Temperature and size dependence of electron magnetic resonance spectra of Ni nanoparticles chemically dispersed in silica

The temperature dependence (5K to 300K) of the electron magnetic resonance (EMR) lines observed at 9.28 GHz in Ni:SiO$_{2}$ (15:85) nanocomposites with mean diameter D of the Ni nanoparticles (NPs) of D=3.8, 11.7, 15 and 21 nm are reported. The sizes of the Ni NPs were determined by TEM and XRD, with SiO$_{2}$ being in the amorphous state. The procedures for the synthesis of the samples along with their DC and AC magnetization behavior were reported recently [1]. In EMR, three resonance lines are observed: (i) Line 1 with linewidth $\Delta $H $\simeq $ 50 Oe and g $\simeq $ 2, and Curie-like variation of the line-intensity, with $\Delta $H and g being temperature and size-independent; (ii) Line 2 with $\Delta $H $\simeq $ 950 Oe and g $\simeq $ 2.2 for D=3.8nm at 300K with both $\Delta $H and g increasing with decreasing T and $\Delta $H size-dependent; and (iii) weak line 3 with g $\sim $ 4 at 300K, with g also increasing with decreasing T. We argue that the line 1 is due to dangling bonds in SiO$_{2}$ as a similar line with $\Delta $H $\simeq $ 9 Oe is also observed in SiO$_{2}$ without Ni doping. Lines 2 and 3 are attributed to majority Ni NPs and large Ni clusters respectively whose anisotropy is both size and temperature-dependent [2], leading to the observed $\Delta $H and g values of the lines. \\[0pt] [1]. Singh, Seehra and Bonevich, J. Appl. Phys. \underline {103}, 070524 (2008) and ibid (in press). \\[0pt] [2]. R. S. de Biasi and T. C. Devezas, J. Appl. Phys. \underline {49}, 2466 (1978).