Effect of Bi Substitution on the FCC to L1$_{\mathrm{\mathbf{0}}}$ Phase Transformation in CoPt(Bi) Nanoparticles

The transformation from the fcc to fct structure L1$_{\mathrm{0}}$ in CoPt requires annealing at temperatures over 600$^{\mathrm{0}}$ C, as compared to FePt which can occur at 550$^{\mathrm{0}}$ C. In the past, similar attempts to lower the transformation temperature in CoPt have been unsuccessful. In this work, we report for the first time a decrease in the phase transformation temperature of chemically synthesized CoPt nanoparticles by the addition of a small amount of bismuth. Our studies have shown that the phase transformation occurs in as-made CoPt(Bi) nanoparticles at refluxing temperatures as low as 330 $^{\mathrm{0}}$C, which is significantly lower than previously reported values in CoPt nanoparticles and thin films. The as-made CoPt nanoparticles with 5{\%} atomic weight Bi show partial L1$_{\mathrm{0}}$ ordering with an average size of 11.7 nm, as shown by TEM imaging, and have a coercivity of 1 kOe and saturation magnetization of 32 emu/g. Annealing of the CoPt(Bi) nanoparticles produced maximum coercivities of 12.4 kOe when annealed at 700 $^{\mathrm{0}}$C for 1 hour. The effect of amount of Bi addition on the formation and ordering of L1$_{\mathrm{0}}$ structure will be discussed.