Abstract

Background: Spinel ferrites are compound oxides with a general chemical formula of AB2O4, where A and B are divalent and trivalent ions, respectively. They can be a transparent conductor or ferromagnetic insulator and show many outstanding properties that are applicable in high technological devices, such as microwave absorbers, photocatalytic activities, lithium-ion battery, spintronics and so forth. Their magnetic properties are sensitive to fabrication and processing conditions, particularly for nanostructured spinel ferrites due to the redistribution of Fe ions at A and B sites as discussed in relevant patents.

Objective: We have prepared Zn0.5Co0.5Fe2O4 nanoparticles (ZCFO NPs) and studied in detail the influence of the annealing temperature (Tan) and ambience on their structural and magnetic properties at room temperature.

Method: ZCFO NPs prepared by a hydrothermal method were annealed at temperatures Tan = 393-1573 K in air and Ar gas for 4 h at atmospheric pressure. The surface morphology and chemical composition of NPs were analyzed by using a field-emission scanning electron microscope equipped with energy dispersive X-ray spectroscopy. The crystal structure was checked by an X-ray diffractometer. Vibration spectra were acquired with a micro-Raman spectrometer. The magnetic properties at RT were studied by using a vibrating sample magnetometer.

Results: Careful analyses of X-ray diffraction patterns and Raman spectra indicate that the samples with Tan < 873 K are single phase and stable in the cubic-spinel structure. The annealing at higher temperatures, Tan ≥ 873 K, leads to the formation of a secondary phase (α-Fe2O3, hematite). This reduces rapidly the saturation magnetization (Ms) of ZCFO NPs. Particularly, around Tan = 1473 K, we have found a partial recovery of cubic-spinel NPs, making Ms slightly increased. The recovery ability in the Ar ambience is better than that in air.

Conclusion: Results showed that ZCFO NPs prepared by a hydrothermal method were unstable versus heat treatment. While the cubic-spinel structure remains unchanged at Tan < 873 K, the annealing at temperatures Tan ≥ 873 K leads to the formation of α-Fe2O3, and around Tan = 1473 K, there was a partial recovery of the cubic spinel phase. Notably, the co-presence of Co and Zn in ZCFO NPs restricts remarkably the recovery ability of the spinel structure. Such structural changes influence directly the exchange interactions between Fe ions at the A and B sites of the spinel structure, leading to the variations of Ms and Hc versus Tan and annealing ambience.

Keywords: Co-doped zinc ferrites, crystal structure, hydrothermal synthesis, heat treatment, magnetic properties, nanoparticles.