Sonochemical preparation and characterization of nanosized amorphous Co–Ni alloy powders

Abstract

Nanosized amorphous alloy powders of Co₂₀Ni₈₀ and Co₅₀Ni₅₀ have been prepared by sonochemical decomposition of solutions of volatile organic precursors, Co(NO)(CO)₃ and Ni(CO)₄ in decalin, under an argon pressure of 100 to 150 kPa, at 273 K. The amorphous nature of these particles was confirmed by various techniques, such as SEM, TEM, SAED, and XRD. A transmission electron micrograph of the heated Co₂₀Ni₈₀ sample showed near uniform particles with sizes less than 10 nm. Magnetic measurements indicated that the as-prepared amorphous CoNi alloy particles were superparamagnetic. The observed magnetization, measured up to a field of 15 kG, of the annealed Co₂₀Ni₈₀ sample (54 emu g^–1) was significantly lower than that for the reported multidomain bulk particles (75 emu g^–1), reflecting the ultrafine nature of our sample. Thermogravimetric measurements of Co₂₀Ni₈₀ with a permanent magnet yielded a glass transition temperature of 338 °C for the amorphous form, and a Curie temperature of 565 °C for the crystallized form. The differential scanning calorimetry showed crystallization temperatures of 400 °C for Co₂₀Ni₈₀ and 365 °C for Co₅₀Ni₅₀ amorphous samples.