pH‑Controlled Sol–Gel Synthesis of Nanocrystalline Strontium Ferrite for Low‑Temperature Permanent Magnet Applications

Abstract

Nanocrystalline strontium ferrite (SrFe₁₂O₁₉) was synthesized by a sol–gel auto‑combustion route with citric acid as a chelating agent. The precursor pH was varied from 0 to 8 and the powders were calcined at a low temperature of 900 °C. Structural, microstructural and magnetic properties were evaluated by XRD, FESEM, and VSM. The most acidic precursor (pH 0) produced a single‑phase product with the highest density, grain size (≈73 nm), saturation magnetization (44.2 emu g⁻¹), and coercivity (6.4 kOe). A slight increase in pH promoted the formation of Fe₂O₃ impurities at pH 8 and reduced magnetic performance. The results demonstrate that pH control is a powerful tool for tailoring SrFe₁₂O₁₉ properties at temperatures suitable for co‑fired ceramic permanent magnets.

Highlights

• Sol–gel auto‑combustion synthesis of SrFe₁₂O₁₉ nanoparticles.
• Single‑phase ferrite obtained at 900 °C with pH ≤ 7.
• Optimal magnetic parameters (Ms = 44.2 emu g⁻¹, Hc = 6.4 kOe) achieved at pH 0.

Background

Strontium hexaferrite (SrFe₁₂O₁₉) is prized for high coercivity, electrical resistivity and corrosion resistance, making it a key material for microwave devices, high‑density recording media and permanent magnets. Conventional solid‑state synthesis requires high temperatures (~1300 °C) and often yields large, polydisperse particles. The sol–gel method offers a low‑temperature route (≤ 900 °C) that produces homogenous, nanocrystalline powders with controllable stoichiometry and particle size. The acidity of the precursor strongly influences metal–oxide polymerization and, consequently, the crystallinity and magnetic performance of the final ferrite. This study explores the impact of precursor pH on the structure, microstructure and magnetism of SrFe₁₂O₁₉ prepared by a citric‑acid‑based sol–gel process.

Methods

Synthesis

Sr(NO₃)₂ (98 %) and Fe(NO₃)₃ (99 %) were dissolved in 100 mL deionized water at 60 °C under stirring (250 rpm). Citric acid (C/N = 0.75) was added and the temperature raised to 80 °C. Ammonia (NH₄OH) was introduced to adjust the solution pH from 0 to 8. The mixture was stirred at 90 °C until a green gel formed, then heated to 200 °C for one hour to induce auto‑combustion. The resulting powders were calcined at 900 °C for 6 h (5 °C min⁻¹ heating rate). A schematic of the synthesis is shown in Figure 1.

Characterization

Phase purity and crystallography were assessed by X‑ray diffraction (Cu Kα, 20°–80°). Lattice parameters and theoretical density were calculated from the XRD data. Grain size and morphology were examined with FESEM (100 k×, 5 kV). Porosity was derived from Archimedes density measurements. Magnetic hysteresis loops were recorded at room temperature with a VSM (±12 kOe). All data are reported in Tables 1–3.

Results and Discussion

Structural Analysis

XRD patterns (Figure 2) confirm the formation of the hexagonal SrFe₁₂O₁₉ phase (JCPDS 98‑004‑3603) for pH 0–7. At pH 8, a minor Fe₂O₃ impurity (≈28 %) appears, reducing phase purity to 87.8 %. Lattice constants (a ≈ 5.88 Å, c ≈ 23.02 Å) are consistent with literature values, indicating well‑crystallized ferrite. Experimental density decreases with increasing pH, reaching a minimum at pH 8, while porosity rises correspondingly (Figure 4). The most acidic precursor (pH 0) yields the highest density (4.69 g cm⁻³) and lowest porosity (8.2 %).

Microstructural Analysis

FESEM images (Figure 5) reveal spherical grains at pH 0 with an average size of 73.6 nm, while higher pH values produce elongated, agglomerated grains up to 133.3 nm (Figure 6). Grain size distributions shift toward larger diameters as pH increases, correlating with the observed decrease in magnetic parameters. Porosity from combustion gases (NH₃) further disrupts grain packing at higher pH.

Magnetic Properties

Hysteresis loops (Figure 7) show that Ms, Mr and Hc decline monotonically with increasing pH. The optimal sample (pH 0) achieves Ms = 44.19 emu g⁻¹ (226.2 emu cm⁻³), Mr = 27.59 emu g⁻¹ and Hc = 6403.6 Oe. These values surpass many sol–gel reports and approach single‑crystal benchmarks. The high coercivity is attributed to strong magnetocrystalline anisotropy and dense grain packing, while the high Ms reflects minimal porosity and phase purity. At pH 8, reduced Ms and Hc (5118 Oe) are linked to Fe₂O₃ contamination and larger grains.

Conclusions

Low‑temperature (900 °C) sol–gel synthesis of SrFe₁₂O₁₉ nanoparticles is feasible when the precursor pH is maintained at ≤ 7. The most acidic condition (pH 0) yields a single‑phase, dense, nanocrystalline ferrite with superior magnetic properties, making it a promising candidate for co‑fired ceramic permanent magnets operating at reduced temperatures.

Abbreviations

• SEM – Scanning Electron Microscope
• FESEM – Field Emission Scanning Electron Microscope
• VSM – Vibrating Sample Magnetometer
• XRD – X‑ray Diffraction
• Ms – Saturation Magnetization
• Mr – Remanence
• Hc – Coercivity
• Fe₂O₃ – Hematite