Magnetic Properties Enhancement of FeNi45 Soft Magnetic Composites Based on Ni _0.5 Zn _0.5 Fe ₂ O ₄ Ferrite-Phosphate Insulation Layer

Soft magnetic composites (SMCs) have found extensive application in electrical energy conversion. Enhancing the high-frequency operational efficiency and reducing the core loss of the materials are critical for improving energy conversion efficiency and minimizing energy consumption. Here, we develop a methodology to enhance the effective permeability and its frequency stability under high-frequency operation and reduce core loss, through optimized particle size distribution and improved insulation layer morphology. A Ni–Zn ferrite-phosphate coating approach is proposed. Results indicate that optimizing the particle size distribution of FeNi45 to 53–75 μm leads to a 44.3% increase in effective permeability (f = 50 kHz) and an 82.0% reduction in core loss (f = 50 kHz, Bm = 50 mT). As the serrated grain morphology and crack of the Ni–Zn ferrite layer proved insufficient for insulation, phosphoric acid with good wettability was subsequently applied over the ferrite layer, forming a continuous, complete, and dense Ni–Zn ferrite-phosphate composite insulation layer. Scanning electron microscopy analysis revealed that the phosphate effectively fills the gap and crack within the ferrite, blocking eddy current paths. With a 1 wt % phosphoric acid addition, yielding a core loss of 146.71 mW/cm3, particularly the eddy current loss decreased significantly from 386.21 mW/cm3 to 26.47 mW/cm3 (f = 50 kHz, Bm = 50 mT). Concurrently, exhibited an effective permeability of 82.85 (f = 50 kHz) and a DC bias performance of 53% (f = 50 kHz, HDC = 7.96 × 103 A/m (1 Oe ≈ 79.6 A/m)). These findings provide insights and practical methodologies for optimizing coating strategies in SMCs, which is beneficial for enhancing their high-frequency working efficiency in electronic applications and reducing energy consumption.