Two-phase separation facilitating the ordering of L10-FePt: experimental and thermodynamic investigation on Fe-Pt-Cu phase diagrams

The L10-FePt nanocrystals exhibit unique magnetic and catalytic properties, but suffer from the ordering difficulty. The addition of Cu can facilitate the L10-FePt ordering for some Fe-Pt-Cu alloys, however, the action mechanism is far from being well understood. In this work, a systematic study has been performed to the phase diagrams of the Fe-Pt-Cu system. The isothermal sections at 600 degrees C, 750 degrees C and 1000 degrees C, as well as the partial vertical sections of Fe50Pt50-Cu50Pt50 and Fe50Pt50-Cu, were constructed. An emphasis has been paid to the L10FePt phase, which has a wide composition range with a solid solubility of Cu up to 40.0 at% at 600 degrees C. Its orderdisorder transition temperature decreases rapidly when deviating from 54 at% Pt, but at different lower rates along (FePt)100-xCux and (Fe50-xCux)Pt50 with the increase of Cu. The ternary miscibility gap which originates from the Fe-Cu side has been studied by both experimental determinations and thermodynamic calculations. The results indicate that, at certain low temperatures, for instance 300 degrees C, the metastable miscibility gap can expand even up to 45 at% Pt, entering the L10-FePt phase region. An ordering transition mechanism of the L10-FePt phase stimulating by a two-phase separation was thus proposed. For a specially designed disordered Fe-Pt-Cu alloy, a metastable two-phase separation can take place firstly in the annealing process, driving the Cu atoms forming Cu-rich particles, leaving a large number of vacancies in the alloy, and consequently, accelerate the atomic diffusion and stimulate the subsequent L10 ordering transition. These results will provide a new perspective for the composition design of Fe-Pt-Cu catalysts and magnetic recording media.