Magnetic dynamics of single-domain Ni nanoparticles

The dynamic magnetic properties of Ni nanoparticles diluted in an amorphous ${\mathrm{SiO}}_2$ matrix prepared from a modified sol–gel method have been studied by the frequency f dependence of the ac magnetic susceptibility $\mathrm{\chi (T).}$ For samples with similar average radii ∼3–4 nm, an increase of the blocking temperature from $T_B\text{∼20}$ to ∼40 K was observed for Ni concentrations of ∼1.5 and 5 wt %, respectively, assigned to the effects of dipolar interactions. Both the in-phase ${\chi }^{\prime }\mathrm{(T)}$ and the out-of-phase ${\chi }^{″}\mathrm{(T)}$ maxima follow the predictions of the thermally activated Néel–Arrhenius model. The effective magnetic anisotropy constant $K_{\mathrm{eff}}$ inferred from ${\chi }^{″}\mathrm{(T)}$ 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.