The synthesis of FePt/Fe3O4 nanocomposite particles with high coercivity

Summary form only given. High-performance permanent magnets for energy-related applications require a high energy product (BH)_max. It has been proposed that the two-phase materials exchange-coupled between magnetically hard and soft phases may greatly enhance the energy products.[1,2] More recently, self-assembly of two-component with hard magnetic phase FePt and soft magnetic phase Fe₃Pt nanocomposites has successfully enhanced the energy product about 50%.[3] It has research on the synthesis of bimagnetic FePt/M (M=Fe-based metal oxide, such as Fe₃O₄, et.al) core/shell nanostructures. [4,5] Whereas, there has fewer report on the successful synthesis of monodisperse FePt/M composite nanoparticles still with high coercivity. The synthesis and magnetic properties of pure FePt and FePt/Fe₃O₄ composite nanoparticles are studied. Monodisperse nanoparticles of about 5nm for single phase and 10nm for compsite phase where intimate contact between hard and soft phases is evidenced . The structure details of the composite nanoparticles are characterized using high resolution TEM . It shows heterojunction structure with certain lattice match between FePt and Fe₃O₄; and the volum ratio of two phases is 1:1. Most important, the coercivitiy of the composite nanoparticles reaches an exciting value of nearly 20 kOe without any loss comparing with that of corresponding single FePt nanoparticles . The results in this work will give a good reference for the researcher in the field of synthesis of hard/soft nanocompistes. Figure 1 shows the TEM images of the FePt nanoparticles and the heterodimer FePt/Fe₃O₄ nanoparticles (NPs). It can be seen from the images that the synthesized NPs have a good dispersibility. Figure 1a-b reveal the as-synthesized FCC FePt NPs and the annealed FCT FePt NPs. The TEM image of Figure 1c-d give a good dispersion of the as synthesized and annealed- FePt/Fe₃O₄ nanoparticles respecively. Besides, the FePt/Fe₃O₄ nanocomposite particles show a dumbbell heterojunction structure . The detailed structure of the FePt/Fe₃O₄ particles will also be discussed. Figure 2 shows the room-temperature hysteresis loops of the annealed FePt NPs and FePt/Fe₃O₄ heterodimer nanocrystals . It can be seen that the coercivity of the signal FePt phase reached to nearly 20 kOe. It is interesting to find that the coercivity of heterodimer FePt/Fe₃O₄ did not reduce much, and remained almost the same compared to FePt NPs . This work may give us a lot of confidence on the preparing of bipolar composite magnets without covercivity deteration .