Threshold-like features in the magnetic response of iron-filled multi-walled carbon nanotube and polymer composite

The magnetic properties of iron-filled multi-walled carbon nanotubes dispersed in polystyrene (Fe-MWNT/PS) have been investigated as a function of Fe-MWNT concentration (0.1-15 wt%) from 300 to 10 K. Electron microscopy studies indicate that Fe nanorods (aspect ratio similar to 5) remain trapped at various lengths of MWNT and are thus, prevented from oxidation as well as aggregation. The magnetization versus applied field (M-H loop) data of 0.1 wt% of Fe-MWNTs in PS show an anomalous narrowing at low temperatures which is due to the significant contribution from shape anisotropy of Fe nanorods. The remanence shows a threshold feature at 1 wt%. The enhanced coercivity shows a maximum at 1 wt% due to the dominant dipolar interactions among Fe nanorods. Also the squareness ratio shows a maximum at 1 wt%.