RESEARCH PAPER Anisotropic magnetic field observed at 300 K in citrate- coated iron oxide nanoparticles: effect of counterions Sushil K. Misra . Lin Li . Sudip Mukherjee . Goutam Ghosh Received: 4 September 2015 / Accepted: 8 December 2015 Ó Springer Science+Business Media Dordrecht 2015 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-potas- sium 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 ion ), in the order of Li ? \ Na ? \ K ? . Our aim was to investigate the effect of counterions on magnetic interactions between CS-NCs in different powder samples at 300 K, using vibrating sample magne- tometer and electron magnetic resonance (EMR) techniques. The hysteresis loops showed negligible coercivity field (H c ) in all samples. The saturation magnetization (M S ) was the highest for TLC-coated CS-NCs. The blocking temperature (T B ), obtained from zero-field-cooled measurements, was [ 300 K for TLC-coated CS-NCs and \ 300 K for TSC- and TKC-coated CS-NCs. The EMR linewidth (DB PP ), measured at 300 K, was also the broadest for TLC- coated CS-NCs. At low temperatures, DB 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. Keywords Core–shell magnetic nanoparticles Á Counterions Á Magnetic anisotropy Á EMR spectroscopy Á Magnetization Introduction Nano-sized particles have physical and chemical properties that are neither the characteristics of the atom, nor those of their bulk counterparts (Sohn and Cohen 1997). Quantum-size effects and large surface area of magnetic nanoparticles change dramatically some magnetic properties and are responsible for exhibiting superparamagnetic phenomena and quan- tum tunneling of magnetization, because each particle can be considered as a single magnetic domain. The Electronic supplementary material The online version of this article (doi:10.1007/s11051-015-3301-1) contains supple- mentary material, which is available to authorized users. S. K. Misra Á L. Li Department of Physics, Concordia University, Montreal, QC H3G 1M8, Canada S. Mukherjee Á G. Ghosh (&) UGC-DAE Consortium for Scientific Research, Mumbai Centre, Mumbai 400 085, India e-mail: ghoshg@yahoo.com; ghoshg@csr.res.in 123 J Nanopart Res (2015) 17:487 DOI 10.1007/s11051-015-3301-1