An unusual temperature dependence of the coercive field in the melt-spun Fe80-xSi20NbxFe80-xSi20Nbx(x=12,20)(x=12,20)

The coercive field of the as-cast Fe68Si20Nb12Fe68Si20Nb12 and Fe60Si20Nb20Fe60Si20Nb20 ribbons decreases with increasing temperature abruptly up to 35 K, then slowly up to around 200 K. Above this temperature the coercive field increases. Qualitatively this is attributed to the point that this unusual coercive field curve behavior is related mainly to the nano-crystalline material containing two nanocrystalline phases (BCC Fe–Si and hexagonal NbFe2-ySiyNbFe2-ySiy), where exchange interactions among these small particles occur. The fraction of the hexagonal phase increases with increasing Nb and after annealing decomposes in two hexagonal phases with similar composition. For the Fe60Si20Nb20Fe60Si20Nb20 alloy the quantity of NbFe2-ySiyNbFe2-ySiy nanocrystallites is almost the same of that BCC Fe–Si particles.