Magnetic Properties and Chemical Synthesis of FePt and CoPt Nanoclusters

Much interest has been paid to self-assembled L1$_{0}$ FePt and CoPt nanoclusters for basic science and potential applications in ultra-high density magnetic recording media. In this study, thermal decompostion of a Fe and Pt acetylacetonates mixture in trioctylamine was employed to produce FePt nanoclusters with average size about 4 nm and a narrow size distribution. The atomic ratio of Fe and Pt for the as-synthesized FePt clusters is 50:50. Using the same procedure, CoPt nanoclusters were obtained with the atomic ratio of 40:60. With the addition of oleic acid into the reaction solution the atomic ratios were adjusted to 50:50. As-synthesized FePt and CoPt nanoclusters have fcc structures, but after rapid thermal annealing in forming gas the nanoclusters transform into hard magnetic L1$_{0}$ ordered structures. The nanostructures of the clusters were characterized by TEM and XRD. The magnetic properties were measured by a SQUID magnetometer. The coercivity of FePt and CoPt clusters reached the maximum value of 23 kOe and 17kOe respectively after annealing at 750 $^{o}$C for 5mins. Beyond 750 $^{o}$C, the coercivity decreased slightly because of the sintering effect and a different magnetic reversal mechanism. This work is supported by NSF-MRSEC, DOE, INSIC, NRI and NCMN.