Synthesis and magnetic properties of cobalt nickel nanoparticles prepared by chemical reduction methods

• Main Author: Patel, Jalpa Dipesh
• Published: University College London (University of London) 2007
• Subjects: 546.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.502372

The purpose of this work was to prepare and characterise CoxNii.x/CoNiO core-shell magnetic nanoparticles which showed magnetic exchange bias. The particles were synthesised using a variety of stabilising surfactants and nucleating seeds, via the polyol reduction method. The surfactants were used to coat nanoparticles of various diameters, to prevent agglomeration and oxidation. A mixture of 1:1 oleic acid: oleylamine was found to be the best stabilising agent for the particles as it protected against complete oxidation whilst allowing a partial oxide shell to form. Phosphine-based surfactants yielded particles with spherical morphologies. However, these particles were too small to support oxide-shell growth, and oxidised fully to antiferromagnetic phases. The nucleation of particles was probed using homogeneous and heterogeneous methods. Homogeneous nucleation resulted in particles which had predominantly oxidised to the core, and therefore did not yield pronounced exchange bias effects. Heterogeneous nucleation was attempted using various seeding techniques and seed materials. Platinum seeds were found to be the most effective in controlling the size of CoxNii_x nanoparticles. They yielded larger particles with core-shell morphology. Following optimisation of the synthesis conditions, a compositional series of CoxNii.x nanoparticulate composite alloys were made. All samples were analysed using TEM to determine the size and structure of the individual particles. A number of other techniques including X-ray diffraction, X-ray photoelectron spectroscopy, Energy-dispersive X-ray analysis, Electron energy loss spectroscopy and Thermogravimetric magnetic analysis, were also used to fully characterise the phase, crystallinity, composition and oxidation in individual particles. The magnetic properties of the particles, made using the various reaction conditions, were measured using the SQUID technique. Exchange bias has been observed in several of the alloyed samples in additional to the cobalt particles made using the polyol technique. Nickel particles did not display any characteristic exchange bias properties.