X-ray absorption spectroscopy to study metallic multilayers

In this work, X-ray absorption spectroscopy, and more precisely Extended X-ray Absorption Fine Structure (EXAFS), was used to determine the atomic structure of Mn films with various thicknesses deposited on (111) and (100) Ir lattices via molecular beam epitaxy. The goal is to reach new magnetic properties by forcing a material to a specific atomic arrangement. Mn is a good candidate as theoretical calculations predict a high spin bulk ferromagnetic state. EXAFS is a well adapted technique to characterize the local environment around a given type of atom in a sample. Measurements were performed at 80 K in the total electron yield mode, with the electric field parallel or perpendicular to the sample surface in order to detect eventual lattice deformations. As Mn can adopt a large variety of crystallographic structures displaying distances between first and second neighbours with very close values, a simulation of the first peak of the Fourier transform of the oscillations recorded after the edge in XAS and corresponding to the first neighbours of Mn, is not accurate enough to determine the actual structure. Thus, using the FEFF code, we calculated the oscillations corresponding to several shells of neighbours for the different possible crystallographic structures taking into account the multiple scattering theory; A direct comparison between calculated and experimental oscillations allowed us to assign an atomic structure to a deposited sample. The results are the following : - for thin Mn films (t = 10 nm), a trigonal structure is adopted on (111) Ir whereas a tetragonal structure is adopted on (100) Ir; - thick samples (t = 35 nm) relax to the a bulk Mn state. We also showed that the superstructure (root 3 x root 3)R30 degrees observed for Mn epitaxially grown on (111) Ir is due to a mixture of fee and alpha Mn and not to structure close to Laves phases (Cu,Mg or Zn,Mg type) as ascertained by some authors. However no ferromagnetic behaviour was observed in all these samples, whatever the structural state. The crystallographic structure of the FexMn1-x/(100)lr superlattices was also identified as a function of Fe concentration. There is an evolution of these systems strained by the Ir lattice. For less than 70 % Fe in the alloy a tetragonal structure is observed, close to a fee lattice with a c/a ratio = 1.32. For larger Fe concentrations, a clear structural transition to a cc lattice occurs, which is in fact a magnetovolumic transition. For c(Fe) < 70 %, the samples are antiferromagnetic or non magnetic whereas above 70 % the alloys are ferromagnetic.