Superparamagnetism and microstructural properties of carbon encapsulated Ni nanoparticle assemblies

Carbon encapsulated Ni nanoparticles (Ni(C)) were synthesized by modified arc-discharge reactor under methane atmosphere. The presence of carbon encapsulation is confirmed by HR-TEM imaging, and Nano-diffraction. The average particle radius is typically 10.5 nm with spherical shape. The intimate and contiguous carbon fringe around these Ni nanoparticles is good evidence for complete encapsulation by carbon shell layers. Superparamagnetic property studies were performed using SQUID magnetometer for the assemblies of Ni(C) nanoparticles. The blocking temperature (T_B) is determined to around 115K at 1000Oe applied field. Above T_B, the magnetization M (H, T) can be described by the classical Langevin function L using the relation, M/M_s(T=0) = coth(μH/kT)-kT/μH. The particle radius can be inferred from Langevin fit (particle moment μ) and blocking temperature theory (T_B), which values are a little bigger than HR-TEM observations. It is suggested, these assemblies of carbon encapsulated Ni nanoparticles have been showed typical single-domain, field-dependent superparamagnetic relaxation properties.