Thermal Annealing Tailors Crystallinity and Magnetism in Silica-Coated Ni–Zn Ferrite (SiO₂@NiZnFe₂O₄) Nanoparticles

Materials

The precursors used in this study include iron(III) chloride hexahydrate (FeCl₃·6H₂O), zinc sulfate heptahydrate (ZnSO₄·7H₂O), nickel(II) chloride hexahydrate (NiCl₂·6H₂O), and sodium hydroxide (NaOH), all of analytical grade and purchased from Merck (Germany). Hydrochloric acid (HCl, 37%) was also obtained from Merck and used without further purification. Deionized water (aquadest) was used throughout the synthesis process.

Fabrication of Ni_0.5Zn_0.5Fe₂O₄ Nanoparticle

Ni_0.5Zn_0.5Fe₂O₄ nanoparticles were synthesized via co-precipitation. Stoichiometric amounts of ZnSO₄·7H₂O and NiCl₂·6H₂O were dissolved in 20 mL deionized water. Separately, FeCl₃·6H₂O and NaOH were each dissolved in 50 mL deionized water. The metal-salt solutions were combined, and 3.37 mL of concentrated HCl (37%) was added to assist homogenization and pH control. The mixed solution was introduced dropwise into 1.5 M NaOH under continuous stirring (1000 rpm) at 90 °C for 1 h to induce precipitation. The precipitate was washed with deionized water six times and dried at 90 °C for 5 h. For silica coating, 10 wt% SiO2 (relative to ferrite mass) was added to selected wet powders prior to heat treatment to obtain SiO₂@ Ni_0.5 Zn_0.5 Fe₂O₄ composites. All powders (coated and, where applicable, uncoated controls) were subsequently annealed in air using a programmable muffle furnace with a heating rate of 5 °C min^-1, held at the target temperature for 2 h, and then furnace-cooled to room temperature. Samples are denoted SiO₂@ Ni_0.5 Zn_0.5 Fe₂O₄-xxx, where xxx is the annealing temperature in °C (e.g., SiO₂@ Ni_0.5 Zn_0.5 Fe₂O₄-200, -400, -600, and -800). Uncoated reference samples follow the analogous notation which named SiO₂@ Ni_0.5 Zn_0.5 Fe₂O₄-90.

Materials Characterizations

The crystallographic structure of the samples was analyzed using X-ray diffraction (XRD, Shimadzu XD) equipped with Cu Kα₁ radiation (λ = 1.5406 Å ). Magnetic properties were measured using a Vibrating Sample Magnetometer (VSM, Riken Denshi Co. Ltd.) under a maximum applied magnetic field of 15 kOe at room temperature. Morphological features were examined via Transmission Electron Microscopy (TEM, JEOL JEM-4000). Fourier Transform Infrared Spectroscopy (FTIR, Shimadzu Prestige-21) was employed to investigate the chemical bonding and functional groups present in the samples.

Structural Morphology of Ni_0.5Zn_0.5Fe₂O₄ Nanoparticles

Figure 1a illustrates the spinel crystal structure of Ni_0.5Zn_0.5Fe₂O₄, in which metal cations occupy both tetrahedral (A) and octahedral (B) sites within a face-centered cubic lattice formed by oxygen anions. In this inverse spinel configuration, Ni²⁺ ions predominantly occupy octahedral sites, Zn²⁺ ions preferentially occupy tetrahedral sites, while Fe³⁺ ions are distributed over both. This specific distribution strongly affects the material’s magnetic and structural properties, which are sensitive to annealing temperature and compositional adjustments. Figure 1b displays the FTIR spectra of SiO₂, Ni_0.5Zn_0.5Fe₂O₄ nanoparticles, and the silica-coated composite SiO₂@ Ni_0.5 Zn_0.5 Fe₂O₄ over 3750–500 cm^-1. All spectra exhibit a broad envelope near 3400 cm^-1 together with a band at ~1630–1650 cm^-1, attributable to O–H stretching and H–O–H bending of adsorbed/lattice water, respectively; a weak feature around  2350 cm^-1. The SiO₂ reference shows the characteristic Si–O–Si asymmetric stretching centered at ~1080 cm^-1, accompanied by the symmetric stretching near ~800–850 cm^-1 and the Si–O rocking mode below ~470 cm^-1, consistent with amorphous silica 23. In contrast, the bare ferrite lacks the silica signatures at ~1080 and ~800 cm^-1 and exhibits a low-frequency band emerging in the 550–600 cm^-1 region, assignable to Fe–O stretching in the spinel lattice 24. The spectrum of SiO₂@ Ni_0.5 Zn_0.5 Fe₂O₄ combines these features: the pronounced ~1080 and ~800 cm^-1 bands confirm the silica shell, while a weaker Fe–O band persists around ~560–590 cm^-1, indicating retention of the ferrite core. The slightly stronger O–H envelope in the coated sample is consistent with surface silanol (Si–OH) groups.

The XRD patterns of Ni_0.5Zn_0.5Fe₂O₄ nanoparticles annealed at 90°C, 400°C, and 800°C are shown in Figure 1c. The diffraction peaks are indexed to the cubic spinel structure of Ni–Zn ferrite, consistent with JCPDS card No. 08-0234, confirming phase purity even at the lowest annealing temperature. At 90°C (SiO₂@Ni_0.5Zn_0.5Fe₂-90), the broad and weak peaks indicate poor crystallinity. As the annealing temperature increases, the diffraction peaks become sharper and more intense, suggesting improved crystallinity and grain growth. Crystallite size was estimated using the Scherrer equation from the full width at half maximum (FWHM) of the (311) peak. As shown in Figure 1d, the average crystallite size increases from approximately 14 nm at 400₄-90), the broad and weak peaks indicate poor crystallinity. As the annealing temperature increases, the diffraction peaks become sharper and more intense, suggesting improved crystallinity and grain growth. Crystallite size was estimated using the Scherrer equation from the full width at half maximum (FWHM) of the (311) peak. As shown in Figure [FIGREF:1]d, the average crystallite size increases from approximately 14 nm at 400°C to about 45 nm at 800°C. The silica-coated samples annealed at lower temperatures maintain smaller grain sizes (10–13 nm), indicating that the presence of silica may inhibit grain growth during thermal treatment. These results emphasize that annealing temperature significantly influences the crystallinity and grain size of Ni_0.5Zn_0.5Fe₂O₄ nanoparticles, with higher temperatures promoting larger, well-crystallized structures.

The morphology and microstructure of SiO₂@Ni_0.5Zn_0.5Fe₂O₄-90 nanoparticles were examined using Transmission Electron Microscopy (TEM), with corresponding Selected Area Electron Diffraction (SAED) patterns, as shown in Figure 2. Figures 2a–c present the TEM and SAED results for the sample annealed at 400°C (SiO₂@ Ni_0.5 Zn_0.5-400). The images reveal highly agglomerated and uniformly distributed nanoparticles with near-spherical shapes and sizes in the nanometer range. The SAED pattern (Figure 2c) shows diffuse concentric rings, indicating the polycrystalline nature of the sample but with relatively poor crystallinity, consistent with the broad XRD peaks observed at this temperature.₂O₄-400). The images reveal highly agglomerated and uniformly distributed nanoparticles with near-spherical shapes and sizes in the nanometer range. The SAED pattern (Figure [FIGREF:2]c) shows diffuse concentric rings, indicating the polycrystalline nature of the sample but with relatively poor crystallinity, consistent with the broad XRD peaks observed at this temperature.

In contrast, the nanoparticles annealed at 800°C (SiO₂@ Ni_0.5 Zn_0.5 Fe₂O₄-800, Figures 2d–f) exhibit significant grain growth and coalescence. The particles become larger and more irregular in shape, with clearer grain boundaries. The corresponding SAED pattern (Figure 2f) displays distinct and sharp diffraction spots, confirming enhanced crystallinity and well-defined lattice planes at elevated annealing temperatures. These observations are in agreement with the XRD results and crystallite size analysis (Figure 1d), further supporting the role of thermal treatment in promoting crystal growth and structural order. Overall, the TEM and SAED analyses demonstrate that annealing temperature strongly influences both particle morphology and crystallinity. Lower temperatures yield small, poorly crystallized particles, while higher temperatures promote larger, well-ordered grains with higher structural coherence.

The TEM/SAED evolution from agglomerated nano crystallites at 400°C (diffuse rings) to faceted, well-defined grains at 800°C (spotty pattern) reflects thermally driven coarsening that minimizes total interfacial energy 25. As temperature rises, desorption of adsorbed water and residual ligands reduces electrostatic/steric barriers between primary particles, enabling particle–particle contact and neck formation. Enhanced surface and grain-boundary diffusion then promote sintering/coalescence and occasional oriented attachment of crystallites with low misorientation 26. In parallel, Ostwald ripening operates high-curvature (smaller) particles, which possess higher chemical potential, partially dissolve and redeposit onto larger ones, advancing grain boundary migration and growth 27. The 5 °C min^-1 ramp and 2 h hold provide sufficient diffusion time for these processes, converting loosely aggregated nanoparticles into larger, less defective crystallites with clearer lattice order—consistent with the transition from diffuse rings to discrete diffraction spots in SAED and with the improved crystallinity inferred from XRD.

Magnetic Properties of Ni_0.5Zn_0.5Fe₂O₄ Nanoparticles

The magnetic behavior of Ni_0.5Zn_0.5Fe₂O₄ nanoparticles was investigated using vibrating sample magnetometry (VSM) at room temperature, as shown in Figure 3 and summarized in Table 1. The magnetic hysteresis loop of the as-prepared (non-annealed) Ni_0.5Zn_0.5Fe₂O₄ sample (Figure 3a) displays a narrow loop with a low saturation magnetization (Ms) of 22.93 emu/g, suggesting the presence of a soft magnetic nature with superparamagnetic tendencies due to its small crystallinity and poor crystallinity. Figure 3b and 3c reveal the evolution of the magnetic hysteresis loops as a function of annealing temperature. A noticeable enhancement in magnetic performance is observed with increasing annealing temperature. The Ms values increased significantly from 16.87 emu/g at 90°C to 55.15 emu/g at 800°C. This increase is attributed to improved crystallinity, reduction in surface spin disorder, and grain growth, as previously evidenced by XRD and TEM results.

Similarly, remanent magnetization (Mr) and coercivity (Hc) also exhibit upward trends with increasing temperature. Mr increases from 0.29 emu/g (90°C) to 14.57 emu/g (SiO₂@Ni_0.5Zn_0.5Fe₂O₄-800), while Hc increases from 45.22 Oe to 253.23 Oe. These changes indicate a transition from superparamagnetic behavior at low temperatures toward ferrimagnetic behavior at higher temperatures, likely due to enhanced magnetic domain alignment and reduced surface effects in larger crystallites. The squareness ratio (Mr/Ms), which provides insights into the magnetic domain state, also increases with annealing temperature, from 0.017 at 90°C to 0.264 at 800°C. This suggests a shift toward multi-domain particle behavior and more stable magnetic configurations at higher temperatures. Overall, the data confirm that thermal treatment plays a critical role in modulating the magnetic properties of SiO₂@Ni_0.5Zn_0.5Fe₂O₄-90 nanoparticles. Higher annealing temperatures lead to increased magnetic ordering, improved crystallinity, and larger grain sizes, which collectively enhance the material’s magnetic performance.

With increasing annealing temperature, the hysteresis loops in Figure 3b (low-field zoom in Figure 3c) evolves from narrow, S-shaped curves into wider, squarer loops with higher saturation, reflecting three coupled mechanisms. First, annealing heals structural defects and relaxes strain at surfaces and grain boundaries 28, while driving cation redistribution toward the equilibrium spinel configuration (Zn²⁺ → A sites, Ni²⁺ → B sites); both effects restore A–B super exchange and align previously canted surface spins, yielding a higher net moment Ms 28. Second, grain growth increases the particle volume and reduces the surface-to-volume ratio, diminishing the fraction of disordered “dead-layer” spins and thereby raising both Ms and the remanence Mr 29.

These annealing-driven improvements map directly onto common device needs. Higher crystallinity and grain size yield larger Ms and blocked behavior at room temperature, enabling efficient magnetic capture/actuation of particulate systems (e.g., magnetically recoverable adsorbents/catalysts) and stronger signals in magnetic sensing. For RF/EMI uses, the more ordered Ni–Zn ferrite produced at 400–800°C offers higher permeability while retaining the intrinsically high resistivity of Ni–Zn ferrites, which helps limit eddy-current loss—relevant to inductor cores, EMI-suppression beads, and microwave absorbers. Conversely, lower-temperature samples with smaller grains and lower Hc approach superparamagnetic response, advantageous where rapid “on/off” magnetization is required (e.g., magnetic separation or targeting when appropriately surface-passivated). Thus, tuning annealing temperature provides a practical handle to select magnetic performance for specific application windows 30,31,32.