Changes in the magnetic moment and other physical properties of a CoPt alloy induced by a new type of ion-beam mixing in an external magnetic field were investigated. This process induces the formation of a metastable phase through extremely rapid quenching from well above the ordering temperature. The measured magnetic moment per Co atom was 2.63 mu(B), larger by 55% and 35% than that of the bulk Co and stable CoPt film, respectively, which is one of the highest values ever observed in the ferromagnetic bulk phase.