Suppressing MHz loss and improving Bs in nanocrystalline cores through alloy design and nanoscale molybdenum oxide insulation

High-frequency magnetic components demand core materials that simultaneously exhibit low core loss ( P cv ), high saturation magnetic flux density ( B s ) and high permeability ( μ e ). Here, we report the design and fabrication of Fe 70.3 Co 3.9 P 4.9 C 2.0 B 7.8 Si 8.8 Cu 0.8 Nb 1.5 nanocrystalline alloy, which exhibits enhanced glass-forming ability (GFA), enabling the production of fully amorphous spherical powders via gas atomization. To suppress high-frequency losses, a nanoscale molybdenum oxide insulating layer with high resistivity was constructed through in-situ thermal decomposition of ammonium molybdate tetrahydrate (AMT). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analyses reveal that an optimal AMT concentration (5 wt.%) leads to the formation of uniform nanoscale dual-layered coatings, comprising an inner amorphous SiO 2 -rich layer (∼5 nm) and an outer polycrystalline MoO 2 /MoO 3 shell (∼60 nm). This engineered interface minimizes eddy currents and high-frequency excess losses. Consequently, the AMT-5 magnetic powder cores exhibit superior comprehensive magnetic properties: a low P cv of 1079 mW/cm 3 (0.02 T, 2 MHz), a high effective B s of 0.98 T, a high μ e of 47 at 20 MHz and robust DC-bias stability (61% under 100 Oe). These findings demonstrate a promising surface engineering strategy combined with alloy design for developing nanocrystalline magnetic composites tailored for advanced power electronics and high-frequency magnetic applications.