Effect of cooling rate on size-dependent atomic ordering of CoPt nanoparticles

We report on the effect of cooling rate on the size-dependent atomic ordering of CoPt nanoparticles using aberration corrected high-resolution transmission electron microscopy. It was found that cooling rate plays a crucial role in promoting atomic ordering during the cooling process after annealing. Nanoparticles of 3 nm in diameter show the A1-disordered phase after annealing at 873K for 1 h followed by rapid cooling (110 K/min), while the L10-ordered phase is obtained when the cooling rate is slow (1.5 K/min). The disordered phase is also obtained by rapid cooling after annealing at 973K for 1 h. These results unambiguously indicate that the order-disorder transformation temperature is reduced to a temperature at least lower than 873K for CoPt nanoparticles smaller than 3 nm in diameter. The slow cooling process promotes the atomic ordering, which resulted in an enhancement of magnetic coercivity as high as 2200 Oe. This study demonstrates that hard magnetic properties of the CoPt nanoparticles can be improved by controlling the cooling rate after heat treatments.