Magnetic nanoparticles with hybrid shape

Co50Ni50 particles with very unusual shapes, resembling dumbbells or diabolos, are obtained by reducing mixtures of cobalt and nickel acetates in sodium hydroxide solution in 1,2-propane diol. These particles consist of a central column richer in cobalt than the overall composition, capped with two terminal platelets that are richer in nickel. These hybrid shapes are the result of a two-step growth mechanism due to a difference in reactivity of the two metal ions. The sodium hydroxide concentration controls the length and diameter of the column, in the ranges 50-250 nm and 5-15 nm, respectively, and the diameter of the platelets in the range 25-50 nm. The X-ray diffraction patterns show a mixture of hcp and fcc phases in various proportions depending on the particle shape. High-resolution electron microscopy shows that the hcp phase is located mainly in the central column and the fcc phase mainly in the terminal platelets. The particles are ferromagnetic at room temperature. When the volume fraction of the central column is high enough and the hcp phase is predominant and not much faulted, high coercivity (up to 1900 Oe) is observed.