Spinel ferrites are magnetic oxides whose crystal structure, described by the general formula AB₂O₄, allows metal cations to occupy two distinct interstitial sites within a framework of oxygen atoms: tetrahedral A sites and octahedral B sites. This architectural flexibility is what makes the family so versatile. Nickel ferrite is an inverse spinel with moderate saturation magnetisation and strong super-exchange interactions between the two sublattices, while magnesium ferrite is a partially inverse spinel prized for its high electrical resistivity, low magnetic losses and thermal stability. Blending the two produces a material already attractive for high-frequency applications, and the non-magnetic character of magnesium ions improves biocompatibility, opening the door to biomedical uses.

The researchers synthesised their nanoparticles using the sol-gel auto-combustion technique, a low-cost route in which metal nitrates dissolved in deionised water are chelated by citric acid in a one-to-one molar ratio. Liquid ammonia adjusts the pH to around seven, and gentle heating at 80 to 90 degrees Celsius transforms the solution into a thick gel. Further heating triggers a self-propagating ignition reaction, releasing gases and leaving behind a porous, fluffy ash-like powder. After grinding and calcination at 900 degrees Celsius for five hours in air, the team obtained phase-pure ferrite samples across all five cerium concentrations.

X-ray diffraction confirmed that every composition crystallised in the cubic spinel structure belonging to the Fd-3m space group, with characteristic reflections from the (220), (311), (400), (422), (511) and (440) planes. The cerium-doped samples also showed weak secondary peaks from iron oxide and cerium dioxide, a signature of the limited solubility of large Ce³⁺ ions in the spinel lattice. The most telling evidence came from the (311) peak, which shifted steadily toward lower angles as cerium content increased. Because Ce³⁺ carries an ionic radius of 1.02 angstroms, substantially larger than the 0.67 angstroms of the Fe³⁺ ions it replaces at octahedral sites, each substitution stretches the lattice. The lattice constant grew from 8.331 to 8.533 angstroms, the unit cell volume expanded from 578.16 to 621.69 cubic angstroms, and lattice strain rose from 2.31 to 4.00 times ten to the minus three.

That same strain suppresses crystal growth. Crystallite sizes calculated with the Debye-Scherrer equation fell monotonically from 40.77 nanometres for the undoped sample to 24.29 nanometres at the highest cerium loading, while Williamson-Hall analysis, which separates size broadening from strain broadening, yielded consistently larger values and confirmed the accumulating microstrain. Field-emission scanning electron microscopy revealed quasi-cubic grains whose average size actually increased with doping, from 57 to 88 nanometres, indicating that each visible grain is an aggregate of many finer crystallites. Porosity dropped from roughly 4.9 percent to 2.5 percent at the highest substitution levels, and energy-dispersive X-ray spectroscopy verified the presence of nickel, magnesium, iron, cerium and oxygen without contaminants.

Optical measurements showed that cerium does more than distort the lattice; it rewires the electronic structure. Ultraviolet-visible spectroscopy revealed strong absorption across the 200 to 800 nanometre range, with the absorption edge shifting toward longer wavelengths as doping increased. Tauc analysis showed the direct band gap narrowing from about 2.2 electronvolts for the undoped ferrite to roughly 2.0 electronvolts at x equals 0.09. The team attributes this red shift to localised cerium 4f states near the conduction band, oxygen vacancies created by charge imbalance, and enhanced iron two-plus to iron three-plus charge-transfer transitions, all of which introduce intermediate energy levels that allow optical transitions at lower photon energies. Urbach tail states arising from lattice disorder extend this visible-light absorption further.

Photoluminescence spectra displayed intense blue emission centred at 408 and 430 nanometres, arising from defect-assisted recombination, oxygen-vacancy charge transfer and cerium 5d to 4f transitions. Emission intensity peaked at the x equals 0.05 composition, which the researchers identify as the optimum concentration of radiative defect centres, before declining at higher loadings through non-radiative concentration quenching. Chromaticity analysis placed all samples firmly in the blue region, with colour purity reaching 91.3 percent for the x equals 0.05 composition and dominant wavelengths near 463 nanometres, characteristics consistent with blue-emitting oxide phosphors for solid-state lighting and optoelectronic devices.

Electrical characterisation painted an equally coherent picture. The dielectric constant was high at low frequencies due to Maxwell-Wagner interfacial polarisation at resistive grain boundaries, then fell to a frequency-independent plateau governed by the grain interior, in line with Koops’ two-layer model. Dielectric loss tangent and alternating-current conductivity followed the classic patterns of hopping conduction between iron ions at octahedral sites, obeying Jonscher’s universal power law at high frequencies. Impedance spectroscopy showed depressed semicircular Nyquist plots characteristic of non-Debye relaxation, with semicircles growing larger as cerium content rose, meaning the dopant increases grain-boundary resistance and makes the material more resistive overall, a useful trait for high-frequency and shielding applications where eddy-current losses must be minimised.

Magnetically, all compositions displayed narrow hysteresis loops typical of soft ferrimagnets, but cerium substitution steadily enhanced the saturation magnetisation, magnetic moment, coercivity, magnetocrystalline anisotropy constant and anisotropy field, while reducing remanence and the squareness ratio below the 0.5 threshold that marks multidomain, soft magnetic behaviour. The team interprets the rising saturation magnetisation through the Yafet-Kittel model: cerium at octahedral sites strengthens A-B super-exchange interactions and reduces the canting angle of B-site spins, aligning the magnetic moments more collinearly and increasing the net moment per formula unit. Meanwhile, lattice strain, grain boundaries and cerium-induced distortions act as domain-wall pinning sites, raising coercivity and anisotropy.

Perhaps the most consequential result came from the antibacterial tests. Using the agar well diffusion method against Staphylococcus aureus and Escherichia coli, the composition Ni₀.₈Mg₀.₂Ce₀.₀₇Fe₁.₉₃O₄ emerged as the standout, producing inhibition zones of 10.2 plus or minus 0.5 millimetres against the Gram-positive S. aureus and 6.1 plus or minus 0.6 millimetres against the Gram-negative E. coli, the only sample active against both organisms. The undoped ferrite and the x equals 0.05 composition showed no activity at all. The researchers caution that diffusion effects in the assay mean the comparison with the streptomycin control, which produced zones of 7.9 and 12.0 millimetres respectively, should be interpreted carefully. The mechanism, they argue, lies in the defect chemistry: cerium substitution increases oxygen vacancies and surface defect sites that catalyse the generation of reactive oxygen species such as hydroxyl and superoxide radicals, which attack bacterial membranes, proteins and genetic material, while the coexisting iron and cerium redox couples facilitate the electron-transfer processes that sustain this oxidative assault. Taken together, the results position cerium-doped nickel-magnesium ferrites as a rare single-material platform spanning microwave components, electromagnetic interference shielding, blue-light optoelectronics and antimicrobial technology, all tuned by one substitution.

Subject of Research: Cerium-doped nickel-magnesium spinel ferrite nanoparticles synthesized by sol-gel auto-combustion for enhanced magnetic, optical, dielectric and antibacterial performance

Article Title: Enhanced magnetic, electrical, optical and antibacterial performance of Cerium doped Nickel-Magnesium ferrites prepared by sol gel method

Article References: Shruthi, M., Gadwala, N., Pradyutha, A., Sridhar, A., Balaganesh, D., & Prasad, M. (2026). Enhanced magnetic, electrical, optical and antibacterial performance of Cerium doped Nickel-Magnesium ferrites prepared by sol gel method. Results in Chemistry, 30, Article 103853. https://doi.org/10.1016/j.rechem.2026.103853

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103853

Keywords: spinel ferrites, cerium doping, nickel-magnesium ferrite, sol-gel auto-combustion, antibacterial nanoparticles, reactive oxygen species, photoluminescence, dielectric properties, soft ferrimagnetism, optical band gap, oxygen vacancies, nanomaterials