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 str ucture of Mn films with various thicknesses deposited on (111) and (100) Ir lattices via molecular beam epitaxy. The goal is to reach new magnetic pro perties by forcing a material to a specific atomic arrangement. Mn is a goo d candidate as theoretical calculations predict a high spin bulk ferromagne tic state. EXAFS is a well adapted technique to characterize the local environment aro und a given type of atom in a sample. Measurements were performed at 80 K i n the total electron yield mode, with the electric field parallel or perpen dicular to the sample surface in order to detect eventual lattice deformati ons. As Mn can adopt a large variety of crystallographic structures displaying d istances between first and second neighbours with very close values, a simu lation of the first peak of the Fourier transform of the oscillations recor ded 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 F EFF code, we calculated the oscillations corresponding to several shells of neighbours for the different possible crystallographic structures taking i nto account the multiple scattering theory; A direct comparison between cal culated and experimental oscillations allowed us to assign an atomic struct ure to a deposited sample. The results are the following : - for thin Mn films (t = 10 nm), a trigonal structure is adopted on (111) I r 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 observe d 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 asc ertained by some authors. However no ferromagnetic behaviour was observed in all these samples, whate ver the structural state. The crystallographic structure of the FexMn1-x/(100)lr superlattices was al so identified as a function of Fe concentration. There is an evolution of t hese 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 rati o = 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.