Correlation between magnetism and structural relaxation in thin Fe(001) films patterned by the atomic saw method

We present detailed extended x-ray-absorption fine-structure (EXAFS) studies carried out on 50-Å epitaxial thin films grown by molecular-beam epitaxy on MgO(001) substrate prior and after structuration into ribbons by the so-called “atomic saw” method. Because of interfacial strain due to lattice mismatch (+3.8%), the crystallographic structure of the as-deposited film is demonstrated to be body-centered tetragonal with lattice constants $a=2.915\mathrm{}\pm 0.015\AA$ and $c=2.82\mathrm{}\pm 0.01\AA .$ This structure is 2% expanded in plane and -1.6% compressed along the surface normal as compared to the bulk Fe one. After structuration, the patterned 50-Å Fe films show a strong in-plane uniaxial magnetic anisotropy (close to 2.5 kOe). The EXAFS studies provide the clear evidence that the dislocation slipping process enhances a uniaxial relaxation $\delta =-2.5\%$ of the elastic strain field in the Fe film along the direction perpendicular to the ribbons. Through magnetoelastic effects, this relaxation is clearly demonstrated to be at the source of the observed magnetic anisotropy. This paper emphasizes the power of the “phase derivative” method in the analysis of EXAFS spectra for the determination of crystalline parameters in the case of bcc-type structures.