Trends Sci. 2026; 23(8): 12474

Introduction

Spinel ferrites have attracted tremendous attention in the last decades owing to their remarkable structural versatility and tunable magnetic properties [1,2], which make them suitable for a wide spectrum of technological applications, including magnetic data storage [3], ferrofluids [4], microwave devices [5], catalysis [6], and biomedical fields [7]. Among them, cobalt ferrite (CoFe₂O₄) has been recognized as a particularly promising material due to its high chemical stability, significant coercivity, moderate saturation magnetization, and mechanical hardness [8]. These features position cobalt ferrite as a candidate for multifunctional applications where both durability and magnetic performance are crucial.

One of the most effective approaches to tailor the structural and magnetic properties of cobalt ferrite is cationic substitution at tetrahedral (A) or octahedral (B) sites within the spinel lattice [9,10]. Such substitutions can induce profound modifications in cation distribution, exchange interactions, and spin alignment, thereby altering the overall magneto-structural behavior. In this context, zinc (Zn²⁺) substitution has been widely explored because of its strong site preference for tetrahedral coordination and its non-magnetic nature [11-14]. The introduction of Zn²⁺ ions into CoFe₂O₄ is expected to displace magnetic Co²⁺ ions from tetrahedral to octahedral sites, which in turn modifies the superexchange interactions between Fe³⁺ ions across A and B sites [15]. This redistribution mechanism not only governs the balance between ferromagnetic and antiferromagnetic contributions but also directly impacts domain structure evolution.

Previous studies have reported that zinc incorporation generally enhances saturation magnetization while reducing coercivity, which is beneficial for soft magnetic applications [16-18]. However, the underlying microscopic mechanism, particularly the emergence of antiferromagnetic (especially spin-down) ordering within the tetrahedral sublattice, remains insufficiently clarified. Most reports primarily address macroscopic magnetic measurements without combining them with structural refinements and magnetic symmetry analysis, which are essential to unambiguously establish the correlation between cation redistribution and magnetic ordering.

On the other hand, synthesis is a crucial step in modulating the structural phenomena and functional properties of CoFe₂O₄ nanoparticles, so that the regulation of thermal conditions, especially the synthesis temperature and annealing temperature, is an aspect that cannot be ignored. The study reported by Fatimah et al. [19] on Zn-substituted CoFe₂O₄ nanoparticles showed through TG-DTA analysis that the precursor decomposition process and the formation of a stable spinel phase began to be achieved at a temperature of around 400 C. This finding indicates that this temperature is sufficient to produce initial thermal stability, but still requires further treatment to improve crystallinity. Therefore, in this study an annealing temperature of 450 C was chosen, which is expected to encourage the development of a better crystal structure without causing excessive particle growth or undesirable phase changes.

In this study, the structural and magnetic behavior of zinc-substituted cobalt ferrite nanoparticles synthesized via the sol-gel method was systematically investigated. X-ray diffraction combined with Rietveld refinement and FTIR spectroscopy was employed to confirm phase purity and spinel formation. Magnetic measurements, supported by magnetic structure simulations using the BasIreps program, provide insights into the spin configuration induced by Zn²⁺ substitution, particularly at tetrahedral and octahedral sites. This work contributes to a deeper understanding of the role of zinc in modulating cation distribution, which supports the orientation of the magnetic moment in cobalt ferrites, offering valuable guidelines for designing advanced soft magnetic materials with controllable properties.

Materials and methods

Materials

Analytical grade ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), and citric acid (C₆H₈O₇) were ob­tained from Merck (≥ 99% purity) and used without further purification. Deionized water was employed as the solvent throughout the synthesis process. Zinc con­centrations were varied with substitution levels of x = 0.0, 0.2, and 0.4 in Zn_xCo_1−xFe₂O₄.

Synthesis of Zn-substituted cobalt ferrite na­noparticles

Zinc-substituted cobalt ferrite nanoparticles were synthesized via a modified sol-gel method. Stoichio­metric amounts of Fe(NO₃)₃·9H₂O, Co(NO₃)₂·6H₂O and Zn(NO₃)₂·6H₂O (according to the desired composition) were dissolved in deionized water under continuous magnetic stirring. Citric acid was introduced as a chelating agent to promote gel formation.

The homogeneous solution was stirred for 5 min and subsequently heated to 90 C until a gel-like structure was formed. The resulting viscous gel was subjected to hydrolysis at 100 C until a dry precursor was obtained. The dried precursor was manually ground to ensure homogeneity before being annealed at 450 C for 6 h in a muffle furnace. After annealing, the powder was reground to achieve uniform particle size and homogeneity, yielding the final Zn_xCo_1-xFe₂O₄ nanoparticles (Figure 1).

Figure 1 Schematic of the synthesis of Zinc-substituted CoFe₂O₄ nanoparticles by sol-gel method.

Characterization techniques

The structural, vibrational, and magnetic properties of the synthesized nanoparticles were comprehensively characterized. Structural analysis was carried out using X-ray diffraction (XRD, PanAnalytical X’Pert Pro, Cu-Kα, λ = 1.5406 Å) combined with Rietveld refinement to determine lattice parameters, crystallite size, and phase composition. Additionally, Crystallite size (D) was calculated by the Debye-Scherrer equation via the hkl (311) strongest peak, which was the result used for determining lattice strain (), lattice parameter (a_cal), and density (d_x) from XRD results. The parameters of the XRD results were calculated as follows [20-22]:

where is the wavelength of the Cu-K source, is the full width at half maximum (FWHM), is the Bragg diffraction angle, d_hkl is the distance between the planes, M is the molecular weight, and N_A is Avogadro’s number.

Fourier transform infrared (FTIR) spectroscopy (Shimadzu IR Prestige 21) in the range of 400 - 4,000 cm⁻¹ was employed to identify metal-oxygen bonding and confirm spinel formation. Magnetic measurements were performed using a vibrating sample magnetometer (VSM, Oxford VSM 1.2H) at room temperature with an applied magnetic field ranging from –10 to +10 kOe to evaluate coercivity (Hc), saturation magnetization (Ms), and remanence (Mr). On the other hand, VSM results were also used for determining magnetic moment (n_B) and magnetocrystalline anisotropy (K), as follows [10]:

For support of VSM analysis, magnetic structure simulations were conducted using the BasIreps program to elucidate further spin-reverse orientation ordering induced by Zn substitution.

Results and discussion

The X-ray diffraction (XRD) patterns presented in Figure 2 confirm that all Zn_xCo_1−x​Fe₂​O₄​ samples with different Zn concentrations (x = 0, 0.2 and 0.4) crystallize in a cubic spinel (FCC) structure with the Fd-3m space group, in agreement with the ICDD 22-1086 database [13,23]. The most intense diffraction peak is observed for the (311) plane, which is the characteristic orientation of the spinel phase. This finding indicates that Zn²⁺ substitution into the crystal lattice does not alter the primary phase, but modifies the lattice parameter and microstructural characteristics through cation redistribution between tetrahedral and octahedral sites.

Figure 2 XRD Pattern of Zn_xCo_1−xFe₂O₄ nanoparticles (x = 0.0, 0.2 and 0.4) by sol-gel method.

The Rietveld refinement results (Figure 3 and Table 1) further support this observation, where the experimental diffraction data are well-fitted with a single spinel phase model. The low goodness of fit (²) magnitude, which is close to 1, together with the reliability factors (R_p, R_wp, R_exp​), demonstrate the consistency of the refined structural model with the experimental data [10,24]. Moreover, the lattice parameter (a) increases progressively with higher Zn concentration, which can be explained by the difference in ionic radii between Zn²⁺ (0.60 Å, tetrahedral site) and Co²⁺ (0.90 Å, octahedral site) [25]. This substitution induces local distortions, leading to lattice expansion. Such a trend is consistent with previous reports on Zn-substituted ferrites, where redistribution of cations within the spinel structure results in similar unit cell enlargement [26].

Furthermore, the calculated crystallite size (D), lattice parameter (a_cal), theoretical density (d_x), and lattice strain (), as shown in Table 2 [10], reveal the significant effect of Zn substitution. The crystallite size, D, decreases with increasing Zn content, while the lattice strain, ε, increases (Figure 4). The reduction in crystallite size is associated with the inhibitory role of Zn²⁺ in grain growth, owing to enhanced lattice distortions [26]. In contrast, the increase in lattice strain is attributed to internal relaxation and structural imperfections induced by cation substitution [24]. The increase in the calculated lattice parameter, a_cal, is consistent with the refinement results, which can be explained by the difference in ionic radii between Zn²⁺ and Co²⁺, leading to local distortions that result in lattice parameter expansion [27]. These findings collectively imply that Zn substitution enhances internal lattice stress and encourages more refined recrystallization, both of which are anticipated to impact the material’s functional characteristics.

Figure 3 Rietveld refinement on XRD results for Zn_xCo_1−xFe₂O₄ nanoparticles (x = (a) 0%, (b) 20%, and (c) 40%) by sol-gel method.

Table 1 The rietveld analysis parameters at crystal system cubic and space group Fd-3m of the samples S1, S2 and S3 (S1 = CoFe₂O₄, S2 = Co_0.8Zn_0.2Fe₂O₄, S3 = Co_0.6Zn_0.4Fe₂O₄).

Hence, Zn substitution in cobalt ferrite is a key factor in altering the structural characteristics, as demonstrated by the combined evidence from XRD patterns, Rietveld refinement, and crystallographic parameter analysis.  These structural changes offer chances to customize the material’s functional characteristics through regulated cation substitution and confirm the stability of the single-phase spinel.

Figure 4 The effect of Zn concentration on crystallite size and lattice strain of cobalt ferrite nanoparticles.

Table 2 Crystallite size (D), lattice strain ( ), lattice parameter (a_cal), and density ( ) of Zn_xCo_1−xFe₂O₄ nanoparticles (x = 0.0, 0.2 and 0.4) by sol-gel method.

Figure 5 The morphology of cobalt ferrite nanoparticles with Zn substitution of (a) 0%, (b) 20%, and (c) 40%.

Figure 5 presents the SEM images of cobalt ferrite nanoparticles with varying Zn substitution levels. The pristine sample (Zn = 0%) exhibits irregularly shaped nanoparticles with pronounced agglomeration, which is commonly attributed to strong magnetic dipole-dipole interactions inherent to cobalt ferrite nanoparticles. These interactions promote particle clustering, resulting in dense and compact agglomerates. Upon substitution of 20% Zn, a noticeable change in morphology is observed. The particles appear relatively more dispersed with reduced agglomeration, although clustering is still present. This behavior can be attributed to the partial replacement of magnetic Co²⁺ ions by non-magnetic Zn²⁺ ions, which weakens interparticle magnetic interactions and suppresses excessive particle coalescence during synthesis. At higher Zn substitution (40%), the nanoparticles tend to form larger, loosely connected aggregates with smoother surfaces. This morphology suggests enhanced particle growth and coalescence, likely driven by lattice expansion and modified cation distribution within the spinel structure. The incorporation of larger Zn²⁺ ions alters nucleation and growth kinetics, leading to increased particle size and the formation of more compact secondary structures [28]. In addition, Figure 5 illustrates the morphological evolution of cobalt ferrite nanoparticles with increasing Zn substitution. The undoped sample (Zn = 0%) consists of relatively small, strongly agglomerated nanoparticles, which are characteristic of the single-domain regime, where strong dipole-dipole interactions dominate. At 20% Zn substitution, particle dispersion improves, and agglomeration is reduced, indicating weakened magnetic interactions and the stabilization of single-to multi-domain behavior due to decreased magnetic anisotropy. At higher Zn content (40%), larger particles and secondary aggregates are formed, favoring the development of multi-domain structures, where domain wall formation becomes energetically favorable. This single-to multi-domain transition correlates well with the observed changes in magnetic properties discussed in the subsequent section.

Figure 6 displays the FTIR spectra of Zn_xCo_1−xFe₂O₄​ (x = 0, 0.2 and 0.4), exhibiting the 2 typical spinel bands: v₁​ (tetrahedral M–O stretching) at 587 - 575 cm⁻¹ and v₂​ (octahedral M–O stretching) at 399 cm⁻¹ [29,30]. The presence of these bands in all compositions confirms the preservation of the spinel framework, consistent with the XRD/Rietveld results indicating a single-phase structure with space group Fd-3m. Notably, v₁​ shows a systematic red shift with increasing Zn content (587.35 581.56 574.81 cm⁻¹), whereas v₂​ band remains nearly unchanged (≈ 399.3 cm⁻¹). This behavior is consistent with the strong preference of Zn²⁺ for occupying tetrahedral sites, which more significantly influences the local A-site environment compared to the B-site, while maintaining the stability of the overall spinel lattice.

Figure 6 FTIR spectra of Zn_xCo_1−xFe₂O₄ nanoparticles (x = 0.0, 0.2 and 0.4) by sol-gel method.

The red shift of the v₁​ band can be explained by 2 main effects. First, the effective reduced mass, , of the tetrahedral M–O pair increases when the heavier Zn²⁺ replaces Co²⁺, thereby lowering the vibrational frequency for a given force constant [29]. Second, cation redistribution (Zn²⁺ A; Co²⁺, Fe³⁺ B) together with the lattice expansion detected by XRD induces local distortions and slightly elongates the tetrahedral M–O bond length, further enhancing the red shift. Conversely, the stability of the v₂ band indicates that the average B-site environment remains largely unaffected despite the migration of Co²⁺/Fe³⁺.

The calculated force constants (Table 3) reveal a slight increase in the tetrahedral constant k_t​ (146.80 → 148.84 → 150.20 N/m), accompanied by a small decrease in the octahedral constant k_o (97.14 → 96.62 → 96.10 N/m). Considering the relation ​, the increase in  due to Zn substitution at the A-site offsets—and even dominates—the modest strengthening of k_t​, thus producing the observed red shift of v₁​. The slight reduction in k_o suggests subtle relaxation in the B-site network as Fe³⁺ becomes more dominant, consistent with lattice expansion and strain observed in the XRD results [31-33]. Consequently, the v₁/v₂ shifts and the k_t/k_o trends complement the crystallographic image of cation redistribution derived from XRD/Rietveld analysis and offer spectroscopic proof of Zn²⁺ inclusion at the tetrahedral locations.

Table 3 FTIR parameters of Zn_xCo_1−xFe₂O₄ nanoparticles (x = 0.0, 0.2 and 0.4) by sol-gel method.

Figure 7 shows the M–H hysteresis curves of Zn_xCo_1−xFe₂O₄ (x = 0, 0.2 and 0.4), and Table 4 lists the corresponding magnetic parameters. The data indicate changes in the magnetic properties with increasing Zn concentration, which are associated with cation redistribution, domain structure, and magnetocrystalline anisotropy.

Figure 7 M H hysteresis curve of Zn_xCo_1−xFe₂O₄ nanoparticles (x = 0.0, 0.2 and 0.4) by sol-gel method.

The coercivity (Hc​) decreases significantly from 1,130 Oe (x = 0) to 247 Oe (x = 0.4), indicating a transition from a single-domain (SD) to a multi-domain (MD) state. This trend is characteristic of a reduction in magnetocrystalline anisotropy (K) due to the incorporation of Zn²⁺ ions. Since Zn²⁺ is non-magnetic and preferentially occupies tetrahedral (A) sites, the A–B superexchange interaction is weakened, making the system soft magnetic and reducing anisotropy barriers. The decline in the squareness ratio (Mr/Ms) from 0.52 to 0.17 further supports the SD MD transition, consistent with cation redistribution and spin canting effects within the B sublattice [34,35].

In contrast to the coercivity trend, the saturation magnetization (Ms​) increases steadily from 75.77 emu/g (x = 0) to 104.77 emu/g (x = 0.4). This enhancement can be explained within the Néel model [36], where the net magnetic moment arises from the difference between the octahedral (B) and tetrahedral (A) sublattices (Ms = M_B M_A) [37,38]. Additionally, the results are supported by BasIreps data results, which emphasize the magnetic configuration opposite to pristine CoFe₂O₄, which allows the possibility of octahedral and tetrahedral magnetic moment orientation in the same direction/aligned with increasing Zn value, which plays a role in rotating the orientation of the magnetic moments. The substitution of non-magnetic Zn²⁺ ions into the A-sites drives the migration of magnetic Fe^3+/Co²⁺ ions to the B-sites. As a result, the magnetic moment at the B sublattice increases, strengthening the B–O–B superexchange interaction and thereby enhancing M_B​. Consequently, higher Zn concentrations yield larger Ms​ magnitudes. The observed increase in Ms​ is consistent with XRD results showing lattice parameter expansion due to cation redistribution and FTIR data indicating local distortions through vibrational frequency shifts.

The effective magnetic moment (n_B​) increases from 3.18 _B​ (x = 0) to 4.45 _B (x = 0.4), supporting the notion of enhanced total magnetic moments induced by cation redistribution. These results also support the presence of the highest saturation magnetization (104.77  emu/g) with 40% Zn concentration, consistent with a previous study that shows an increase in saturation magnetization with 0.4 concentration, after which there is a decreasing tendency above 0.4 concentration [12,39]. Conversely, the magnetocrystalline anisotropy constant (K) decreases significantly from 4.62 10⁵ to 1.40 10⁵ erg/cm³, confirming that the system becomes soft magnetic close to superparamagnetic (x = 0.4) [40]. The strong correlation between the reduction in Hc, Mr/Ms, and K with the enhancement in Ms​ and n_B​ demonstrates that Zn substitution produces ferrite materials with high saturation magnetization but low coercivity.

Thus, the magnetic results show that Zn substitution efficiently modifies the structural and magnetic environment of CoFe₂O₄, encourages cation redistribution, and produces materials with low Hc and high Ms. Applications in microwave absorbers, spintronic components, and electromagnetic devices—where soft magnetic behavior with increased saturation magnetization is essential—benefit greatly from these properties.

Table 4 Magnetic parameters of Zn_xCo_1−xFe₂O₄ nanoparticles (x = 0.0, 0.2 and 0.4) by sol-gel method.

Rietveld refinement analysis indicates that cobalt ferrite (CoFe₂O₄) crystallizes with a lattice parameter of 8.3898 Å, with Fe atoms located at fractional coordinates (x,y,z) = (0.6250, 0.6250, 0.6250). Upon Zn substitution, the lattice parameter systematically expands to 8.3971 Å for 20% Zn and 8.4124 Å for 40% Zn. This expansion is attributed to the larger ionic radius of Zn²⁺ (0.60 Å) compared with Co²⁺ (0.56 Å). In accordance with the spinel cation distribution, Zn²⁺ preferentially occupies the tetrahedral (A) sites, replacing Co²⁺ ions, which are predominantly located in the octahedral (B) sites of the inverse spinel structure. Meanwhile, Fe³⁺ ions are distributed over both A- and B-sites, playing a central role in mediating the overall magnetic interactions.

Figure 8 Possible spin orientations of Fe₁ atoms: (a) oriented out of the z-plane in CoFe₂O₄, (b) oriented into the z-plane in Zn_0.2Co_0.8Fe₂O₄, and (c) oriented into the z-plane in Zn_0.4Co_0.6Fe₂O₄.

Simulation results obtained using the BasIreps program [41] (following ) with the cubic space group Fd-3m reveal systematic changes in the spin orientations of Fe atoms upon Zn substitution. In pristine CoFe₂O₄, Fe₁ atoms exhibit a spin orientation of (1,1,1), while Fe₅ atoms show a spin orientation of (1,1,−1). At 20% Zn substitution, the spin orientation of Fe₁ changes to (1,1,−1), whereas Fe₅ adopts (−1,1,−1). At 40% Zn, these orientations become more stabilized, indicating a tendency toward an antiferromagnetic arrangement (rising spin-down orientation) compared to the pristine sample.

The spin configuration visualizations (Figure 8) further support this interpretation. In pure CoFe₂O₄, Fe₁ spins tend to orient outside the z-plane. At 20% Zn substitution, Fe₁ spins shift into the z-plane, while at 40% Zn they become more firmly stabilized along this axis. This behaviour can be explained by the non-magnetic nature of Zn²⁺ at the tetrahedral sites, which weakens the superexchange interaction between Fe³⁺(A)–O² –Fe³⁺(B). Consequently, the contribution of Fe³⁺(B)–O² –Fe³⁺(B) interactions within the octahedral sublattice becomes dominant, leading to the experimentally observed increase in saturation magnetization (Ms). These results confirm that Zn can rotate the orientation of the magnetic moment in the spinel ferrite structure, so that the greater the quantity of Zn, the greater the magnetic configuration that is opposite to pristine CoFe₂O₄.

In addition, Zn²⁺ substitution in the CoFe₂O₄ lattice induces a measurable lattice expansion due to the larger ionic radius of Zn²⁺ compared to Co²⁺, which in turn alters the local crystal field environment of neighboring Fe³⁺ ions. As illustrated schematically in Figure 9, the preferential occupation of Zn²⁺ at tetrahedral sites triggers a redistribution of Fe³⁺ cations toward octahedral sites, leading to a reorientation of Fe spin moments, particularly at the Fe₁ and Fe₅ positions. This spin reorganization modifies the balance between antiparallel sublattices and enhances intra-octahedral (B–B) magnetic interactions, which become more dominant as the Zn content increases. Consequently, the reduction in magnetic moment at the tetrahedral sublattice and the stabilization of spin alignment within the octahedral sublattice contribute to the observed increase in saturation magnetization (Ms), establishing a clear correlation between lattice expansion, cation redistribution, spin reorientation, and magnetic enhancement in Zn-substituted CoFe₂O₄ nanoparticles.

Overall, the distribution of Zn²⁺ ions in the tetrahedral sites, Co²⁺ ions in the octahedral sites, and Fe³⁺ ions across both tetrahedral and octahedral sites provides a consistent explanation for the structural expansion, spin reorientation, and enhanced magnetic properties. Zn substitution thus promotes lattice expansion, modifies Fe³⁺ spin alignment, and strengthens intra-octahedral magnetic interactions, in agreement with the observed rise in Ms magnitudes.

Figure 9 Illustration of the orientation magnetic moment in cobalt ferrite nanoparticles at Fe1 and Fe5 atoms of (a) without and (b) with Zn substitution.

Conclusions

Cobalt ferrite nanoparticles substituted with zinc (Zn_xCo_1−xFe₂O₄, x = 0, 0.2 and 0.4) were successfully synthesized via the sol-gel method followed by annealing at 450 C for 6 h. XRD characterization confirmed that all samples crystallized in a cubic spinel (FCC) structure with space group Fd-3m, consistent with ICDD 22-1086. FTIR analysis further verified the presence of characteristic absorption bands at the tetrahedral (v₁) and octahedral (v₂) sites, supporting the structural stability of the spinel phase. Magnetic measurements revealed a decrease in coercivity (Hc) and an increase in saturation magnetization (Ms) with higher Zn content, attributed to Zn²⁺ occupation at tetrahedral sites, which weakens A–B superexchange while enhancing B–B interactions. BasIreps simulations complemented these findings by showing spin reverse-orientation of Fe atoms and a tendency toward spin-down ordering of trivalent cation in the tetrahedral site with Zn substitution. Overall, the results demonstrate that Zn substitution effectively modifies the structural and magnetic properties of cobalt ferrite, making it a promising material for spinel-based magnetic and photocatalytic applications.

Acknowledgements

This study was financially supported by Penguatan Kapasitas Grup Riset (PKGR-UNS) A Universitas Sebelas Maret, Indonesia contract number: 371/UN27.22/PT.01.03/2025.

Declaration of generative AI in scientific writing

The authors acknowledge the use of generative AI tools (Grammarly) in the preparation of this manuscript, specifically for language editing and grammar correction. No content generation or data interpretation was performed by AI. The authors take full responsibility for the content and conclusions of this work.

CRediT author statement

Nurdiyantoro Putra Prasetya: Writing - Original Draft; Investigation; Formal analysis. Siti Nurjanah: Methodology; Formal analysis. Retna Arilasita: Data Curation; Formal analysis. Utari: Supervision; Validation. Riyatun: Supervision; Project administration. Suharno: Software; Validation. Suharyana: Formal analysis. Budi Purnama: Conceptualization; Funding acquisition; Writing - Review & Editing; Supervision.

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