Controlling Devitrification in FeSiB Amorphous System for Advanced Soft Ferromagnets

The performance of advanced soft ferromagnets relies on the formation of an optimized nanocrystalline structure through controlled devitrification from amorphous precursors. This structure is conventionally achieved via small additions of early/late transition metal elements (Nb, Cu, <i>etc</i>.) that allow for tuning of devitrification kinetics but also degrade saturation magnetization. Here, building from an in-depth study of as-quenched amorphous nanostructures produced by different rapid solidification conditions, we identify that specifics of small quenched-in crystallites strongly determine devitrification kinetics in FeSiB system free of transition metal additions.<br/>Samples of composition Fe₇₉Si₁₁B₁₀ were fabricated into two forms: 1) melt-spun ribbons of ~25 μm in thickness and 2) water-quenched microwires of ~150 μm in diameter. While identical chemical composition and overall X-ray amorphous structure of both as-quenched forms are confirmed, nanoscale differences are revealed by high-resolution transmission electron microscopy. In specific, the water-quenched microwires feature small (~30 nm in diameter) Fe(Si) crystallites dispersed throughout their volume, in contrast to the melt-spun ribbons that exhibit amorphous structure with Fe(Si) nanocrystalline islands (~2 nm in thickness, ~50 nm in diameter) confined to the surfaces. Analysis of calorimetric data reveals that the microwires devitrify via nucleation from pre-existing nuclei, resulting in reduced activation energy, followed by diffusion-controlled grain growth. On the other hand, the ribbons exhibit continuous nucleation followed by interface-controlled growth. It is contemplated that the different devitrification behavior exhibited by the two sample types is attributed to their unique nanostructural state, which is subtly affected by the rapid solidification conditions. These results provide insights into the control of devitrification to achieve high-moment, high-performance nanocrystalline soft ferromagnets without alloying additions.<br/>Acknowledgments: IEEE Magnetics Society Educational Seed Funding, Northeastern University, and ICMM/CSIC, Spain