Approach-to-magnetic saturation of Fe nanoparticle/nanorod encapsulated multi-walled carbon nanotubes under heavy ion irradiation

Abstract

Effects of irradiation with swift heavy ions at different fluences on the structural and magnetic properties of multi-walled carbon nanotubes filled with Fe nanorods and nanoparticles (Fe-MWCNTs) are investigated. An up-shift in the ‘G’- band of the Raman spectra with the increasing fluence designates the development of stresses in Fe-MWCNTs. During the ion-irradiation, the disorder parameter that signifies the defect concentration initially decreases at lower fluence and then picks-up with a further rise in fluence. Structural characterization by electron microscopy reveals damaged walls and formation of defects in Fe-MWCNTs irradiated at the highest fluence. The average diameter of Fe-MWCNTs inferred from high-resolution TEM shows a reduction from 56 (±2) nm to 36 (±2) nm upon increasing the irradiation fluence from 5 × 1012 to 1 × 1014 ions/cm2. The ferromagnetic character of Fe nanoparticles/nanorods embedded in the MWCNTs swamps the diamagnetic nature of bare MWCNTs and hence a saturation behaviour in magnetization is seen for the Fe-MWCNTs. A significant reduction in the saturation magnetization (≈20%) is observed in the ‘irradiated’ Fe-MWCNTs when compared to their ‘as-prepared (pristine)’ counterparts and is attributed to the increase in surface disorder and weakening of exchange interactions upon irradiation. The coercive field follows a monotonous thermal decline in both ‘pristine’ and ‘irradiated’ Fe-MWCNTs. Contrary to the expectation, the coercivity of the ‘irradiated’ Fe-MWCNTs is less than nearly half the value for ‘pristine’ samples, in the entire temperature range 2 K–300 K.