Design of FeCoSiBPCu nanocrystalline alloys with high Bs and tunable soft magnetic properties

The Fe81.3-xCoxSi4B8P5Cu1.7 nanocrystalline alloys (NAs) were synthesized by longitudinal magnetic field annealing (MF) with the aim of improving the saturation magnetic flux density (Bs) while clarifying the magnetic softness mechanism. It is found that Co addition effectively increases the thermal stability, Curie temperature and Bs of alloys. The soft magnetic properties (SMPs) after MF exhibit abnormal variation with microstructure, that is, the no or low Co contented NAs with smaller grain size (D) have a high coercivity (Hc) and high effective permeability (mu e) whereas the high Co concentration NAs with larger D show a low Hc and ultra-low mu(e). Magnetic domain structure observation reveals that the magnetic softness mechanism after MF is dominated by dual anisotropy with mu e mainly determined by average anisotropy whereas Hc and relative permeability (mu(r)) mainly dependent on field induced anisotropy. The Fe66.3Co15Si4B8P5Cu1.7 NA was successfully prepared with a high B-s of 1.84 T, low H-c of 4-5 A/m and high mu(r) of 41,300, as well as low iron loss with 200 J/m(3) at 1T/1 kHz and 209 J/m(3) at 0.5T/20 kHz. The results provide guide for the design of NAs with high Bs and tunable SMPs.