Structure and magnetism of ultra-small cobalt particles assembled at titania surfaces by ion beam synthesis

Highlights: • Ion beam synthesis was used to produce ultra-small cobalt particles with sizes below 2 nm embedded at the surface of titania thin films. • The structure and magnetism of the particles in the composite was studied using high-resolution transmission electron microscopy, X-ray absorption spectroscopy and polarised neutron reflectometry. • The cobalt particles are shown to be predominantly metallic and exhibit strong temperature-dependent superparamagnetism with a large moment per cobalt atom. • A thermodynamic argument is presented to explain how the sub stoichiometric titania layer passivates the cobalt particles, and stabilizes unoxidized cobalt particles even under ambient exposure. Metallic cobalt nanoparticles offer attractive magnetic properties but are vulnerable to oxidation, which suppresses their magnetization. In this article, we report the use of ion beam synthesis to produce ultra-small, oxidation-resistant, cobalt nanoparticles embedded within substoichiometric TiO_2-δ thin films. Using high fluence implantation of cobalt at 20–60 keV, the particles were assembled with an average size of 1.5 ± 1 nm. The geometry and structure of the nanoparticles were studied using scanning transmission electron microscopy. Near-edge X-ray fluorescence spectroscopy on the L_2,3 Co edges confirms that the majority of the particles beneath the surface are metallic, unoxidised cobalt. Further evidence of the metallic nature of the small particles is provided via their high magnetization and superparamagnetic response between 3 and 300 K with a low blocking temperature of 4.5 K. The magnetic properties were studied using a combination of vibrating sample magnetometry, element-resolved X-ray magnetic circular dichroism, and depth-resolved polarised neutron reflectometry. These techniques provide a unified picture of the magnetic metallic Co particles. We argue, based on these experimental observations and thermodynamic calculations, that the cobalt is protected against oxidation beneath the surface of titania owing to the enthalpic stability of TiO₂ over CoO which inhibits solid state reactions.