Curie temperatures of annealed FePt nanoparticle systems

The chemically synthesized FexPt100−x nanoparticles with controlled compositions were annealed to transfer the disordered face-centered-cubic structure to the ordered structure. It was found that the L10 FePt structure can be formed in the wide compositional region of x=40–68, and lower or higher Fe content leads to formation of the L12 FePt3 or Fe3Pt phase, respectively. The Néel temperature of L12 FePt3 phase and Curie temperatures (TC) of L10 FePt and L12 Fe3Pt phases are all strongly composition dependent. The room-temperature saturation magnetization has an abnormal dependence on x which is caused by the low TC of Fe3Pt phase with x=75–79. The big difference in TC between the heating and cooling thermomagnetic curves of the Fe–Pt alloys with x=79–90 can be attributed to the difference of α⇔γ phase transition temperature during heating and cooling. On the other hand, TC of the L10 FePt nanoparticles was found to be strongly size dependent.