Halide Ion-Mediated Synthesis of L1-FePt Nanoparticles with Tunable Magnetic Properties.

L1-FePt nanoparticles (NPs) have great potential in areas of advanced magnetic and catalytic applications. Here, we present a facile control route for synthesis of hard magnetic L1-FePt NPs in which halide ions (Cl, Br, or I) were added to the synthetic process to promote the phase transformation. It is confirmed that the strong ionic binding force between halide ions and Fe or Pt ions could facilitate the formation of L1-FePt phase due to favoring growth of FePt NPs in a more thermodynamically stable way, which enables the formation of an ordered structure. L1-FePt NPs with the highest coercivity of 8.64 kOe and saturation magnetization of 64.21 emu/g at room temperature can be directly obtained by controlling the amount of the halide ions. In comparison with conventional solution phase reduction methods, the halide ion-assisted method shows enhanced capability to tune the growth of hard magnetic bimetallic NPs, particularly Pt-based bimetallic NPs.