Temperature and size dependence of electron magnetic resonance spectra of Ni nanoparticles embedded in an amorphous SiO2matrix

Investigations of spin dynamics in Ni nanoparticles (NPs) with diameters D = 3.8, 11.7, 15 and 21 nm embedded in an amorphous SiO(2) matrix of composition 15/85 (Ni/SiO(2)) are reported using EMR (electron magnetic resonance) spectroscopy at 9.28 GHz. Three resonance lines are observed whose EMR parameters, namely linewidth ΔH, resonance field H(r) and intensity I(o), are measured from 5 to 300 K. Line 1 with temperature-independent ΔH = 50 Oe and g≈2, and intensity varying as 1/T, is shown to result from paramagnetic defects in the SiO(2) matrix. Lines 2 and 3, with g≈2.2 and 8, respectively, and temperature-dependent EMR parameters are assigned to Ni NPs. While line 2 with g≈2.2 is due to the majority of Ni NPs, the source of line 3 is discussed in terms of two possibilities: (i) large clusters of blocked Ni NPs and (ii) the inherent part of the composite asymmetric line made up of lines 2 and 3 predicted by the Raikher-Stepanov (RS) model for dispersed ferromagnets. The temperature dependence of ΔH (full width at half-maximum) of the composite line obtained by integration of the EMR spectra decreases with the increase in temperature, reaching a minimum near 300 K in agreement with the RS model. The observed decreasing asymmetry of the composite absorption spectra with increasing temperature is also in agreement with the predictions of the RS model, thus providing a satisfactory explanation for the observed temperature dependence of the EMR spectra of Ni NPs. Large clusters of blocked Ni NPs as the source of line 3 are ruled out and additional tests for the RS model are proposed. The decreasing magnetization with decreasing particle size of Ni NPs observed here is discussed in terms of the possible roles of the surface disordered spins and Ni-SiO(2) interaction.