Anisotropic magnetic field observed at 300 K in citrate-coated iron oxide nanoparticles: effect of counterions

Abstract

Iron oxide nanoparticles (IONPs) have been synthesized by chemical co-precipitation method and coated with three citrates, namely, tri-lithium citrate (TLC), tri-sodium citrate (TSC), or tri-potassium citrate (TKC). In these ‘core–shell’ structures, the ‘core’ is a cluster of average 3 IONPs which is enveloped by a ‘shell’ of citrate molecules and counterions, and thus called ‘core–shell’ nano-clusters (CS-NCs), of average size ~20 to 22 nm. The counterions in the three CS-NCs differ in ionic radii (\(r_{\text{ion}}\)), in the order of Li+ 300 K for TLC-coated CS-NCs and <300 K for TSC- and TKC-coated CS-NCs. The EMR linewidth (∆B PP), measured at 300 K, was also the broadest for TLC-coated CS-NCs. At low temperatures, ΔB PP was found to increase more significantly for TSC- and TKC-coated CS-NCs than for TLC-coated CS-NCs. These results indicate a significant anisotropic field effect; arising due to thermal motion of counterions at 300 K, on the magnetic interactions in TLC-coated CS-NCs. To our knowledge, this is the first report on the effect of counterions on magnetic interactions between CS-NCs.