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 degreesC, well below the transformation temperature, for 10-80 min after quenching from 900 degreesC. 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. (C) 2002 American Institute of Physics.