Influence of nanoscale order-disorder transitions on the magnetic properties of Heusler compounds for spintronics

Modifications in nanoscale chemical order are used to tune the magnetic properties, namely T_C, of Co₂FeSi_xAl_1-x (0 < x < 1). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with Z-contrast reveals nanoscale regions of L2₁ order within a B2 matrix in the off-stoichiometry samples. Perhaps surprisingly, the latter, more chemically disordered structure, exhibits a higher T_C. Upon annealing, the off-stoichiometry samples become more homogeneous with the fraction of L2₁ order decreasing. The short-range order was also investigated using X-ray absorption fine structure (XAFS) measurements at the Co and Fe K edges. Since the local atomic environments of Co atoms in the L2₁ and B2 structures are identical, the features presented in the Co K edge XAFS data are the same in both cases. By contrast, the L2₁ and B2 structures exhibit different signatures at the Fe K edge owing to the different chemical environments. Fitting of these spectra confirms the nanoscale chemical disorder observed by HAADF-STEM and the expected role this disorder plays on T_C. Our results point to a methodology that might be extended to modify the magnetic and electronic properties of any Heusler compound; chemical disorder can be an engineering tool to realize highly tailored properties.