This investigation addresses the effect of nanocrystallization on ferromagnetic ordering in a series of (Fe72-XCox)Si 4B20Nb4, (X = 10, 20, 36, 50 at%) alloy ribbons prepared by the melt-spinning technique. Partial substitution of Fe by Co throughout this series affected the primary and secondary crystallization onsets observed from differential scanning calorimetry and the consequent generation of bcc-FeCo and (CoFe)3B, respectively, as observed from x-ray diffractometry. The reduction in the size of nanoparticles with increasing cobalt content was observed from transmission electron microscopy. At an elevated temperature, the alloy with X = 36 at% was found to reveal superior soft magnetic properties compared with other alloys. In the nanostructured state, this alloy also showed enhanced ferromagnetic ordering and low power losses exhibited its scope as a material for high temperature soft magnetic application. © 2009 IOP Publishing Ltd.