Size control of cobalt nanoparticles by adjusting the linear carboxylic acid ligand chain length

•Chain length of carboxylic acid ligands give sizes control of cobalt nanoparticles.•Formation of cobalt oxide shells causes exchange bias.•Magnetic properties depend on the size of nanoparticle.•Strongest effects of exchange bias observed for the smallest nanoparticle size. Size control in the synthesis of Co nanoparticles (NPs) through thermolysis of Co2(CO)8 is achieved by adjusting the chain length of linear carboxylic acids used as ligands. As the chain length increases, steric effects reduce the reactivity of the carboxylic acids, thereby slowing the nucleation rate. In the subsequent growth phase, completing deposition of Co precursor onto fewer nuclei yields an increased NP size. Oxidation of Co NPs under ambient atmosphere causes formation of an antiferromagnetic CoO shell that can exchange couple to the Co core, resulting in exchange bias. The superparamagnetic blocking temperature (TB), coercivity (HC), and exchange shift (HEB) strongly depend on the size of the NP, and the most pronounced effects of exchange bias are observed for the smallest Co core size.