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

作者：Misra Sushil K; Li Lin; Mukherjee Sudip; Ghosh Goutam^*

来源：Journal of Nanoparticle Research, 2015, 17(12): 487.

DOI：10.1007/s11051-015-3301-1

• Core-shell magnetic nanoparticles
• Counterions
• Magnetic anisotropy
• EMR spectroscopy
• Magnetization

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 similar to 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 magnetometer 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 (Delta B-PP), measured at 300 K, was also the broadest for TLC-coated CS-NCs. At low temperatures, Delta B-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.

• 出版日期2015-12-18