Synthesis and Magnetic Properties of CoPt Nanoparticles

Xiangcheng Sun, Z.Y. Jia, Y.H. Huang, J. W. Harrell and D. E. Nikles
Center for Materials for Information Technology, The University of Alabama, Tuscaloosa Alabama, 35487-0209
K. Sun and L. M. Wang
Nuclear Engineering and Radiological Sciences, University of Michigan,
Ann Arbor, MI, 48109.

There has been considerable interest in recent years in chemically synthesized magnetic nanoparticles as potential media for storage densities beyond 1 Tb/in2. Following the work by Sun and co-workers at IBM, most emphasis has been on FePt nanoparticles that can be transformed to the high-anisotropy L10 phase after thermal annealing. CoPt also forms a high-anisotropy L10 phase; however, previous reports on chemically synthesized CoPt nanoparticles have not shown the high coercivities expected for particles with a high degree of chemical ordering. The reason may be in part due to the greater need to accurately control the stoichiometry in CoPt as compared with FePt. We have developed a new procedure for synthesizing CoPt nanoparticles using the superhydride reduction of CoCl2 and Pt(acac)2 in the presence of oleic acid and oleyl amine that enables accurate control of the composition. Using this new procedure we have synthesized ~8 nm Co50Pt50 nanoparticles that can be transformed to the L10 phase with high degree of chemical order. After heat treatment at 650oC, the nanoparticles have a room temperature coercivity of 13 kOe and anisotropy energy ~1.7 x 107 erg/cc, the largest values thus far reported for chemically synthesized CoPt nanoparticles. This work demonstrates the potential for CoPt nanoparticles for ultra-high density recording media.