Superparamagnetic Ni:SiO2–C nanocomposites films synthesized by a polymeric precursor method

In this work, we report on the magnetic properties of nickel nanoparticles (NP) in a SiO2–C thin film matrix, prepared by a polymeric precursor method, with Ni content x in the 0–10 wt% range. Microstructural analyses of the films showed that the Ni NP are homogenously distributed in the SiO2–C matrix and have spherical shape with average diameter of ~10 nm. The magnetic properties reveal features of superparamagnetism with blocking temperatures T B ~ 10 K. The average diameter of the Ni NP, estimated from magnetization measurements, was found to be ~4 nm for the x = 3 wt% Ni sample, in excellent agreement with X-ray diffraction data. M versus H hysteresis loops indicated that the Ni NP are free from a surrounding oxide layer. We have also observed that coercivity (H C) develops appreciably below T B, and follows the H C ∝ [1 – (T/T B)0.5] relationship, a feature expected for randomly oriented and non-interacting nanoparticles. The extrapolation of H C to 0 K indicates that coercivity decreases with increasing x, suggesting that dipolar interactions may be relevant in films with x > 3 wt% Ni.