Refining the formation of nanocrystals in soft magnetic amorphous alloy via EXAFS spectroscopic analyses

Excerpt

The amorphous metallic alloy with the composition Fe_73.5Nb₃Cu₁Si_13.5B₉ is now quite well known as FINEMET, mainly owing to its excellent magnetic properties that are supposed to be due to the formation of small nanocrystals consisting of Fe and Si, i.e., DO₃ phases, during simple heat treatment [1‐3]. The evolution of such nanocrystals and their effects on the magnetic properties were comprehensively described in previous reports [4‐6]. With using mainly the Fe K-edge and Cu K-edge EXAFS spectra, Ayers et al. carefully studied nucleation kinetics of samples that were heat-treated at various durations [7, 8]. In particular, the role of Cu on nucleation processes in this noncrystalline metal was largely proven by experimental observations of a series of Fourier-transformed EXAFS spectra taken at the Cu K-edge of differently heat-treated samples. A key point in their nucleation model for this amorphous metal is that Cu clusters act as nucleation sites of the DO₃ nanocrystallites [8]. It is noteworthy that not all Cu atoms are involved in the generation of the DO₃ nanocrystals. Nonetheless, Ayers et al.’s analyses were generally qualitative since they explained the local structural evolution of Cu based on the fingerprint comparison between the partial radial distribution functions of the samples. Thus, though successful, their analyses did not give a quantitative fraction of Cu participating in the nucleation processes of DO₃ nanocrystals in their samples. …