Fe–Co–B Nanocrystalline Soft Magnetic Alloys with High Saturation Induction: a Review

Abstract

The development of nanocrystalline soft magnetic alloys with saturation induction (B_s) approaching or exceeding 2 T remains a major materials science challenge for modern electromagnetic energy conversion systems. This review critically examines the compositional design, crystallisation kinetics, and dynamic magnetic performance of Fe–Co–B high-induction alloys, with particular emphasis on Fe₆₇Co₂₀B₁₃ as a model metalloid-lean composition. The interplay between chemical composition, crystallisation pathway, and nanostructural evolution is discussed as the key framework for reconciling high saturation induction with low coercivity and an adequate dynamic magnetic response. The roles of individual alloying elements are reviewed: metalloids govern glass-forming ability and crystallisation pathways, whereas transition-metal additions influence nucleation and grain growth. Special attention is given to the critical influence of saturation magnetostriction (λ_s) on alternating-current initial permeability and dynamic losses. The available data indicate that λ_s should be regarded as a primary design variable, since alternating-current permeability at 1 kHz correlates negatively with λ_s but does not correlate reliably with direct-current coercivity. Recent advances from 2024-2026 demonstrate that saturation induction above 1.9 T can be achieved in Fe-rich and Fe–Co-based nanocrystalline alloys through ultra-rapid annealing, although only within a narrow processing window. Fe₆₇Co₂₀B₁₃ represents a promising near-steel-loading nanocrystalline platform whose main remaining limitations are insufficient dynamic magnetic softening and limited processing window tolerance.