Abstract
We report an investigation of the structural, morphological, the ac-dc electrical, magnetic, and Mössbauer spectroscopy properties of 20MnFe2O4-80SiO2 nanocomposite prepared using a one-step and facile auto-combustion approach. XRD pattern shows the formation of MnFe2O4 nanocrystallite without any crystallization of the SiO2 phase. However, the presence of a secondary phase of the nanosized α-Fe2O3 particles was also detected. The morphological analysis showed aggregation of polygonal magnetic nano-crystallites dispersed non-uniformly in a silica matrix. The dc electrical measurements performed on a wide range of temperatures from 120 to 400 K showed the semiconducting nature of the nanocomposite. The temperature dependence of dc conductivity could be perfectly fitted to the nearest neighborhood hopping model with activation energy, ΔENNH, of 0.45 eV. The Nyquist plots demonstrated a nonmonotonous thermally activated trend and non-Debye relaxation behavior. An equivalent circuit was successfully fitted to the complex impedance spectra. The variation of both grain and grain boundary conductivities as a function of temperature exhibited three distinct regions, semiconducting-metallic-semiconducting with different activation energies over the measured temperature window. The provided description of such behavior is further advocated by the ac conductivity and dielectric modulus studies. VSM measurements revealed that the nanocomposite magnetic behavior deviates from the ideal non-interacting superparamagnetic picture, due to the presence of α-Fe2O3 nanocrystalline impurities and relatively intensive exchange interactions between ions. Mössbauer spectra showed the presence of Fe3+ ions with sixfold environment and also confirmed the existence of a sextet related to α-Fe2O3 with a quantity of about 20% out of magnetic components. Fe3+ superparamagnetic doublets were also found within the fitting procedure.
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