Role of the amorphous SiO2/C matrix on the anomalous saturation magnetization of Ni nanoparticles

Ni nanoparticles (NPs) with a mean diameter ∼ 5 nm, embedded in an amorphous SiO ₂ and C matrix (SiO ₂ /C) were produced through a modified sol–gel method. Magnetization measurements at temperatures ranging from 2 to 350 K and under an applied magnetic field of 70 kOe showed two different temperature regimes: (i) for temperatures from 2 to 100 K, where an exponential-like decrease in the temperature dependence of the saturation magnetization M S ( T ) is observed; and (ii) a slower decrease in M S ( T ) with increasing T above 100 K, attributed to spin waves, and obeying the predictions of the Bloch’s law. Fittings performed to the data adding appropriate terms to the Bloch’s law indicated that the low temperature behavior of M S ( T ) can be satisfactorily explained as the result of magnetic contributions from the amorphous SiO ₂/C matrix and from the surface of the NPs. The high temperature behavior of M S ( T ) follows the Bloch’s law with an exponent α = 3/2 and an increased B value, compared to the Ni bulk.