A magnetic core material made of a Fe-Co base alloy for use at high frequencies, and has excellent high-frequency alternating magnetic properties. The core material composition is Co: 45-53% by weight, C: 0.3-3% by weight, one or both of Mn and Si: 0.1-2% by weight, at least one of Mg, Ca, and Ce: 0.1-0.2% by weight, and the remainder Fe and unavoidable impurities. The structure of the alloy comprises a ferrite matrix containing graphite in an amount of from 1-20% by volume dispersed therein. The material is applied, for example, in a dot impact printer and to a pulse motor.