Remanence enhancement 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 in ice–water from 900°C. The results of X-ray diffraction, transmission electron microscopy, alternating-current (ac) susceptibility and magnetization measurements indicate that the strong remanence enhancement effect originates from exchange coupling between the face-centered-tetragonal (fct) ordered hard-magnetic precipitated phase and the face-centered-cubic (fcc) disordered soft-magnetic matrix phase during the initial ordering transformation. The non-uniform distribution of the magnetic anisotropy results in a lower reversibility, unlike the so-called exchange spring magnet.