Study of morphology and magnetism of MnFe2O4–SiO2 composites

Abstract

Mixed magnetic composites of manganese ferrite and cristobalite (MnFe2O4–SiO2) were produced by the solid-state reaction method, using as starting materials in natura ores found in the state of Rio Grande do Norte – Brazil. Based on the stoichiometric proportions of the precursor ores, various composites in the form of powders and pellets were synthesized at a temperature of 1200 °C/12h under ambient atmosphere. Samples in the form of pellets were made at compression pressures of 370 and 740 MPa to investigate the effect of pressure on the physicochemical, and dielectric properties of the material. Structural, morphological and magnetic analyses were studied for the series of samples. X-ray diffractogram (XRD) results confirmed the formation of the mixed spinel phase MnFe2O4 with cubic symmetry and space group Fd-3m:1, together with peaks corresponding to SiO2–cristobalite. The Rietveld refinements identified a predominance of about 90% of the MnFe2O4 phase with crystallite variations of 130–190 nm due to the effect of compaction pressure. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electronic microscopy (TEM) mappings show clusters of triangular crystals in the pellets series. Magnetic hysteresis cycles at room temperature show a fast magnetization saturation (MS) response in a field of ∼4 kOe, with parameters dependent on the size and shape of the crystallites. The magnetically soft ferrites exhibited very low coercive field values. The hysteretic curves measured at 5 K showed a significant gain in the MS of ∼30 emu/g. Analyzes of magnetization as a function of temperature identified a blocked regime with a maximum ∼97 K (powder) and ∼170 K (pellets), and an irreversibility of the ZFC–FC curves up to room temperature. An indication of a spin-glass (SG) like state close to 20 K can also be seen in these results. Dielectric characterization of the pellets was performed from 1.0 to 8.5 GHz, and presented well-defined behavior, where real permittivity can be highly related to Rietveld refinement of XRD, and SEM/EDS results. This research suggests the feasibility of producing manganese ferrite composites directly from minerals, without the use of high-purity chemical reagents, allowing their obtainment on a large scale for commercial use.