Development of nanocrystalline materials for soft-magnetic applications

The structural and soft magnetic properties of Fe and FeCo-based nanocrystalline alloys have been investigated. These alloys were initially prepared in the form of amorphous ribbons with consistent properties by optimising different melt-spinning parameters. The structural behavior and soft magnetic properties depended on the alloy chemistry. The effect of metalloids in the Fe73.5Nb3Cu1Si22.5-XBX (X = 5, 9, 10, 11.25 and 19 at %) was studied. X-ray diffractograms showed that formation of a-Fe(Si) and /orFe3Si nanoparticles were responsible for the superior soft magnetic properties of the alloy with 9 at% Boron. All other alloys (X=5,10,11.25 and 19at%) exhibited early appearance of highly magnetocrystalline anisotropic boride phases leading to deterioration in soft magnetic properties. The role of extra alloying elements Al and Mn in the FeNbCuSiB system was also investigated. The alloy exhibited superior soft magnetic properties with a coercivity value of 0.32A/m (~ 4mOe) when heat-treated at 790K for 15min. Transmission electron microscopy study showed that this was due to the formation of ~ 6.0nm sized a-Fe(Si,Al) and /or Fe3(Si,Al) nanoparticles. However, this alloy has a limitation on its use for high temperature soft magnetic application due to its low Curie temperature in nanocrystalline state. Hence, a new Fe40Co40Cu0.5Al2Zr9Si4B4.5 alloy system was developed in which the Curie temperature of 736K in amorphous state increased above 1000K on annealing to nanocrystalline state. The saturation magnetization of the annealed alloy was also found to increase due to the formation of nanocrystalline -(Fe,Co)(Si,Al) phase with higher magnetization suggesting the suitability of the alloy for high temperature soft magnetic applications