Magnetic hardening in nanocrystalline FeCoSiBCuNb alloy

Abstract

The crystallization process and magnetic properties of Fe 73.5−x Co x Si 13.5B 9Cu 1Nb 3 ( x=0, 5, 15, 29, 36, 45, 69, 73.5) melt-spun alloys are analysed as a function of x. For x⩽45 ultrafine BCC α-FeSi (Co) crystallites (mean grain diameter in the range ∼13–16 nm) in an amorphous matrix are obtained after thermal treatments at 550°C (1 h) while, for higher Co contents the precipitation of FCC β-Co rich crystallites is observed. For x up 29, a slow increase in coercivity with x is observed that can be correlated, within the framework of the random anisotropy model, with the increase in the mean grain size of the α-FeSi(Co) crystallites with x. However for x=36 and 45, the nanocrystalline alloys manifest a noticeable magnetic softening compared with the alloys of lower Co contents, which can be ascribed to a reduction in the magnetocrystalline anisotropy of the precipitated α-FeSi(Co) phase. Finally beyond x=45, the coercivity of the nanocrystalline alloys increases drastically, eventually by several orders of magnitude, reflecting the change to β-Co rich nanocrystallites.