• Cryogenic treatment at 213 K optimally enhances soft magnetic properties of (Fe0.5Co0.5)75B21Nb4 metallic glasses while maintaining fully amorphous structure.
• Appropriate CT and relaxation annealing synergistically increase saturation magnetization and reduce coercivity, with linear correlations to free volume and packing density.
• CT promotes uniform nanocrystallization during annealing by lowering nucleation activation energy and increasing growth activation energy.
• Corrosion resistance is improved via stabilization of Nb-rich passive film in 3.5wt% NaCl solution, attributed to CT and relaxation annealing.