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S5.8.1

Particle Size and Oxidation in CoNi Nanoparticles Jalpa Patel1, Dorothy Farrell2, Ivan P. Parkin1, and Quentin A. Pankhurst2 Department of Chemistry, University College London, 20 Gordon Street London WC1H 0AJ, United Kingdom 2 London Centre for Nanotechnology, 2-16 Torrington Place London WC1E 7HN, United Kingdom 1

ABSTRACT Cobalt nickel nanoparticles with an oxide shell were prepared by polyol reduction syntheses with varying heating rates, reflux times, surfactants, and reagent concentrations. Differences in particle formation temperature, as evidenced by the reaction medium changing color from violet to black, the UV-Vis absorption spectra of aliquots of reaction media extracted throughout the synthesis reaction, and the colloidal stability of the particle dispersions were observed when the surfactant was changed between otherwise identical reactions. These changes in the reaction kinetics, and therefore the particle sizes of the different samples, also resulted in varying magnetic properties and air stabilities of the samples. Particles with diameters of 15 nm, synthesized using oleic acid, displayed exchange biasing and enhanced coercivity below 50 K, with Hex = 650 Oe and an increase in Hc from 570 Oe to 910 Oe after cooling in field to 2 K. These particles were stable in air over a period of at least five months, showing no change in magnetic moment. Smaller (6 nm) particles synthesized with the same surfactant did not exhibit exchange biasing after field cooling and experienced degradation of their magnetic properties upon exposure to air for one month. Particles synthesized with trioctylphosphine and mixtures of trioctylphosphine and oleic acid were 5 nm and formed the most stable dispersions, but oxidized upon drying in air to non-ferromagnetic phases. INTRODUCTION The polyol method of synthesis for submicron particles is well established, and in the last decade modifications of the technique have been used to produce monodisperse nanoparticles of numerous magnetic metals (e.g., Fe, Co, Ni, FeCo, CoNi) [1]. Cobalt nickel nanoparticles can be synthesized by the reduction of hydrated cobalt and nickel acetate salts in high boiling point organic solvents in the presence of a surfactant, with the final alloy composition determined by the [Co]:[Ni] precursor ratio. Previous work using oleic acid as the surfactant with a 3:1 surfactant:metal salt molar ratio to synthesize CoNi nanoparticles 10 nm and larger suggests that each particle consists of an oxide shell around an alloy core. Exchange coupling between core and shell leads to exchange anisotropy in the particles and therefore enhanced coercivity, and the core/shell composition is stable for up to one year [2]. High quality cobalt nanoparticles are standardly synthesized using mixtures of trialkyphosphines or phosphine oxides and oleic acid [3, 4], and alkylamine stabilized nickel particles have also been reported [5]. However, these particles are generally air sensitive and carefully stored to prevent oxidation. CoNi particles were synthesized in