Synthesis and self-assembly of monodisperse CoxNi100−x (x = 50, 80) colloidal nanoparticles by homogenous nucleation

► Monodispersed Co50Ni50 and Co80Ni20 alloy nanoparticles are prepared by reductive thermal decomposition of Co(II) (acac)2 and Ni(II) (acac)2. ► Ni nuclei act as catalyst for the growth of CoNi nanoparticles. ► Significant high coericivity is observed for Co50Ni50 nanoparticles at 5 K due to the random freezing of surface spins. ► The magnetization of CoNi nanoparticles increases with increase of Co content.