Influence of Co substitution for Fe on magnetic properties and crystal structure of soft magnetic Fe81.3Mo0.2Cu0.5Si4B14 alloy

•Small addition of Co = 2 at.% enhanced the magnetic saturation from 1.48 T for Co-free alloy up to 1.58 T at 500 A/m.•The strong correlation between the nanocrystallites size and magnetic saturation has been shown for Co-content alloys.•For Co substituted alloys the grain growth is similar over a wider temperature range. The magnetic properties correlated with crystal structure of soft magnetic ribbons of the nominal composition Fe81.3−xCoxMo0.2Cu0.5Si4B14 synthesized by a melt spinning technique are reported. The crystallization process was monitored by differential scanning calorimetry (DSC) that allows identification of the primary crystallization temperature. Thereafter crystallization process is being optimized by changing heat treatment parameters in order to get best possible magnetic properties. The resulting crystal structure of the properly annealed cores has been investigated by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods. It was found that the structure consists of α-Fe(Co,Si) nanograins with the mean size of 20 nm. Additionally, the magnetic saturation and coercivity from the measured hysteresis loops of studied samples have been determined. The microstructure was further correlated with Co content and nanograins size in order to discuss its impact on magnetic properties.