Superparamagnetism and magnetic properties of Ni nanoparticles embedded inSiO2

We have performed a detailed characterization of the magnetic properties of Ni nanoparticles embedded in a ${\mathrm{SiO}}_2$ amorphous matrix. A modified sol-gel method was employed which resulted in Ni particles with average radius $\sim 3\mathrm{nm},$ as inferred by TEM analysis. Above the blocking temperature $T_B\approx 20\mathrm{K}$ for the most diluted sample, magnetization data show the expected scaling of the ${M/M}_S$ vs $H/T$ curves for superparamagnetic particles. The hysteresis loops were found to be symmetric about zero field axis with no shift via exchange bias, suggesting that Ni particles are free from an oxide layer. For $T<\mathrm{}{T}_B$ the magnetic behavior of these Ni nanoparticles is in excellent agreement with the predictions of randomly oriented and noninteracting magnetic particles, as suggested by the temperature dependence of the coercivity field that obeys the relation $H_C{\mathrm{}\left(T\right)=H}_{C0\mathrm{}}\left[1-{(T/T}_B{)}^{1/2}\right]$ below $T_B$ with $H_{C0\mathrm{}}\sim 780\mathrm{Oe}.$ The obtained values of $H_{C0\mathrm{}},$ considering the temperature dependence of the magnetic anisotropy constant, are discussed within the scenario of isolated randomly oriented and noninteracting single-domain particles.