Magnetic, mechanical, and chemical properties of amorphous Co-based alloys (Fe-Co)75(SizB1z)25 and (Fe-Co-Ru)75(Si0.2B0.8)25 having zero magnetostriction were studied for use as recording/playback head cores. Preferable Si/(SiB) ratio (z) for high saturation induction and high permeability is between 0.15 and 0.2. Small addition of Ru is found to improve wear resistance. A suitable alloy (Fe-Co-Ru-Cr)75(Si0.2B0.8)25 for audio heads has been developed with values of B108.2 kG, Hc20 mOe, and µ1(1 kHz)8,000. Comparing with a commercial sendust head, three features of the amorphous-alloy head were observed: high wear resistance, low bias current, and high output level at high frequencies.