Effect of Contact Non‑Equilibrium Plasma on Mn_xFe_3−xO_4 Spinel Nanoparticles: Structural and Magnetic Insights

Abstract

Ultrafine manganese ferrites Mn_xFe_3−xO₄ (x = 0–1.3) were synthesized by contact non‑equilibrium plasma (CNP) in two alkaline media (pH 11.5 and 12.5). The influence of cation ratio and initial pH on phase purity, crystallite size and magnetic behaviour was examined by X‑ray diffraction (XRD), differential thermal analysis (DTA), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating‑sample magnetometry. Monodispersed, faceted ferrite particles were obtained for x ≤ 0.8. FTIR spectra in the 1200–1700 cm⁻¹ range revealed surface‑adsorbed water, while the 400–1200 cm⁻¹ region was most sensitive to composition changes, reflecting Fe(Mn)–O, Fe(Mn)–OH and Fe(Mn)–OH₂ vibrations. XRD confirmed a cubic spinel structure with average crystallite size 48–49 Å; a slight decrease in crystallite size with increasing x was observed.

Background

Spinel ferrites containing polyvalent cations form extensive solid‑solution series, enabling fine control over their physicochemical properties. Manganese ferrites (Fe₃O₄–Mn₃O₄) have attracted sustained interest due to their applications in microwave heating, magnetic recording, catalysis (e.g., methane dehydrogenation) and adsorption.

Numerous synthesis routes exist—ceramic, coprecipitation, hydrothermal, reverse micelle, sol‑gel, combustion, mechanochemical and high‑energy approaches—yet achieving nanometric, phase‑pure products at low temperatures remains challenging. Hydrophase methods, particularly coprecipitation, allow precise compositional control but typically require post‑synthesis treatments to eliminate residual oxides and to induce crystallization.

Contact non‑equilibrium plasma (CNP) offers a unique oxidative environment: the discharge generates radicals, free electrons, and reactive oxygen species (OH, O, H₂O₂) that can oxidize metal ions in situ. Prior work has demonstrated that CNP can activate aqueous solutions, promoting the formation of complex oxides with uniform composition and enhanced crystallinity.

In this study we investigate whether CNP can be combined with coprecipitation to produce nanocrystalline Mn_xFe_3−xO₄ spinels, and how the initial pH and Mn/Fe ratio influence phase formation, microstructure and magnetic behaviour.

Methods

Iron(II) sulfate heptahydrate (FeSO₄·7H₂O) and manganese(II) sulfate pentahydrate (MnSO₄·5H₂O) were dissolved in deionized water to prepare 0.5 M stock solutions. Aqueous NaOH (0.5 M) served as the precipitating agent. Two series of coprecipitation experiments were performed at initial pH = 11.5 and pH = 12.5, respectively. For each Mn/Fe molar ratio (x = 0, 0.2, 0.4, 0.6, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3), the corresponding sulfate solutions were added dropwise under vigorous stirring to the NaOH solution until a clear suspension formed. The precipitates were then subjected to CNP treatment in a cylindrical reactor (inner diameter 45 mm, height 85 mm) equipped with a 4 mm stainless‑steel lower electrode and a 2.4 mm upper electrode 10 mm above the liquid surface. A high‑voltage pulsed supply (up to 15 kV, 1.5 ms width) produced a discharge at 100 Hz, with a typical burning voltage of 750–900 V. Treatment times ranged from 10 to 40 min. After CNP, the precipitates were washed until sulfate‑free, filtered, and dried at 150 °C.

Compositional analysis of Mn and Fe was carried out by complexometric titration (Mn²⁺) and permanganate/bichromate methods (Fe). The Mn/Fe molar ratio was calculated as x/(3‑x). Magnetic properties (saturation magnetization I_S and coercive force H_C) were measured with a vibrating‑sample magnetometer at room temperature.

Phase identification and crystallite size were obtained from XRD patterns (monochromatized Co‑K_α radiation, DRON‑2 diffractometer). Crystallite size and microstrain were calculated by the Scherrer–Williamson method. Morphology and particle size were examined by SEM (REMMA‑102) and TEM (JEOL Jem 1010, 200 kV). Thermal behaviour was assessed by DTA/DTG (Derivatograph Q‑1500D, heating rate 10 °C min⁻¹, 20–1000 °C). FTIR spectra were recorded on a Nicolet iS10 spectrometer (400–4000 cm⁻¹). All reference standards (γ‑Al₂O₃) and sample masses (200 mg) were used as per standard protocols.

Results and Discussion

Phase analysis showed that samples prepared at pH = 12.5 yielded single‑phase cubic spinel ferrites (JCPDS 10‑0467) for x ≤ 0.8, while those at pH = 11.5 contained residual Fe₂O₃ and MnO_x phases, indicating a maghemite‑type ferritization mechanism. The absence of Fe₂O₃ and MnO_x peaks in the XRD patterns of the pH 12.5 series confirmed the efficacy of CNP in promoting complete oxidation and spinel formation.

Crystallite sizes (Scherrer) ranged from 48 to 49 Å for x < 1.0, with a gradual reduction as x increased. TEM images revealed monodispersed, faceted nanoparticles with primary sizes 5–8 nm; the apparent crystallite size from XRD was roughly four times larger due to particle aggregation.

Magnetic measurements (Fig. 2) demonstrated that saturation magnetization peaked at x = 0.8 (M_S ≈ 60 emu g⁻¹) for the pH 12.5 series, surpassing the stoichiometric MnFe₂O₄ value. Coercivity (Fig. 3) increased with Mn content up to x ≈ 0.8, then declined, correlating with the observed phase purity and crystallite size. Samples from the pH 11.5 series exhibited significantly lower M_S and H_C due to non‑magnetic impurity phases.

FTIR spectra (Fig. 8) displayed characteristic Fe(Mn)–O stretching bands (≈700 cm⁻¹) that shifted to lower wavenumbers (≈688 cm⁻¹) as x approached 0.8, reflecting increased Mn occupancy of octahedral sites. A new absorption at 445 cm⁻¹ appeared for x = 0.8–0.9, indicative of octahedral Mn²⁺ and Mn³⁺ incorporation. The 1200–1700 cm⁻¹ region confirmed the presence of adsorbed and lattice water.

Thermal analysis (Fig. 6) showed mass losses at 100 °C (free water) and 160 °C (bound water). Endothermic peaks between 280–360 °C corresponded to Fe²⁺→Fe³⁺ and Mn²⁺→Mn³⁺ oxidation, while exothermic events at 600–700 °C marked further oxidation to Fe³⁺O₄ and Mn³⁺O₄ phases. After calcination at 1000 °C, XRD confirmed the formation of rhombohedral α‑Fe₂O₃ and α‑Mn₂O₃ in stoichiometric samples, reflecting the limits of Mn²⁺ substitution at high temperatures.

Overall, the data demonstrate that CNP-assisted coprecipitation at high pH (12.5) produces nanocrystalline, phase‑pure Mn_xFe_3−xO₄ spinels with superior magnetic properties, particularly for x = 0.6–0.8.

Conclusions

We have established a scalable route to synthesize ultrafine Mn_xFe_3−xO₄ spinel nanoparticles (x = 0–1.3) via coprecipitation followed by contact non‑equilibrium plasma treatment. The method yields single‑phase cubic spinels for x ≤ 0.8 at pH 12.5, with crystallite sizes 48–49 Å and average particle diameters 70–80 nm. Magnetic performance peaks for x = 0.6–0.8, achieving saturation magnetizations comparable to or exceeding bulk MnFe₂O₄. FTIR and thermal analyses confirm the structural integrity and the role of Mn occupancy in octahedral sites. These findings highlight CNP as a powerful tool for tailoring the structural and magnetic properties of ferrite nanomaterials.

Abbreviations

CNP:

Contact non‑equilibrium plasma

DTA:

Differential thermal analysis

DTG:

Differential thermogravimetric analysis

FTIR:

Fourier transform infrared

I_S:

Saturation magnetization

SEM:

Scanning electron microscopy

T_C:

Curie temperature

TG:

Mass loss

XRD:

X‑ray diffraction

Нс:

Coercive force (Oe)