Enhanced soft magnetic properties of Fe-based nanocrystalline alloy by suppressing surface crystallization through sulfur addition

Soft magnetic materials play a crucial role in electrical devices owing to their high saturation magnetization, low coercivity, and high permeability. To reduce energy loss by achieving lower coercivity, preventing surface crystallization is essential. This study focuses on suppressing surface crystallization and enhancing their soft magnetic properties through sulfur addition. The tendency of sulfur to segregate at metal surfaces is hypothesized to enhance the glass-forming ability (GFA) in the surface region, thereby inhibiting the formation of dendritic structures that contribute to increased coercivity. Two compositions of Fe-based nanocrystalline alloys were prepared: Fe₇₉Si_5.5B_4.5P_3.5C_4.5Cu_1.0Nb_1.5Al_0.5 and Fe_78.9Si_5.5B_4.5P_3.5C_4.5Cu_1.0Nb_1.5Al_0.5S_0.1. The sulfur-free specimen exhibited large dendritic crystallites on the surface, resulting in higher coercivity (10.3 A/m) compared to the sulfur-containing specimen (3.4 A/m). In-situ Lorentz transmission electron microscopy analysis revealed that the movement of domain walls was impeded by the presence of large dendritic crystallites on the surface. Secondary ion mass spectrometry confirmed the segregation of sulfur at the surface, which enhanced the GFA and suppressed dendritic crystallization in the sulfur-containing specimen. This study offers valuable insights into the structure–property relationship of Fe-based nanocrystalline alloys and underscores the significance of sulfur segregation in enhancing surface GFA to optimize soft magnetic properties.