Optimization of high-frequency performance for Finemet nanocrystalline soft-magnetic composites via acid passivation

Abstract

Though Finemet SMCs exhibit excellent high-frequency soft-magnetic performance, their core losses (P_cv) at high frequencies remain to be further reduced. In this work, the effects of HNO₃, H₃PO₄, and H₂C₂O₄ passivation on the Finemet nanocrystalline powders were systematically investigated to decrease the P_cv at ~ 1000 kHz. The results show that P_cv exhibits a valley-shaped variation with increasing acid concentration for all three kinds of acids, while the permeability (μ_e) declines gradually. Among the passivated samples, the SMCs passivated with HNO₃ deliver superior comprehensive soft-magnetic properties, achieving a 36% reduction in P_cv at 1000 kHz (B_m = 20 mT) compared with the untreated sample. Loss separation analysis further reveals that acid passivation mainly reduces the core loss by suppressing eddy current loss (P_e). Compared with H₃PO₄ and H₂C₂O₄, the strong oxidizing property of nitric acid accelerates the rapid formation of denser and more continuous insulating oxide films on powder surfaces, which effectively breaks intergranular conductive paths and remarkably reduces P_e dominating high-frequency P_cv. Meanwhile, the passivation layer derived from HNO₃ contains Fe₃O₄, whose partial ferromagnetic contribution is beneficial to alleviate magnetic dilution. As a result, the SMCs after proper HNO₃ passivation maintain a high μ_e, while achieve favorable electrical insulation.