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Nanocrystalline and nanocomposites of (Mn0.6Zn0.4Fe2O4)(1−z)(SiO2)z (z=0.0,0.10,0.15,0.25) were prepared by sol-gel method. The as-prepared samples were annealed at 200, 400, and 600 °C. The crystallographic phases, particle sizes and their distributions, etc. of the annealed samples were determined by x-ray diffraction and high-resolution transmission electron microscopy (TEM). Zero field cooled (ZFC) and field cooled (FC) magnetizations of some selected samples measured in the temperature range of 300–5 K suggest the presence of superparamagnetic (SPM) relaxation in the samples. For a few selected samples particle sizes were also estimated from the TEM micrographs and dc magnetizations. The particle sizes, magnetic anisotropies and blocking temperatures of some samples have also been computed from the difference of FC and ZFC magnetizations data. The particle sizes of the samples were also worked out from the Langevin fitting of the anhysteretic magnetization curve above the blocking temperature. The SPM relaxations of all the samples have also been confirmed by ac magnetic measurements. The SPM relaxations in the nanocomposite samples annealed at higher temperature (600 °C) is stronger than the bare sample which is attributed to the controlled rate of growth of particle sizes by SiO2 coating. In case of sample annealed at 400 °C, the value of magnetic anisotropy constant determined from the ZFC and FC magnetizations is in agreement with that obtained from the Neel’s equation. However it is a bit smaller for the sample annealed at 600 °C.

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Nanoparticles of GaFeO3 (GFO) and Ni0.4Zn0.4Cu0.2 Fe2O4 (NZCF) and their nanocomposite [(GaFeO3)0.50 (Ni0.4Zn0.4Cu0.2Fe2O4)0.50, GFONZCF] were prepared by chemical route. Nanoparticles of GFO were synthesized by sol–gel route, and those of NZCF were prepared by chemical coprecipitation method. The nanoparticles of GFO were incorporated in the matrix of NZCF by coprecipitating the salts required for NZCF in the presence of GFO particles, followed by subsequent washing and heat treatment at 500 °C. X-ray diffractograms (XRDs) were recorded to confirm the formation of the desired crystallographic phases of the samples. The sizes of the nanoparticles were estimated from the broadening of the well-defined peaks using the Debye–Scherrer equation. The nanoparticle size and its distribution, crystallographic phase, nanocrystallinity, and so on were studied by a high-resolution transmission electron microscope (HRTEM), and the extracted results were in good agreement with those obtained from the XRD patterns. The static and dynamic magnetic measurements were carried out. The observations of field-cooled (FC), zero-field-cooled (ZFC) magnetizations, and hysteresis loops (M-H loop) in the temperature range of 300 to 2 K were carried out in the static measurements. The static magnetic data were analyzed to evaluate the particle size, nanocrystalline anisotropy, and so on, and the agreement of these evaluated data are quite satisfactory, so far as the extracted results obtained from XRD and HRTEM are concerned. The maximum magnetization of the GFO sample has been drastically enhanced by incorporating them in the matrix of NZCF. Also, the nature of variation of the magnetization in all cases of FC, ZFC, and M-H curves of the nanoparticles of GFO has been drastically modulated by the NZCF. The dynamic magnetic measurements include the measurements of ac magnetization versus excitation curves, hysteresis loops at different frequencies at room temperatures, and so on. The remarkable enhancement of magnetization of the multiferroic system of GFO by the encapsulation of NZCF would be quite interesting for various applications.