The paper reports correlation between processing, structural and magnetic properties of cobalt nano particles dispersed in an insulating oxide matrix. High-temperature, chemical decomposition approach was utilized to achieve highly-dispersed, ultra-fine cobalt nanoparticles in silica matrix. Synthesis parameters such as precursor to passivating agent ratio were designed to control the metal-to-oxide ratio so as to promote magnetic interactions between the metal particles while keeping the cobalt particle size in sub-nanometer. High-resolution TEM and XRD studies were carried out to demonstrate face-centered phase of single-crystal cobalt particles of size similar to 8-10 nm monodispersed in thin silica matrix. The particle size variation as a function of precursor to passivating agent ratio was also studied to yield process conditions for ultra-fine nanoparticles. Composites with sub-10 nm metal particles showed superior soft magnetic properties with a lowest coercivity of 10 Oe. The reduction in coercivity with such fine particles is attributed to the transition to superparamagnetic regime. Larger particles showed higher saturation magnetization and higher coercivity of 100 Oe. The coercivity decreased with further increase in particle size from similar to 50 nm to 80 nm because of the transition from single domain to multi-domain structures. (C) 2016 Elsevier B.V. All rights reserved.