We present a detailed magnetic study of the perovskite manganite ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ at low temperatures including magnetization and ac susceptibility measurements. The data appear to exclude a conventional spin glass phase at low fields, suggesting instead the presence of correlated ferromagnetic clusters embedded in a charge-ordered matrix. We examine the growth of the ferromagnetic clusters with increasing magnetic field as they expand to occupy almost the entire sample at $H\sim 0.5\mathrm{T}.$ Since this is well below the field required to induce a metallic state, our results point to the existence of a field-induced ferromagnetic insulating state in this material.

This paper presents the influence of Mn²⁺ substitution by Ni²⁺ on the structural, morphological and magnetic properties of Mn_1−xNi_xFe₂O₄@SiO₂ (x = 0, 0.25, 0.50, 0.75, 1.00) nanocomposites (NCs) obtained by a modified sol-gel method. The Fourier transform infrared spectra confirm the formation of a SiO₂ matrix and ferrite, while the X-ray diffraction patterns show the presence of poorly crystalline ferrite at low annealing temperatures and highly crystalline mixed cubic spinel ferrite accompanied by secondary phases at high annealing temperatures. The lattice parameters gradually decrease, while the crystallite size, volume, and X-ray density of Mn_1−xNi_xFe₂O₄@SiO₂ NCs increase with increasing Ni content and follow Vegard’s law. The saturation magnetization, remanent magnetization, squareness, magnetic moment per formula unit, and anisotropy constant increase, while the coercivity decreases with increasing Ni content. These parameters are larger for the samples with the same chemical formula, annealed at higher temperatures. The NCs with high Ni content show superparamagnetic-like behavior, while the NCs with high Mn content display paramagnetic behavior.

This paper presents the synthesis of metal doped Co ferrites, M_0.2Co_0.8Fe₂O₄ (M = Cu²⁺, Ni²⁺, and Zn²⁺) embedded in SiO₂ matrix by an innovative sol-gel route. The structural and morphological characterization provided information about the crystalline phases, crystallite size, and the shape of the prepared ferrites. The thermal study depicted the thermal decomposition and stability of the obtained ferrites. X-ray diffraction indicated nanocrystalline ferrites with spinel structure and the lack of crystalline phase impurities, while Fourier transform infrared spectroscopy revealed the presence of functional groups in precursors and ferrite powders. The lattice parameters showed a gradual increase indicating a uniform distribution of divalent metal ions in the Co ferrite lattice. The crystallite size, magnetic moment, super-exchange and deflection of magnetic domain were influenced by the dopant metal ion. The room temperature magnetization indicated a ferromagnetic behavior of the ferrites annealed at 1000 °C and a superparamagnetic behavior of the ferrites annealed at 700 °C.

The structure, morphology and magnetic properties of (Ni_0.6Mn_0.4Fe₂O₄)_α(SiO₂)_100−α (α = 0–100%) nanocomposites (NCs) produced by sol-gel synthesis were investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and vibrating sample magnetometry (VSM). At low calcination temperatures (300 °C), poorly crystallized Ni_0.6Mn_0.4Fe₂O₄, while at high calcination temperatures, well-crystallized Ni_0.6Mn_0.4Fe₂O₄ was obtained along with α-Fe₂O₃, quartz, cristobalite or iron silicate secondary phase, depending on the Ni_0.6Mn_0.4Fe₂O₄ content in the NCs. The average crystallite size increases from 2.6 to 74.5 nm with the increase of calcination temperature and ferrite content embedded in the SiO₂ matrix. The saturation magnetization (Ms) enhances from 2.5 to 80.5 emu/g, the remanent magnetization (M_R) from 0.68 to 12.6 emu/g and the coercive field (H_C) from 126 to 260 Oe with increasing of Ni_0.6Mn_0.4Fe₂O₄ content in the NCs. The SiO₂ matrix has a diamagnetic behavior with a minor ferromagnetic fraction, Ni_0.6Mn_0.4Fe₂O₄ embedded in SiO₂ matrix displays superparamagnetic behavior, while unembedded Ni_0.6Mn_0.4Fe₂O₄ has a high-quality ferromagnetic behavior.

The effect of SiO₂ embedding on the obtaining of single-phase ferrites, as well as on the structure, morphology and magnetic properties of (Zn_0.6Mn_0.4Fe₂O₄)_δ(SiO₂)_100−δ (δ = 0–100%) nanoparticles (NPs) synthesized by sol-gel method was assessed. The phase composition and crystallite size were investigated by X-ray diffraction (XRD), the chemical transformations were monitored by Fourier transform infrared (FT-IR) spectroscopy, while the morphology of the NPs by transmission electron microscopy (TEM). The average crystallite size was 5.3–27.0 nm at 400 °C, 13.7–31.1 nm at 700 °C and 33.4–49.1 nm at 1100 °C. The evolution of the saturation magnetization, coercivity and magnetic anisotropy as a function of the crystallite sizes were studied by vibrating sample magnetometry (VSM) technique. As expected, the SiO₂ matrix shows diamagnetic behavior accompanied by the accidentally contribution of a small percent of ferromagnetic impurities. The Zn_0.6Mn_0.4Fe₂O₄ embedded in SiO₂ exhibits superparamagnetic-like behavior, whereas the unembedded Zn_0.6Mn_0.4Fe₂O₄ behaves like a high-quality ferrimagnet. The preparation route has a significant effect on the particle sizes, which strongly influences the magnetic behavior of the NPs.