Abstract: The growing demand for high-efficiency and low-loss energy conversion and transportation techniques urges the development of advanced Fe-Si based soft magnet alloys. Simultaneous achievement of low coercivity (H_c) and large saturation magnetization (M_s) however, remains challenging. In this study, soft magnetic alloys with the composition Fe_82-xSi₁₈Co_x (x = 0 at.%, 4 at.%, 8 at.%, 12 at.%, 16 at.%, and 20 at.%) have been designed followed by microstructural tuning. The Co incorporation results in initially decreased H_c followed by increment due to reduced magnetocrystalline anisotropy and increased saturation magnetostriction from negative to positive values of the alloys. Meanwhile, the M_s raises with subsequent reduction, which origins from competitive mechanisms of increased average moment of Fe atoms and decreased average moment of Co atoms according to first principles calculations. Microstructural evolution during annealing of the Fe₇₀Si₁₈Co₁₂ with synergistically optimized H_c and M_s has been revealed that after elevated-temperature annealing, the DO₃ phase is predominately transformed from the B2 phase accompanied by an increase in the degree of ordering. The growth of the DO₃ phase deteriorates the H_c due to the aggravating pinning effect on the domain wall movement, which arises from the inhomogeneous magnetization distribution caused by increasing antiphase boundaries.

Key words: Fe-Si-Co alloy, Compositional design, Microstructural evolution, Magnetic domain, Magnetic property