Magnetic dynamics of single-domain Ni nanoparticles

The dynamic magnetic properties of Ni nanoparticles diluted in an amorphous SiO2 matrix prepared from a modified sol-gel method have been studied by the frequency f dependence of the ac magnetic susceptibility chi(T). For samples with similar average radii similar to3-4 nm, an increase of the blocking temperature from T(B)similar to20 to similar to40 K was observed for Ni concentrations of similar to1.5 and 5 wt %, respectively, assigned to the effects of dipolar interactions. Both the in-phase chi'(T) and the out-of-phase chi(T) maxima follow the predictions of the thermally activated Neel-Arrhenius model. The effective magnetic anisotropy constant K-eff inferred from chi(T) vs f data for the 1.5 wt % Ni sample is close to the value of the magnetocrystalline anisotropy of bulk Ni, suggesting that surface effects are negligible in the present samples. In addition, the contribution from dipolar interactions to the total anisotropy energy E-a in specimens with 5 wt % Ni was found to be comparable to the intrinsic magnetocrystalline anisotropy barrier. (C) 2003 American Institute of Physics.