Effect of exchange coupling in nanocrystalline CoPt bulk magnets

The effect of the atomic disorder–order transformation on remanence enhancement and coercivity in isotropic CoPt magnets has been studied by isothermal annealing at 675 °C, well below the transformation temperature, for 10–80 min after quenching from 900 °C. The results of x-ray diffraction, alternating-current susceptibility, and magnetization measurements indicate that the strong remanence enhancement effect originates from exchange coupling between the face-centered-tetragonal ordered hard-magnetic precipitated phase and the face-centered-cubic disordered soft-magnetic matrix phase during the initial ordering transformation. The nonuniform distribution of the magnetic anisotropy results in a lower reversibility, unlike the so-called exchange spring magnet.