A new type of high‐resistivity soft magnetic amorphous alloy film with a very fine two‐phase heterostructure was found in cosputtered Fe‐B‐C films. These amorphous films are found to have a number of superior properties in the high‐frequency range (over∼10 MHz) over not only regular amorphous films, but also sendust film cores, which have already been applied to high‐density recording media having high coercivity. They were found to exhibit unique isotropic soft magneticB‐Hcurves with highBs(∼15 000 G), lowHc(∼0.3 Oe), and high &rgr; (∼400 &mgr;&OHgr; cm), and the reduced magnetization versus reduced temperature curve was almost the same as thej=∞ curve of the Brillouin function. A very high permeability with less core loss, especially in the high‐frequency range, was also experimentally observed in those amorphous films.