing film thickness (8 to 45 A) in two crystallographic directions ([100] and [111]) for magnetite and maghemite. The films were prepared in situ by molecular beam epitaxy of Fe in oxygen environment, the chemical characterization was obtained by X-ray absorption spectroscopy (XAS) and the growth direction and the crystallographic order were attested by low energy electron diffraction (LEED). The magnetic behavior was investigated by XMCD at the L2;3 iron absorption edges. Hysteresis loops for each iron ion per site (Fe2+ oct, Fe3+ oct, Fe3+ tet ) were measured, and the sum rules were applied on X-ray magnetic circular dichroism spectra (XMCD) in order to obtain the ms values of a series of films. For both growth directions the magnetic spin moment has shown similar behavior as a function of thickness. We have obtained ms=(3.6+-0:2) B/f.u. for a 35A-thick Fe3O4 lm, close to the bulk (4B/f.u.) value. Our results show that the ferrimagnetic order and the full ms value are preserved at room temperature for Fe3O4 films around the thickness of two unit cells. This work also describes the construction of a home made Kerr magnetometer (MOKE) in ultra-high-vacuum environment, for vectorial study of magnetization (3D magnetometry). The steps of how to extract the mx; my; mz magnetizations of ultrathin films from a set of measurements in various geometries, are explained in detail.