Impact of High Fe Doping on Structure, Optical, and Magnetic Properties of Zinc Oxide Nanostructures Synthesized by Hydrothermal Route

Abstract

Zn_1−xFe_xO nanocomposites (NCs) with varying Fe concentrations (x = 0, 0.1, 0.2, 0.3, and 0.4) were effectively prepared using the hydrothermal approach, and their morphology, structural, optical, and magnetic properties were systematically analyzed. XRD analysis confirmed Fe doping reduced crystallinity and crystallite size. TEM images of Zn_1−xFe_xO NCs exhibited smaller and more agglomerated nanostructures compared to the pure ZnO NPs. Raman and XPS analyses indicated increased lattice disorder, oxygen vacancies, and the coexistence of Fe²⁺/Fe³⁺ species. UV–Vis spectra showed enhanced visible light absorption and a tunable band gap, while PL results reflected defect-induced emission shifts and quenching, associated with zinc vacancies, interstitials, and oxygen-related defects. Magnetic measurements revealed a transition from diamagnetism to ferromagnetic-like behavior at room temperature for Fe content x ≥ 0.2, with magnetization strongly dependent on doping level. These results highlight Zn_1−xFe_xO for advanced optoelectronic and spintronic applications.

1. Introduction

Diluted magnetic semiconductors (DMSs) represent a category of nanomagnetic materials where a limited portion of the host cations is substituted by magnetic ions, potentially inducing room-temperature ferromagnetism (RTFM) [1]. In recent years, DMSs have attracted wide interest owing to their promise in spintronic and multifunctional electronic applications, including quantum computers and non-volatile memory [2], ultraviolet detectors [3], optoelectronic devices [4], spin-based field-effect transistors [5], and photocatalytic and gas-sensing applications [6,7]. Transition metal (TM)-doped ZnO is a promising DMS, due to its wide band gap (3.37 eV), notable piezoelectric effect, and thermal and chemical stability [8]. The incorporation of TM into ZnO lattice led to changes in their electronic structure as well as their optical and magnetic characteristics [4,9].

Iron-incorporated zinc oxide (Zn_1−xFe_xO) has been extensively investigated using a wide range of synthesis approaches, including green, chemical, and physical methods [10,11,12,13,14]. The reported effect of Fe doping on the optical, electronic, and magnetic properties of ZnO remains highly controversial, with inconsistent trends observed in photoluminescence (PL) behavior [15,16,17], optical band gap evolution [17,18,19,20,21,22,23], and magnetic response [20,24,25,26]. For example, Rosowska et al. [15] observed quenching of near-band-edge (NBE) emission and red-shifted deep level PL bands in ZnO doped with 0–10% Fe synthesized via a microwave-assisted hydrothermal method, whereas Maibam et al. [16] observed simultaneous suppression of NBE and green PL emissions accompanied by a blue shift in the UV absorption edge at very low Fe content. Other studies reported redshifted [18,19,20,21], blueshifted [22], or nearly unchanged bandgap energies [23], reflecting the lack of consensus on Fe-induced electronic structure modification in ZnO. Similar discrepancies are evident in magnetic studies, where ferromagnetism has been attributed to intrinsic exchange interactions [20], defect-mediated mechanisms and crystallite size effects [24], Fe-content-dependent magnetic phase transitions [25], or synthesis-condition-sensitive factors such as pH [26].

These discrepancies mainly originate from variations in synthesis routes, growth conditions, dopant incorporation pathways, and the physical form of ZnFeO, particularly when comparing thin films with nanoparticles. Thin films are often influenced by substrate-induced strain, growth kinetics, and interface effects, whereas nanoparticles are dominated by surface states, size-dependent phenomena, and defect-related processes. Recent studies [27,28] have clearly demonstrated that the synthesis method itself plays a decisive role in governing dopant distribution, Fe²⁺/Fe³⁺ ratio, defect density, and magnetic exchange interactions, even at comparable Fe concentrations. As a consequence, direct comparison between ZnFeO nanocomposites prepared by different synthesis routes or between thin-film and powder-based ZnFeO materials are inherently misleading and have contributed significantly to the contradictory trends reported in photoluminescence behavior, optical band-gap evolution, and magnetic response.

Among the available synthesis techniques, the hydrothermal approach offers distinct advantages, including precise control over particle size, morphology, crystallinity, and optical properties through careful tuning of reaction parameters such as temperatures, time, and precursor chemistry [29]. This method provides a well-defined and reproducible platform for investigating intrinsic dopant–defect interactions in ZnO nanostructures while minimizing extrinsic artifacts associated with high-temperature processing or substrate effects. However, despite these advantages, and to the best of the authors’ knowledge, hydrothermal synthesis of Fe-doped ZnO remains relatively underexplored. Existing studies are typically restricted to narrow Fe concentration ranges [16,30] or limited characterization techniques [31], or application-specific investigations such as gas sensing or photocatalysis [32,33]. Consequently, a systematic understanding of how Fe incorporation drives defect evolution and, in turn, governs the coupled optical and magnetic behavior of Zn_1−xFe_xO nanostructures across a wide doping range is still lacking.

In this context, the present work addresses these limitations by providing a systematic and comprehensive investigation of Zn_1−xFe_xO nanostructures synthesized via the hydrothermal route over a broad Fe concentration range (x = 0, 0.1, 0.2, 0.3, and 0.4). A comprehensive suite of characterization techniques, including transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV–Vis spectroscopy, photoluminescence (PL), and vibrating sample magnetometry (VSM), are employed to establish direct correlations between Fe incorporation, structural evolution, defect chemistry, optical response, and magnetic behavior in hydrothermally synthesized ZnFeO nanoparticles. Through this integrated approach, the present work provides a clear correlation between Fe incorporation, structural evolution, defect chemistry, optical response, and magnetic behavior in hydrothermally synthesized ZnFeO nanoparticles.

2. Materials and Methods

2.1. Synthesis of ZnO and ZnO-Fe Nanostructures

All reagents used in this study were of analytical grade and used without additional purification. To synthesize pure ZnO nanoparticles (NPs), a 0.1 M solution of zinc acetate [Zn(CH₃COO)₂] was dissolved in 100 mL of deionized water (DI) under continuous stirring. The pH of the solution was then adjusted to ~11 by adding sodium hydroxide (NaOH). Subsequently, 0.0028 M cetyltrimethylammonium bromide (CTAB) was added as a surfactant and capping agent, and the mixture was stirred thoroughly. The resulting solution was transferred into a hydrothermal reactor (KEJIA FURNACE, Zheng Zhou Shi, He Nan Sheng, China) and heated at 120 °C for 4 h. For the synthesis of Fe-doped ZnO nanostructures (Zn_1−xFe_xO), 0.1 M zinc acetate and 0.5 M iron chloride (FeCl₂) solutions were prepared in 100 mL of DI. These solutions were mixed in appropriate ratios according to the desired Fe doping content and stirred at ambient temperature for 2 h. The mixed solution was then placed in hydrothermal reactor and heated at 120 °C for 4 h. Later, the reactor left to cool to ambient temperature. The resulting precipitate was collected, thoroughly rinsed with DI water to ensure removal of residual ions, and dried at 60 °C for 24 h. The powders were stored for subsequent characterization. The chemical pathways leading to the formation of the ZnO and Fe-doped ZnO nanostructures are outlined as follows.

$$Zn{\left({CH}_{3}C{O}_2\right)}_2+2H_{2}O\rightleftharpoons Zn{\left(OH\right)}_2+2{CH}_{3}COOH$$

$$Fe{Cl}_2+2NaOH\rightleftharpoons Fe{\left(OH\right)}_2+2NaCl$$

$${Zn\left(OH\right)}_2\to ZnO+H_{2}O$$

$${Fe\left(OH\right)}_2\to FeO\left({Fe}^{2+}\right)+H_{2}O$$

Otherwise, during hydrothermal reaction the Fe(OH)₂ possibly partially oxidized.

$${4Fe\left(OH\right)}_2+O_2+{2H}_{2}O\to {4Fe\left(OH\right)}_3({Fe}^{2+}\to {Fe}^{3+})$$

2.2. Characterizations

The ZnO and Fe-doped ZnO nanostructures were comprehensively characterized using multiple techniques. The morphology of the samples was examined with a Transmission electron microscope (JEOL Ltd., Tokyo, Japan). Structural analysis was performed using an X-ray diffractometer XRD (Rigaku International Corp., Tokyo, Japan) with Cu Ka radiation of λ = 1.543 Å and Micro Raman (SENTERRA II, Brucker, Carteret, NJ, USA) spectrometer at λ = 532 nm. Surface chemical composition analysis was performed through X-Ray photoelectron spectroscopy XPS (Thermo Scientific, Waltham, MA, USA) using Thermo K alpha X-rays (1486.6 eV). Optical properties were analyzed with a UV-5200 spectrophotometer (Shanghai Xiwen Biotech, Shanghai, China), while photoluminescent (PL) spectra were recorded using a Cary Eclipse Fluorescence Spectrometer (Agilent, Santa Clara, CA 95051, USA) at excitation λ = 350 nm. The magnetic properties were assessed using MicroSense Vibrating Sample Magnetometers (EZ VSMs) Quantum Design, Taipei, Taiwan.

3. Results

3.1. Morphology of ZnO and ZnO-Fe Nanocomposite

Figure 1 shows the TEM images of the NPs (a,c) correspond to pure ZnO, while (b,d) depict Fe-doped ZnO (Zn_0.6Fe_0.4O) at different magnifications, along with their corresponding SAED patterns in insets of Figure 1c,d. The pure ZnO NPs display a relatively uniform morphology consisting of well-faceted, polyhedral grains with particle sizes of ~80–120 nm, and a mean size of approximately 105 ± 10 nm. The particles are densely packed, with clear, sharp boundaries indicating high crystallinity. The SAED pattern (inset of Figure 1c) exhibits well-defined, sharp concentric rings, confirming the polycrystalline nature and the characteristic wurtzite ZnO hexagonal phase. By contrast, the Zn_0.6Fe_0.4O nanostructures (Figure 1b,d) show a substantial change in morphology. The particles are more spherical, smaller, and more agglomerated, with a broader size distribution and less pronounced faceting. The average particle size is significantly reduced to approximately 30–60 nm, with an average size of ~42 ± 8 nm. This refinement in grain size suggests that Fe doping inhibits crystal growth and promotes defect formation during synthesis. The SAED pattern of Zn_0.6Fe_0.4O (inset of Figure 1d) displays broadened, less intense rings compared to the pure ZnO, indicating reduced crystallinity and increased lattice disorder. A similar observation was reported in ref. [34].

3.2. Structure and Vibrational Analysis

The XRD patterns of Zn_1−xFe_xO NPs are presented in Figure 2. For pure ZnO NPs, x = 0, one can observe sharp peaks positioned at 2θ ~31.6°, 34.3°, 36.1°, 46.8°, 56.0°, 62.5°, and 67.4° corresponded to the hexagonal wurtzite ZnO structure, associated with P6₃mc (card number 96-900-8878) [35]. The observed peaks are then indexed to the (100), (002), (101), (102), (110), (103), and (112) planes, respectively. The sharpness of the XRD peaks indicates a well-defined crystalline structure, while the absence of unknown peaks confirms the purity of the synthesized ZnO NPs. The XRD pattern of Zn_1−xFe_xO (with x ≥ 0.1) nanocomposites displays similar sharp diffracted peaks; however, their sharpness decreases as x increases, implying a decrease in crystallinity. Further, (101) peak position and broadening are slightly altered as the Fe content increases, suggesting the doping of Fe ions in the ZnO structure probably induces changes in the crystal size as well as the lattice spacing. It should be noted that the spinel phase ZnFe₂O₄ has characteristic peaks at 2θ ~35.23°, 56.60°, and 62.14° assigned to 311, 333, and 440 planes [28], which might be a reason for the observed slight peak shifting and broadening.

Williamson–Hall Plot

The Williamson–Hall (W–H) method was employed to evaluate the average crystallite size (G) and microstrain (ε) of the examined samples. This analysis allows the separation of peak broadening effects related to finite crystallite size and those arising from lattice strain, since their dependence on the diffraction angle (θ) differs. The contribution of crystallite size to peak broadening can be calculated using the Debye–Scherrer relation as follows: G$={\displaystyle \frac{0.9·\lambda }{\beta ·\cos \theta }}$, where λ = 1.54 Å and β denote the full width at half-maximum (FWHM). Meanwhile, the broadening due to strain generated by lattice imperfections is given as ${\beta }_s=C\epsilon \tan \theta$ [36]. Thus, the total broadening (β_hkl) is expressed as

$${\beta }_{hkl}\cos \theta ={\displaystyle \frac{0.9\lambda }{G}}+4\epsilon \sin \theta$$

Assuming isotropic behavior of the crystallites, ${\beta }_{hkl}\cos \theta$ was obtained for all peaks and plotted against $4\epsilon \sin \theta$, as illustrated in Figure S1. From the linear W–H fit, the intercept reflects the crystallite size, whereas the slope represents the microstrain. The observed positive slope indicates the presence of tensile strain in the prepared samples. The computed values of G and ε are reported in

Table 1. XRD pattern analysis of Zn_1−xFe_xO nanocomposites.

Sample	Crystal Size G (nm)	Strain ε (10−3)
Pure ZnO	41.1	2.18
Zn0.9 Fe0.1O	43.2	1.59
Zn0.8 Fe0.2O	40.3	1.69
Zn0.7 Fe0.3O	39.1	1.52
Zn0.6 Fe0.4O	37.4	1.43

.

As evident from Table 1, at lower Fe doping levels (x ≈ 0.1), the crystallite size increases to 43.2 nm. However, with a further increase in Fe concentration, the crystallite size decreases, reaching 37.4 nm at x ≈ 0.4. Concurrently, Fe doping reduces the microstrain in the ZnO lattice from 2.18 × 10⁻³ to 1.43 × 10⁻³. These structural changes indicate the partial substitution of Zn²⁺ ions (ionic radius = 0.60 Å) by Fe ions with different valence states and ionic radii. At lower doping levels, the substitution of larger Fe²⁺ ions (0.64 Å) may dominate, leading to lattice expansion. Conversely, at elevated doping concentrations, the substitution of smaller Fe³⁺ ions (0.49 Å) introduces compressive strain. The difference in ionic radii between the Fe dopants and the host Zn²⁺ ions disrupts the lattice symmetry, induces strain, and inhibits crystallite growth, resulting in a reduction in crystallite size. Similar trends have been reported in previous studies [21].

We further investigated the vibrational structure of Zn_1−xFe_xO nanocomposites using Raman spectroscopy, with the results depicted in Figure 3. The Raman spectrum of pristine ZnO NPs (x = 0) revealed a prominent peak at 438 cm⁻¹ corresponding to E_2H mode, and other low-intensity peaks at 98, 333, and 578 cm⁻¹ assigned to E_2L, E_2H-E_2L, and 2(E_2H-E_2L). These modes are characteristic Raman modes of ZnO hexagonal wurtzite [8]. The E_2H mode is an indicator of the crystallinity level in wurtzite-structured ZnO nanomaterials [37]. The broadening and shifts in asymmetric E_2H mode for x > 0.1 imply a decrease in the crystallinity which aligns well with the XRD results in Figure 2. The E_2H mode broadening is likely due to the simultaneous vibrations of multiple cations (Fe²⁺, Fe³⁺, and Zn²⁺) at their respective characteristic frequencies [38]. Meanwhile, similar shifts in the E_2H peak have also been observed in other transition metal-doped ZnO materials and are typically linked to oxygen displacement [39,40]. This frequency shift suggests that Fe doping into ZnO enhances lattice distortion, which, in turn, softens the E_2H phonon mode by reducing the atomic vibration force constant [22]. Moreover, Raman spectra of Zn_1−xFe_xO samples with (x ≥ 0.1) exhibit A_1g mode (680–760 cm⁻¹), which is characteristic of spinel ferrites, arising from the combined Fe–O and Zn–O vibrations [32]. This A_1g mode also exhibits a shoulder (A²_1g), which can be attributed to lattice defects [41]. The E_g mode at 325 cm⁻¹ corresponds to the symmetric bending of oxygen atoms in Fe(Zn)–O bonds located at octahedral sites [38].

3.3. XPS Analysis

The chemical composition at the surface of both pristine ZnO and Fe-doped ZnO (Zn_0.6Fe_0.4O) was analyzed by XPS and is shown in Figure 4. The XP survey spectrum of pristine ZnO exhibits the following characteristic peaks of zinc: Zn 3d, Zn 2p, Zn 3s, Zn (LMN), Zn 2p_3/2, and Zn 2p_1/2, as well as Oxygen O 1s, as depicted in Figure 4a. Incorporation of Fe into the ZnO matrix (Zn_0.6Fe_0.4O) exhibited a relative reduction in the main Zn peak intensities; however, an additional tiny peak can be observed at 710 eV that is assigned to iron Fe 2p, as observed in Figure 4a. The inset of Figure 4a shows the elemental compositions estimated from the XPS survey spectra. The reported compositions % should be taken with caution as XPS is a surface-sensitive technique.

Figure 4b depicts the O 1s spectra of pristine ZnO and Zn_0.6Fe_0.4O samples. The O 1s spectrum of pristine ZnO is asymmetric with a shoulder at higher binding energies and can be deconvoluted into the following three distinct components: a peak at 530.2 eV assigned Zn–O bond; a peak at 531.3 eV corresponding to oxygen vacancies and/or Zn–OH; and a peak at 532.4 eV associated with adsorbed water (H₂O) [8]. In contrast, the O 1s spectrum of Zn_0.6Fe_0.4O exhibits significant broadening, indicative of multiple chemical environments. This broadening mainly arises from the simultaneous presence of Zn–O and Fe–O bonds, along with the coexistence of multiple oxidation states of iron (Fe²⁺/Fe³⁺) and enhanced structural disorder. These factors collectively contribute to an increased concentration of oxygen vacancies and defect states. Deconvolution of the Zn_0.6Fe_0.4O spectrum reveals three components similar to those observed in pure ZnO; however, the fitted peak positions are slightly shifted toward lower binding energies. The peak at 530.2 eV now represents contributions from lattice oxygen in both Zn–O and Fe–O bonds. The peak at 531.3 eV, indicative of oxygen-deficient sites, is more intense in the Zn_0.6Fe_0.4O sample, indicating a higher defect density resulting from Fe incorporation. A similar trend has been reported previously [42].

Fe 2p XPS spectrum of the Zn_0.6Fe_0.4O composition, displayed in Figure 4c, shows two main peaks assigned to Fe 2p₃/₂ and Fe 2p_1/2, along with a characteristic satellite peak at 718.8 eV [38,41]. Peak deconvolution indicates that Fe exists in a mixture of Fe³⁺ and Fe²⁺ oxidation states, suggesting that Fe³⁺ and Fe²⁺ ions substitute Zn²⁺ within the Zn_0.6Fe_0.4O lattice.

XPS analysis of the Zn 2p region of ZnO NPs shows a distinct doublet at 1021.5 eV (Zn 2p₃/₂) and 1044.6 eV (Zn 2p₁/₂) (Figure 4d), giving a spin–orbit splitting of 23.1 eV in accordance with earlier studies [19]. Furthermore, the spectrum confirms that Zn exists in the +2 oxidation state and is tetrahedrally coordinated with O²⁻ ions in the wurtzite ZnO crystal structure [11]. Upon Fe doping to form the Zn_0.6Fe_0.4O sample, notable changes are observed in the Zn 2p spectral profile. The Zn 2p doublet becomes broader accompanied by distinct shoulders at lower binding energies, suggesting alterations in the local chemical environment. The Zn 2p₃/₂ spectrum of the Fe-doped ZnO sample (Zn_0.6Fe_0.4O) is deconvolved, as shown in Figure 5d. The peaks at 1021.1 eV and 1044.3 eV are attributed to Zn²⁺ species in the ZnO wurtzite structure, whereas the lower binding energy peaks at 1018.7 eV and 1041.4 eV are tentatively attributed to Zn⁰ (metallic zinc) [43]. This observation suggests a partial reduction in Zn²⁺ (from ZnO) to Zn⁰, potentially accompanied by the oxidation of Fe²⁺ to Fe³⁺, according to the following redox process: 2Fe²⁺ + Zn²⁺ → 2Fe³⁺ + Zn⁰. The presence of Zn⁰ implies the formation of oxygen defects, known to influence the magnetic and optical responses of Fe-doped ZnO nanocomposites.

3.4. Optical Properties

UV–Vis absorption spectroscopy was employed to examine the optical characteristics of both pristine and Fe-doped ZnO nanostructures. As shown in Figure 5a, the absorption spectra of the synthesized ZnO-based nanostructures exhibit pronounced absorption in the ultraviolet region (below 400 nm), which can be attributed to the intrinsic bandgap absorption of ZnO [8,29]. The UV absorption peak shifted from 3.67 to 3.56 nm (or 3.67 eV to 3.56 eV) as Fe content increased up to x = 0.4 (inset of Figure 5a), potentially due to a Moss–Burstein-type shift in the absorption edge [44,45]. Fe-doped ZnO (Zn_1−xFe_xO) samples with x ≥ 0.1 also showed stronger absorption in the visible spectrum (λ > 400 nm) relative to the undoped ZnO, with the absorption increasing in the order Fe₀.₂ > Fe₀.₃ > Fe₀.₁ > Fe₀.₄, highlighting the critical role of Fe content in modulating the optical response of Zn_1−xFe_xO nanostructures.

Using the Tauc method [8], the optical band gap E_g of the Zn_1−xFe_xO nanostructures was estimated

$$h\nu ·\alpha =A·{\left(h\nu -E_g\right)}^n$$

where A is constant, α is the absorption coefficient, and n = 2 for direct band-gap semiconductor. E_g was determined by extrapolating the linear portion of the (hν.α)² versus hν plot to the energy axis (i.e., hν = 0). As illustrated in Figure 5b–e, introducing Fe into ZnO alters the band gap in a manner that depends on Fe content. The E_g decreases from 3.14 eV in pure ZnO to 3.02 eV for Zn_0.8Fe_0.2O. However, as x increases beyond 0.3, the band gap rises again to 3.21 eV. The observed band gap narrowing for x ≤ 0.3 is due to the substitution of Zn²⁺ ions with Fe²⁺ ions leading to structural defects such as Fe³⁺ and oxygen vacancies, as supported by XPS and Raman analyses. This effect is consistent with the mechanism described by Bylsma et al. [46], where sp–d interactions between band-edge electrons and dopant d electrons further reduce E_g, making Zn_1−xFe_xO (x ≤ 0.3) a promising visible-light photocatalyst for dye degradation.

Room-temperature photoluminescence (PL) spectra of Zn_1−xFe_xO samples were obtained using a 350 nm excitation laser, as shown in Figure 6. The spectrum of pristine ZnO NPs reveals a pronounced ultraviolet (UV) emission peak at 377 nm, a minor shoulder at 390 nm, and a broad, weak emission centered around 575 nm. The UV peak at 377 nm is due to near-band-edge (NBE) excitonic recombination in ZnO [47], whereas the yellow emission at ~575 nm is associated with interstitial oxygen ions (O_i²⁻) [48]. With increasing Fe content (x ≥ 0.1), the NBE emission decreases in intensity and exhibits a red shift up to x ≤ 0.3, beyond which blue shift is observed at x = 0.4, suggesting a change in the dominate recombination pathways. Concurrently, the 575 nm peak shifts to shorter wavelengths and decreases in intensity, indicating changes in defect-related states. The substitution of Fe³⁺ for Zn²⁺ introduces a charge imbalance that may be compensated by reducing negatively charged defects such as interstitial oxygen (O_i²⁻), leading to the observed decline in the 575 nm PL emission.

The PL spectra of Zn_1−xFe_xO (x ≤ 0.1–0.3) exhibit emission humps at 408, 430, and 460 nm in the violet-blue region, which are commonly assigned in the literature to zinc vacancies (V_Zn), oxygen vacancies, and interstitial zinc (Zn_i) [49]. Although the exact origin of visible luminescence in ZnO remains a subject of ongoing debate [50], in this context, the observed violet-blue emissions may be tentatively associated with shallow defect introduced by Fe²⁺/Fe³⁺ substituting at Zn²⁺ lattice sites, which can induce local lattice distortion and alter the defect landscape of the ZnO host. At a higher Fe content (x = 0.4), both the UV and visible emissions reduced, likely due to the formation of Fe–Fe clusters that suppress radiative transitions [51]. These variations indicate modifications in the band structure and defect states of Zn_1−xFe_xO due to Fe incorporation, which is consistent with the band gap changes (Figure 5b–f).

3.5. Magnetic Analysis

The magnetic properties of Zn_1−xFe_xO nanostructures (x = 0.0, 0.1, 0.2, 0.3, and 0.4) were examined at room temperature using a vibrating sample magnetometer (VSM) under an applied magnetic field of ±15 kOe. After subtracting the diamagnetic contribution of the sample holder, the resulting magnetization (M–H) curves are presented in Figure 7. Pristine ZnO NPs exhibit a diamagnetic response similar to bulk ZnO. Upon Fe incorporation, the Zn_0.9Fe_0.1O sample displays a combination of diamagnetic behavior at high fields and weak ferromagnetic features in the low-field regime. With further increase in Fe content (x ≥ 0.2), the M–H curves show enhanced magnetic response across the entire field range, indicating the emergence of room-temperature ferromagnetic-like behavior that strengthens with increasing Fe concentration. Notably, although small but finite coercivity and remanent magnetization values are observed for samples with x ≥ 0.1, the corresponding M–H curves remain narrow and unsaturated, indicating the absence of a well-defined ferromagnetic hysteresis loop and suggesting superparamagnetic-like response, which is commonly reported for Fe-doped ZnO and nanostructured spinel ferrites. The slight loop opening observed for the Zn₀.₈Fe₀.₂O sample is attributed to uncompensated surface spins, arising from finite-size effects and defect-induced magnetic moments.

XRD analysis (Figure 2) reveals the absence of secondary iron-containing phases, indicating that the observed ferromagnetic-like behavior at room temperature is intrinsic rather than arising from metallic Fe clusters. This conclusion is further supported by XPS and Raman analyses, which confirm the incorporation of Fe³⁺ and Fe²⁺ ions at Zn²⁺ lattice sites. The observed magnetic behavior is therefore attributed to a defect-mediated ferromagnetic exchange mechanism, in which intrinsic point defects Zn_i and V_Zn facilitate indirect exchange interactions between localized Fe spins and neighboring oxygen ions [18,19,20,21].

Raman and photoluminescence (PL) analyses reveal that these defect species increase with Fe doping (especially at x ≈ 0.1–0.3), while oxygen interstitials (O_i) decrease. The spatial coexistence of Fe dopants with these defect states enhances carrier-mediated and defect-assisted exchange pathways, therefore enabling long-range ferromagnetic order. At higher Fe content (x = 0.4), the increased density of Fe ions and reduced ion spacing strengthen the magnetic exchange interactions, resulting in enhanced magnetization [52]. However, this behavior is accompanied by increased defect disorder and Fe–Fe interactions, which reduce magnetic remanence and promote superparamagnetic relaxation. This behavior is consistent with the strong quenching of PL emission observed at the same composition, indicating that excessive defect accumulation suppresses both radiative recombination and stable magnetic ordering.

Table 2. Magnetic parameters for the Zn_1−xFe_xO nanostructures.

Sample	Hc (Oe)	Mr (emu/g)	MHF (emu/g)	Mr/MHF
Pure ZnO	---	----	----	----
Zn0.9 Fe0.1O	165	1.1 × 10−3	0.05 × 10−2	0.45
Zn0.8 Fe0.2O	100	0.8 × 10−3	1.4 × 10−2	0.06
Zn0.7 Fe0.3O	71	0.6 × 10−3	2.0 × 10−2	0.03
Zn0.6 Fe0.4O	48	0.65 × 10−3	6.0 × 10−2	0.01

summarizes the magnetic parameters, namely coercive field (H_c), remanent magnetization (M_r), high-field magnetization (M_HF, measured at 15 kOe), and the remanence ratio (M_r/M_HF). It is evident that none of the samples reach complete magnetic saturation within the applied field range (±15 kOe), indicating the coexistence of paramagnetic contributions alongside weak ferromagnetic ordering. With increasing Fe content, M_HF increases markedly, while H_c decreases systematically. The highest Mr/M_HF ratio at x = 0.1 reflects relatively stronger ferromagnetic ordering at low Fe concentrations. In contrast, at higher Fe content (x = 0.4), despite the largest M_HF, the significantly reduced M_r/M_HF ratio indicates increased spin disorder and the dominance of superparamagnetic-like relaxation effects.

Overall, although finite coercivity and remanence values are observed for all doped Zn_1−xFe_xO samples, the narrow and unsaturated loops indicate weak ferromagnetism rather than a well-developed ferromagnetic hysteresis. These magnetic trends closely correlate with the defect evolution revealed by PL and Raman analyses, demonstrating that Fe-induced defect states play a decisive role in governing magnetic interactions in Zn_1−xFe_xO nanostructures. This defect-mediated magnetism provides a consistent explanation for the coupled optical and magnetic behavior, highlighting the potential of these nanostructures for dilute magnetic semiconductors and spintronic applications.

4. Conclusions

Zn_1−xFe_xO nanocomposites with x = 0–0.4 were synthesized using hydrothermal route. Doping ZnO with Fe changes its morphology to be more spherical, smaller, and more agglomerated, along with a reduction in the crystallite size and microstrain in the ZnO lattice. Further, Fe incorporation disrupts the ZnO lattice by introducing structural defects and cation disorder, leading to Raman phonon mode softening, and the emergence of Raman spinel ferrite features at higher Fe content. XPS results reveal that Fe is found mostly in mixed Fe²⁺ and Fe³⁺ oxidation states, while Zn is in the Zn²⁺ oxidation state. The oxygen vacancies V_o increase with Fe doping. The optical band gap of Zn_1−xFe_xO nanocomposites initially decreases from 3.14 to 3.02 eV at x = 0.2, beyond which it increases to 3.21 eV for Fe content of x = 0.4. The PL results confirm that Fe incorporation modifies the optical properties of Zn_1−xFe_xO possibly by introducing defect-related shallow levels and reducing radiative recombination at higher doping levels through the formation of non-radiative centers. Fe doping in ZnO nanostructures induces ferromagnetic-like behavior at room temperature, with optimal magnetic ordering observed at x = 0.2. The enhancement in magnetization and defect-mediated exchange interactions highlights the potential of Zn_1−xFe_xO for spintronic and soft magnetic applications.