A ferromagnetic amorphous alloy ribbon, a method of fabricating a ribbon and a wound transformer core are provided. The ribbon includes an alloy of FeaSibBcCd where 80.5≦a≦83 at. %, 0.5≦b≦6 at. %, 12≦c≦16.5 at. %, 0.01≦d≦1 at. % with a+b+c+d=100, and is cast from a molten state of the alloy, having a-surface tension of greater than or equal to 1.1 N/mi,. A defect length along a direction of the ribbon's length is between 5 mm and 200 mm, a defect depth less than 0.4 ×t μm and a defect occurrence frequency less than 0.05 ×w times within 1.5 m of ribbon length, where t is the ribbon thickness and w is the ribbon width in mm. The ribbon has a saturation magnetic induction exceeding 1.60 T and a magnetic core loss of less than 0.14 W/kg when measured at 60 Hz and at 1.3 T induction level in an annealed straight strip form, and a core magnetic loss of less than 0.3 W/kg and an exciting power of less than 0.4 VA/kg in an annealed wound transformer core form and is suitable for use in transformer cores, rotational machines, electrical chokes, magnetic sensors and pulse power devices.