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L1 0 -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 0 -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 3+ or Pt 2+ ions could facilitate the formation of L1 0 -FePt phase due to favoring growth of FePt NPs in a more thermodynamically stable way, which enables the formation of an ordered structure. L1 0 -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.

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