Critical role of the low-magnetostriction regime in high-frequency loss of nanocrystalline alloys

To clarify the effect of small saturation magnetostriction in the order of a few ppm on high-frequency soft magnetic properties of Fe-based nanocrystalline alloys, a wide range of Fe–Nb–B–Cu alloy samples were prepared by varying Nb content as well as annealing conditions using ultra-rapid and conventional annealing techniques. Loss separation analysis reveals that the optimum annealing condition to obtain the lowest core loss is often frequency-dependent due to the competing contributions of hysteresis and excess losses. While the hysteresis loss increases with grain coarsening and secondary crystallization, the excess loss decreases with the suppression of magnetostriction through the fractional reduction of the residual amorphous phase. A strong correlation between excess loss ( P ev ) and saturation magnetostriction (λ s ) is confirmed even in a low λ s regime below 3.5 ppm and P ev is reduced by about 90% as λ s decreases from 3.5 ppm to below 1 ppm. Furthermore, analysis based on an extended Bertotti model suggests that the number of dynamically active magnetic regions increases more readily with the driving field as λ s approaches near zero. Alloys with near-zero magnetostriction exhibit improved high-frequency performance; a high initial permeability (at 0.05 A/m) around 20,000 at 100 kHz and a low core loss about 2 W/kg at 1 kHz and 1.0 T are obtained with saturation polarization around 1.6 T. Our results confirm that minimizing magnetostriction, in addition to grain refinement, is critical for realizing excellent core properties in nanocrystalline soft magnetic materials.