Oriented magnetic nanowires with high coercivity

CoNi magnetic nanowires with a mean diameter of 7 nm and a mean length in the range 100-300 nm have been prepared by reduction of a mixture of cobalt and nickel salts in a sodium hydroxide solution in a liquid polyol. The shape control is related to the particle growth rate which is strongly dependent on the basicity and on the Co-Ni composition. We show that high growth rate favours cobalt urchin-like particles while lower growth rate favours dumbbell-like bimetallic cobalt-nickel particles. Wire formation corresponds to an optimization of the growth conditions and is obtained in a narrow range of basicity and Co-Ni composition. Structural studies showed well crystallized wires with their long axis corresponding to the crystallographic c-axis of the hexagonal close-packed (hcp) phase. Multipods are also observed in which several wires have grown from a single nucleus presenting a cubic structure. Addition of surfactants (trioctyl phosphine and oleic acid) allows the modification of the wire surface and favours their dispersion in toluene. Square hysteresis curves are obtained on wires aligned under magnetic field and frozen in toluene or in PMMA with remanence to saturation ratio close to 1 and coercivity of 6.5 kOe. This value is much higher than was previously obtained on anisotropic particles prepared by the polyol process and results from an improvement of the shape control and the good crystallinity of the wires.