Effect of phase transformation on remanence enhancement in bulk Fe-Pt magnets

We have studied the atomic disorder-order transformation occurring around t he equiatomic composition in Fe-Pt ingots by isothermal annealing at temper atures well below the face-centered-cubic-face-centered-tetragonal (fcc-fct ) transformation temperature. We have determined the relative amount of the low-temperature hard-magnetic fct phase precipitated in the high-temperatu re magnetically soft fcc phase by means of x-ray diffraction. As a function of annealing time, the particle size and the relative amount of the fct ph ase increase at the expense of the fcc phase. We have followed these change s by means of magnetic measurements. The coercivity was found to increase c ontinuously with annealing time, eventually reaching a maximum. The occurre nce of remanence enhancement and the continuously changing degree of exchan ge coupling between the magnetically soft and the hard phase are explained in terms of the Kneller-Hawig model [E. F. Kneller and R. Hawig, IEEE Trans . Magn. MAG-27, 3588 (1991)]. (C) 2001 American Institute of Physics.