Effect of grain size on the core loss of nanocrystalline Fe86B13Cu1 prepared by ultra-rapid annealing

Nanocrystalline alloy samples with the same chemistry (Fe86B13Cu1) but with a range of mean grain sizes were prepared in order to investigate the effect of the grain size (D) on the core losses in nanocrystalline alloys. The core loss was measured on an Epstein frame in a frequency range between 10 Hz and 30 kHz. The measured core loss was separated into 3 parts, i.e. the hysteresis loss, classical eddy current loss and the excess loss. The classical eddy current loss is only a minor component due to the small thickness and moderately high electrical resistivity of the samples. Both the coercivity and the cycle hysteresis loss are found to follow roughly a D3 dependence. The excess loss at the maximum polarization (Jm) between 0.6 and 1.0 T is described universally by a simple power dependence of (Jm·f)n with an exponent of about 1.4 over a wide frequency (f) range between 10 Hz to 30 kHz. The excess loss at low frequencies appears almost independent of the grain size. Thus, the grain size effect on the total loss at low frequencies is attributable to the change in the hysteresis loss which reflects the static coercivity. However, the excess loss at 30 kHz shows a clear increase with an increase in D, indicating that the excess loss also becomes influential to the grain size dependence of the total core loss at high frequencies.