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Article

Microstructural, Morphological, and Magnetic Effects of NiFe2O4 Shell Formation Around Nanospherical ZnFe2O4 Cores

by
Marija Šuljagić
1,
Vuk Uskoković
2,3,
Lukasz Kilanski
4,
Sabina Lewinska
4,
Abdul Khaliq
4,
Anna Ślawska-Waniewska
4,
Aleksandar Kremenović
5,
Vladimir Pavlović
6,
Dejan A. Jeremić
7 and
Ljubica Andjelković
1,*
1
Department of Chemistry, Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia
2
TardigradeNano LLC, Irvine, CA 92604, USA
3
Division of Natural Sciences, Fullerton College, 321 East Chapman Avenue, Fullerton, CA 92832, USA
4
Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, PL-02668 Warsaw, Poland
5
Faculty of Mining and Geology, University of Belgrade, Djušina 7, 11000 Belgrade, Serbia
6
Faculty of Agriculture, University of Belgrade, Nemanjina 6, 11000 Belgrade, Serbia
7
Innovation Center of the Faculty of Chemistry, University of Belgrade, Studentski Trg 12–16, 11000 Belgrade, Serbia
*
Author to whom correspondence should be addressed.
Magnetochemistry 2025, 11(1), 2; https://doi.org/10.3390/magnetochemistry11010002
Submission received: 2 December 2024 / Revised: 17 December 2024 / Accepted: 3 January 2025 / Published: 5 January 2025
Browse Figures
Versions Notes

Abstract

:
First-row transition metal oxides have relatively modest magnetic properties compared to those of permanent magnets based on rare earth elements. However, there is a hope that this gap might be bridged via proper compositional and structural adjustments. Bi-magnetic nanostructures with homogeneous interfaces often exhibit a combination or synergy of properties of both phases, resulting in improved performance compared to their monophasic magnetic counterparts. To gain a deeper insight into these complex structures, a bi-magnetic nanostructured material composed of superparamagnetic nanoparticles comprising a zinc ferrite core and a nickel ferrite shell was synthesized using the seed-mediated growth approach. The resulting ZnFe2O4@NiFe2O4 core–shell nanoparticles were characterized using a series of experimental techniques and were compared to the ZnFe2O4 cores. Most importantly, the formation of the NiFe2O4 shell around the ZnFe2O4 core improved the net crystallinity of the material and altered the particle morphology by reducing the convexity of the surface. Simultaneously, the magnetic measurements demonstrated the coherence of the interface between the core and the shell. These effects combined led to improved spin coupling and stronger magnetism, as evidenced by higher saturation magnetization and the doubling of the blocking temperature for the ZnFe2O4@NiFe2O4 core–shell particles relative to the ZnFe2O4 cores.

1. Introduction

Magnetic spinel oxide nanoparticles have been intensely and thoroughly studied since their discovery because of their functional properties, such as coercivity, electrical conductivity, and saturation magnetization, that can be easily tuned to address competing demands for specific applications in high technologies [1,2,3,4,5,6,7,8]. Usually typified by the combination of low coercivity and high saturation magnetization, such compositionally simple and highly stable magnetic nanomaterials have also become an area of ever-growing interest in biomedicine [9]. These properties make them suitable for targeted drug delivery, hyperthermia, and magnetic resonance imaging (MRI). In fact, with the help of magnetic spinel oxides, new methods for diagnosing and treating a variety of diseases have sprung to life [10,11,12,13].
The most comprehensively studied of all magnetic spinel oxides have been ferrites, the general formula of which is [M][Fe(III)2]O4, where M is a tetrahedrally coordinated cation, typically a transition metal, while B is octahedrally coordinated by oxygen atoms. Zinc ferrite (ZnFe2O4) is one of these heavily investigated mixed ferrites with a normal spinel structure and an antiferromagnetic nature in its pure form [14], which, however, severely limits its application as a magnetic material. The long-range magnetism in normal ZnFe2O4, in fact, can be achieved only below a Néel temperature of approximately 1.5 K [15]. However, ZnFe2O4 particles in the nanometric range can express ferrimagnetic behavior [16,17,18], the reason for this being the inversion in the crystal structure, where ferric ions begin to occupy tetrahedral positions and share them with Zn ions. As a result, superexchange interaction becomes promoted between ferric ions at the tetrahedral and the octahedral sites, which increases the magnetism of the material. Nickel ferrite (NiFe2O4), in contrast, has an inverse spinel structure and soft magnetic nature by default. It was recently recognized as a prominent candidate for magnetic hyperthermia applications [19,20,21] since its low coercivity leads to relatively high specific absorption rates during the switching of the magnetic field.
Bi-magnetic nanoheterostructures (NHSs), which include core–shell structures that consist of two spinel compounds, have attracted increasing scientific interest due to their oftentimes superior magnetic properties in comparison to those of bare oxides. For example, Faramawy and El-Sayed revealed that the magnetization of CuFe2O4@ZnFe2O4 was approximately 63% higher than that of bare CuFe2O4, while the magnetic loss decreased [22]. Some studies have shown a large increase in coercivity in core–shell-structured magnetic ternary nanocubes combining Fe3O4 cores and Mn-Zn ferrite nanoparticles as shells [23], as well as in FexO@CoFe2O4 cubic-shaped nanoparticles [24]. Song and Zhang reported that bi-magnetic MnFe2O4@CoFe2O4 core–shell nanocrystals differed in both blocking temperature and coercivity compared to their bare ferrite analogs [25]. These materials have the potential to revolutionize the development of novel magnetic materials with tailored properties and there is an open possibility for their advanced application in many fields, from biomedicine to catalysis [26,27,28,29]. In the medical arena, these complex nanoscale structures may be powerful platforms for drug delivery and may serve as MRI contrast agents [30]. To overcome challenges in the fundamental understanding of magnetic interactions, extensive studies on various NHSs composed of different magnetic materials have been conducted [31,32,33,34,35,36]. Most of them have dealt with combinations of hard and soft magnetic materials [25,37,38,39,40] as well as the influence of structure on functional properties [38,41,42,43]. Particularly, poor crystallinity and high structural disorder in systems with a core–shell arrangement tend to cause unsatisfactory magnetic properties.
Among the plethora of synthesis procedures, the seed-mediated growth approach [44,45] presents a commonly applied synthesis method to obtain NHSs with interesting physical and chemical properties [46,47]. In this method, the heterogeneous nucleation of the surface crystal phase is made to occur, usually with the assistance of organic additives, on previously prepared particles, i.e., seeds. Since this relatively simple organic phase synthesis does not require a low-yield fractionation procedure to fulfill the desired size distribution, it is easily adaptable for large-scale production. Furthermore, this procedure allows the different magnetic phases to be combined by maximizing the contact interface. It also enables an independent control of core and shell thicknesses during synthesis, resulting in variable crystallinities. Depending on whether the crystalline phases of the two components are the same or different, there are two possible scenarios for NHS formation via the seed-mediated growth. When crystalline phases of the components are isostructural, well-defined core–shell heterostructures form [48,49,50]. In the second scenario, the growth along a particular direction of the seed occurs, generating Janus- [51,52,53], dumbbell- [54,55], or flower-like [56,57,58] NHSs. The magnetic properties of such species intrinsically depend on the grain size and grain size distribution, but also on morphology, chemical composition, and particle–particle interaction. All of these factors altogether determine the final NHS architecture.
In this study, ZnFe2O4 and core–shell ZnFe2O4@NiFe2O4 NHSs were synthesized using a seed-mediated growth approach. The synthesized materials were characterized with the use of X-ray powder diffraction (XRPD), Fourier-transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). To test their potential for advanced medical applications, their magnetic properties were investigated too.

2. Materials and Methods

2.1. Chemicals

Zinc (II) acetylacetonate (for synthesis), iron (III) acetylacetonate (97%), nickel (II) acetylacetonate (95%), oleic acid (≥99%), oleylamine (≥98%), and benzyl ether (98%) were supplied by Merck, Darmstadt, Germany. Fischer Scientific, Waltham, MA 02451 USA supplied 1,2-hexadecanediol (98.0+%).

2.2. Synthesis of ZnFe2O4

The synthesis reaction was carried out in a 500 cm3 round-bottomed flask equipped with a magnetic stirrer and a 1 m long air-cooled reflux condenser, which was submerged in a silicone oil bath. Zinc (II) acetylacetonate (0.52 g), iron (III) acetylacetonate (2.82 g), 1,2-hexadecanediol (15.48 g), oleic acid (11.4 cm3), and oleylamine (11.8 cm3) were added into 120 cm3 of benzyl ether. The reaction mixture was first equilibrated in an ultrasonic bath for 15 min at 70 °C. Heating was conducted slowly and incrementally over 2 h and the mixture remained for an additional 1.5 h at 296 °C. The final product was washed with absolute ethanol (600 cm3). The obtained precipitate was dried in an oven at 105 °C, furtherly demagnetized in a microwave oven, and pulverized in an agate mortar.

2.3. Synthesis of ZnFe2O4@NiFe2O4

To synthesize ZnFe2O4@NiFe2O4, ZnFe2O4 nanoparticles were used as seeds. Ni(II) acetylacetonate (1.76 g), iron(III) acetylacetonate (4.44 g), 1,2-hexadecanediol (16.13 g), oleic acid (3.9 cm3), oleylamine (4.1 cm3), and ZnFe2O4 (0.25 g) were added to 125 cm3 of benzyl ether. The reaction mixture was then equilibrated and homogenized in an ultrasonic bath for 15 min at 70 °C prior to heating. Heating was conducted slowly and incrementally for 1 h and the mixture was refluxed for an additional hour at 250 °C. The final product was further treated as described above.

2.4. Characterization of the Synthesized Nanoparticles

The XRPD patterns were collected using a Rigaku SmartLab automated powder X-ray diffractometer (Applied Rigaku Technologies, Cedar Park, TX, USA) with Cu Kα1,2 (λ = 1.54059 Å) radiation (U = 40 kV and I = 30 mA) that was equipped with a D/teX Ultra 250 stripped 1D detector working in the XRF reduction mode. The diffraction angle range was 10–100° 2θ with a step size of 0.01° at a scan speed of 1°/min. Structural and microstructural investigations of the samples were conducted by the Williamson–Hall method.
The FT-IR spectra were recorded on a Nicolet 6700 FT-IR instrument (Thermo Scientific, Waltham, MA, USA) in the range of 4000–400 cm−1 using the ATR technique with a Smart Orbit accessory (diamond crystal).
TEM analysis was performed on a JEOL JEM-1400 Plus Electron microscope (JEOL Ltd., Peabody, MA, USA) at a 120 kV voltage and using a LaB6 filament at a magnification of 100,000×.
Magnetic measurements were conducted using a Quantum Design Physical Property Measurement System (PPMS) (Quantum Design, San Diego, CA, USA) equipped with a 9 T superconducting magnet and a vibrating sample magnetometer (VSM) option. The temperature dependence of the magnetization, M(T), was measured upon heating in the zero-field cooled (ZFC) and the field-cooled (FC) regimens at 50 Oe, 200 Oe, and 500 Oe from 5 K to 300 K. In the ZFC mode, the magnetization is measured after cooling a material without any external magnetic field present. Conversely, in the FC mode, the magnetization is measured after cooling the material while it is exposed to an external magnetic field. Hysteresis loops, M(H), were measured at 4.5 K, 25 K, 50 K, 75 K, 100 K, 200 K, and 300 K in the field range of ±9 T.

3. Results and Discussion

The XRPD patterns for ZnFe2O4 and core–shell structured ZnFe2O4@NiFe2O4 are shown in Figure 1. The presence of a single-phase cubic spinel structure with no traces of impurity phases was confirmed for both investigated powders. Relatively broad peaks were in agreement with the ultrafine nature and small crystallite size of the particles (≈5.3 nm for ZnFe2O4 and ≈8.2 nm for ZnFe2O4 @NiFe2O4, as seen in Table 1).
With the introduction of the NiFe2O4 shell, the intensity of the diffraction peaks increased while their half-widths decreased (Figure 1), suggesting an increase in the net crystallinity [59]. The lattice parameter of the core–shell structure was lower than that of ZnFe2O4 due to the difference in the ionic radii between Zn2+ (0.82 Å) and Ni2+ (0.78 Å), as seen in Table 1, providing further evidence that the core–shell structure was formed. In addition, the higher crystallite size of ZnFe2O4@NiFe2O4 than that of ZnFe2O4 led us to believe that the growth process took place. Finally, the higher value of the microstrain parameter obtained for ZnFe2O4@NiFe2O4 in comparison to the microstrain parameter for bare ZnFe2O4 ferrite (Table 1) was a sign of the strains likely originating at the core–shell interface [60], presenting further indirect evidence of the successful implementation of the coating procedure.
The FT-IR spectra of ZnFe2O4 and ZnFe2O4@NiFe2O4 showed bands at ∼560 cm−1 and ∼430 cm−1, which are characteristic for metal–oxygen vibrations in the spinel structure (Figure 2). Here, the vibration mode at the lower of the two wavenumbers corresponds to the stretch along the bond between oxygen and a metal cation located at the octahedral lattice site, while the vibration mode at the higher of the two wavenumbers corresponds to the stretch along the bond between oxygen and a metal cation located at the tetrahedral lattice site. The shift of these stretching vibration modes to higher frequencies was observed to follow the addition of the NiFe2O4 shell onto the ZnFe2O4 core. The previously evidenced increase in crystallinity typically leads to compaction of the lattice, which results in the bonds becoming shorter and their stretching modes more energetic, shifting to higher frequencies. Moreover, two bands in the region of approximately 2900 cm−1 were assigned to asymmetric and symmetric stretching modes of the –CH2 group. The relatively broad band near 3200 cm−1 originated from the stretching modes of –NH2 entities. The peaks at ∼1525 cm−1 and ∼700 cm−1 are related to deformation vibrations of the primary amino group of oleylamine used in the synthesis. The interaction between such groups and ferrite surface was previously responsible for the shift of these bands to lower positions [42,55]. The band near 1525 cm−1 can be associated with the asymmetric stretching vibration of the carboxylate from the free tetramethylammonium 11-aminoundecanoate, while its symmetric stretching mode was located at approximately 1390 cm−1.
TEM images, along with the corresponding size distribution charts, are presented in Figure 3. Since the core and the shell had the same density, they cannot be distinguished by the contrast of the TEM image as can be done for core–shell particles, for which this contrast is obvious [61]. However, the most direct evidence of the formation of the NiFe2O4 shell over the ZnFe2O4 core came from the obvious increase in size and change in the shape of the nanoparticles before and after the shell deposition process: while the ZnFe2O4 particles were spherical and had diameters close to 5 nm, the core–shell particles had ~15 nm in size on average and were mostly polygonal in shape. This obvious difference in particle size and shape between the core and the core–shell structure suggests that shell formation was successful. The notable influence on morphological characteristics and the particle size distribution that the shell addition had on the core particles can be seen by comparing the the TEM images and particle size distribution histograms in Figure 3. The different morphologies observed for the synthesized samples obviously influenced the relative peak intensities in XRPD, although their positions remained quite close [62].
To determine the blocking temperature (TB), the ZFC and FC modes of magnetization as the function of temperature were recorded. The behavior of the magnetic nanoparticles in the external field was highly influenced by external perturbations and experimental conditions at TB, where thermal energy became comparable to anisotropy energy. Below TB, the magnetic moments of most particles were frozen, with their preferable orientations being determined by magnetic anisotropy. At temperatures above TB, on the other hand, where the thermal energy overcame the magnetic energy barrier, the magnetic moments of the majority of particles could be considered as freely fluctuating, and thus the materials became superparamagnetic, following the Curie–Weiss law.
The temperature dependencies of magnetization measured in the ZFC and FC regimens at different applied fields (50 Oe, 200 Oe, and 500 Oe) for both samples are depicted in Figure 4. All the measured ZFC-FC relations had similar curvatures. As expected for uniaxial, single-domain nanoparticles, the ZFC and FC curves diverged when the temperature decreased below TB, with the FC curves decreasing smoothly toward zero values with the temperature [63]. A slight increase in the FC magnetization curves with the decrease in temperature below TB points out weak interparticle interactions.
The average TB for ZnFe2O4 nanoparticles was 111 K, 77 K, and 57 K for the applied field strengths of 50 Oe, 200 Oe, and 500 Oe, respectively. For the core–shell structure, the TB values were 241 K, 183 K, and 116 K for the fields of 50 Oe, 200 Oe, and 500 Oe, respectively. The introduction of the shell consistently led to considerably higher TB values than those of the core nanoparticles. The appearance of a single TB for ZnFe2O4@NiFe2O4 indicates that the spins of the core and the shell were strongly coupled and responded jointly to the changes in temperature and the magnetic field [29,63].
The separation temperature presented the maximum TB of the particles of maximum size in their heterogeneous distribution (Figure 3b), whereas the maximum of the ZFC curve corresponded to the average blocking temperature. A very small difference between these two temperatures was observed at all the applied fields (Figure 4), which supports the narrow particle size distribution observed under TEM and the formation of ferromagnetic clusters in both cases. However, the nature of the FC and ZFC curves indicates that both the core and the core–shell nanoparticles synthesized by the seed-mediated growth approach were superparamagnetic at room temperature.
Magnetization, as a function of the magnetic field in the temperature range of 4.5–300 K, and magnetic hysteresis loops up to 100 K for ZnFe2O4 and ZnFe2O4@NiFe2O4 powders are shown in Figure 5. The hysteresis curves show characteristics of soft ferrites with low coercive fields and magnetization values at saturation. Their shapes become narrower, i.e., both coercivity and remanent magnetization lowered as the measurement temperature increased, conforming to their superparamagnetic nature at room temperature. The hysteresis loops obtained for the core–shell magnetic structures were almost comparable to the loops of the ZnFe2O4 core, indicating single-phase-like magnetic material behavior. This suggests that a coherent interface formed between the core and the shell, with undetectable disturbances in the ordering of the magnetic moments.
In bulk materials, achieving saturation magnetization is straightforward, typically characterized by a plateau in the high-field region of the hysteresis loop. However, at the nanoscale, magnetization dynamics become more complex. Surface disorder and the finite size of ultrasmall particles significantly impact magnetic behavior, making complete magnetization saturation challenging to achieve [64]. For the samples investigated, magnetization continued to show a slight increase even under very intense magnetic fields, as illustrated in Figure 5 [64]. The key magnetic parameters, including saturation magnetization (Ms), coercive field (Hc), and remanent magnetization (Mr), are compiled in Table 2. To determine the Ms values, the point where the forward and reverse branches of the magnetic hysteresis loop converge was identified [64]. Generally, a slight increase in the MS values was noticed for the core–shell material relative to the core alone, which can be explained by the higher crystallinity of the former systems (Figure 1) and also by their polygonal shapes, as opposed to spherical cores (Figure 3). Higher crystallinity ensures a better superexchange coupling between the magnetic moments, while polygonal morphologies minimize the amount of the “dead”, spin-canted surface layer present on the surface of their more convex, spherical counterparts [65,66]. The possibility that this enhancement of magnetization was caused by an exchange interaction between the magnetic moments across the core–shell interface cannot be excluded either. The bulk magnetization values for the two phases comprising the core–shell nanoparticles, namely ZnFe2O4 and NiFe2O4, were not taken into account because, at the nanoscale, a variety of trends in magnetic properties of these two phases could be observed. Thus, some of the literature reports show considerably higher saturation magnetization of ZnFe2O4 than that of NiFe2O4 [67]; some show considerably higher saturation magnetization of NiFe2O4 than that of ZnFe2O4 [68]; and some show nearly identical values [69].
The change of the MS values with temperature for both investigated samples is presented in Figure 6. As expected, magnetization decreased with temperature, an effect that can be attributed to the thermal agitation of magnetic moments with an increase in temperature [70,71]. However, the lower slope of this curve for the core–shell particles indicates that their magnetism had greater thermal stability than that of bare ZnFe2O4 cores, which is in agreement with the aforementioned crystallinity and morphology arguments. It is also possible that an additional cause for this improved stability was due to the pinning of the magnetic moments at the interface between the core and the shell. More detailed examinations of this interface are required to confirm these ideas and are anticipated to be a part of future research on this topic.

4. Conclusions

The present study dealt with a detailed investigation of the structural and magnetic properties of ZnFe2O4 nanoparticles and of a ZnFe2O4@NiFe2O4 core–shell NHSs prepared by a seed-mediated growth approach. To confirm the formation of the core–shell structure, ZnFe2O4@NiFe2O4 was thoroughly characterized using XRPD, FT-IR, and TEM, and compared to bare ZnFe2O4 cores. The XRPD data showed the formation of a pure spinel phase in both cases. The introduction of the NiFe2O4 shell around the ZnFe2O4 core, however, led to an increase in the average crystallite size and microstrain. TEM analysis revealed the presence of nanoparticles smaller than 20 nm for both samples. However, the core was composed of spherical particles, while the bi-magnetic powder consisted mainly of polygonal particles, which almost tripled in size. The synthesized materials were further subjected to a detailed magnetic characterization, where the shape of the FC and ZFC curves indicated that both ZnFe2O4 and ZnFe2O4@NiFe2O4 were superparamagnetic at room temperature. The hysteresis loops not only confirmed their superparamagnetic nature under ambient conditions, but they were almost the same in shape, indicating a coherent interface formed between the core and the shell in ZnFe2O4@NiFe2O4, with undetectable disturbances in the ordering of the magnetic moments. The higher crystallinity and the polygonal shape of ZnFe2O4@NiFe2O4 nanoparticles led to a slight increase in the MS values as a consequence of the superexchange coupling between the magnetic moments and the minimal amount of the spin-canted surface layer. Bearing in mind the obtained functional properties of the synthesized NHSs, these materials should be considered for biomedical applications. Future studies will continue to explore the structural alterations in core–shell systems in search of a more superior magnetic nanomaterial.

Author Contributions

Conceptualization, L.A. and M.Š.; methodology, L.A. and M.Š.; validation, L.A. and L.K.; investigation, M.Š., V.U., L.K., S.L., A.K. (Alekandar Kremenović), A.Ś.-W., A.K. (Abdul Khaliq), V.P., D.A.J. and L.A.; resources, M.Š., V.U., L.K., S.L., A.K. (Alekandar Kremenović), A.Ś.-W., A.K. (Abdul Khaliq), V.P., D.A.J. and L.A.; writing—original draft preparation, L.A., V.U. and M.Š.; visualization, L.A. and M.Š.; supervision, L.A.; project administration, L.A.; funding acquisition, L.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science, Technological Development, and Innovation of the Republic of Serbia (Contract No: 451-03-66/2024-03/200026, 451-03-66/2024-03/200116, 451-03-65/2024-03/200126, and 451-03-47/2024-03/200288), as well as by the Science Fund of the Republic of Serbia, #7551, Development of epsilon-iron oxide-based nanocomposites: Towards the next-generation rare-earth-free magnets—DOMINANTMAG.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author(s).

Acknowledgments

The authors would like to acknowledge Predrag Vulić, our late colleague, for his expertise and assistance throughout XRPD analyses.

Conflicts of Interest

Author Vuk Uskoković was employed by the company TardigradeNano LLC. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Magnetochemistry 11 00002 g001
Figure 1. XRPD patterns of the synthesized powders. The d-values and intensities of the diffraction maxima match the literature data of ZnFe2O4 (ICDD PDF 22–1012) and NiFe2O4 (10–0325).
Figure 1. XRPD patterns of the synthesized powders. The d-values and intensities of the diffraction maxima match the literature data of ZnFe2O4 (ICDD PDF 22–1012) and NiFe2O4 (10–0325).
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Figure 2. FT-IR spectra of the synthesized powders.
Figure 2. FT-IR spectra of the synthesized powders.
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Figure 3. TEM micrographs, along with the corresponding size distribution charts, for the synthesized powders (a) ZnFe2O4 and (b) ZnFe2O4@NiFe2O4.
Figure 3. TEM micrographs, along with the corresponding size distribution charts, for the synthesized powders (a) ZnFe2O4 and (b) ZnFe2O4@NiFe2O4.
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Figure 4. MFC(T) and MZFC(T) dependencies obtained in the fields of 50 Oe, 200 Oe, and 500 Oe for the core ZnFe2O4 particles (a) and for the core–shell ZnFe2O4@NiFe2O4 particles (b).
Figure 4. MFC(T) and MZFC(T) dependencies obtained in the fields of 50 Oe, 200 Oe, and 500 Oe for the core ZnFe2O4 particles (a) and for the core–shell ZnFe2O4@NiFe2O4 particles (b).
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Figure 5. Magnetization as a function of the magnetic field in the temperature range 4.5–300 K for the core ZnFe2O4 particles and for the core–shell ZnFe2O4@NiFe2O4 particles.
Figure 5. Magnetization as a function of the magnetic field in the temperature range 4.5–300 K for the core ZnFe2O4 particles and for the core–shell ZnFe2O4@NiFe2O4 particles.
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Figure 6. MS value as a function of temperature for the core ZnFe2O4 particles and for the core–shell ZnFe2O4@NiFe2O4 particles.
Figure 6. MS value as a function of temperature for the core ZnFe2O4 particles and for the core–shell ZnFe2O4@NiFe2O4 particles.
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Table 1. Unit cell and microstructure parameters of the synthesized powders.
Table 1. Unit cell and microstructure parameters of the synthesized powders.
ZnFe2O4ZnFe2O4@NiFe2O4
Unit cell parametera = 8.4111 (18) Åa = 8.3721 (16) Å
Average crystallite size53 (1) Å82 (1) Å
Average strain0.14 (2) %0.4 (1) %
Table 2. The magnetization values (MS and MR) and coercive field (HC) values for all investigated samples.
Table 2. The magnetization values (MS and MR) and coercive field (HC) values for all investigated samples.
ZnFe2O4ZnFe2O4@NiFe2O4
T
(K)		MS (emu/g)MR (emu/g)HC
(Oe)		MS (emu/g)MR (emu/g)HC
(Oe)
4.548.6715.88222.7749.5315.87167.78
2547.626.0556.9049.409.1673.21
5046.091.9311.3248.905.7137.21
7544.721.002.3048.423.5219.17
10042.181.9113.7647.831.756.77
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Šuljagić, M.; Uskoković, V.; Kilanski, L.; Lewinska, S.; Khaliq, A.; Ślawska-Waniewska, A.; Kremenović, A.; Pavlović, V.; Jeremić, D.A.; Andjelković, L. Microstructural, Morphological, and Magnetic Effects of NiFe2O4 Shell Formation Around Nanospherical ZnFe2O4 Cores. Magnetochemistry 2025, 11, 2. https://doi.org/10.3390/magnetochemistry11010002

AMA Style

Šuljagić M, Uskoković V, Kilanski L, Lewinska S, Khaliq A, Ślawska-Waniewska A, Kremenović A, Pavlović V, Jeremić DA, Andjelković L. Microstructural, Morphological, and Magnetic Effects of NiFe2O4 Shell Formation Around Nanospherical ZnFe2O4 Cores. Magnetochemistry. 2025; 11(1):2. https://doi.org/10.3390/magnetochemistry11010002

Chicago/Turabian Style

Šuljagić, Marija, Vuk Uskoković, Lukasz Kilanski, Sabina Lewinska, Abdul Khaliq, Anna Ślawska-Waniewska, Aleksandar Kremenović, Vladimir Pavlović, Dejan A. Jeremić, and Ljubica Andjelković. 2025. "Microstructural, Morphological, and Magnetic Effects of NiFe2O4 Shell Formation Around Nanospherical ZnFe2O4 Cores" Magnetochemistry 11, no. 1: 2. https://doi.org/10.3390/magnetochemistry11010002

APA Style

Šuljagić, M., Uskoković, V., Kilanski, L., Lewinska, S., Khaliq, A., Ślawska-Waniewska, A., Kremenović, A., Pavlović, V., Jeremić, D. A., & Andjelković, L. (2025). Microstructural, Morphological, and Magnetic Effects of NiFe2O4 Shell Formation Around Nanospherical ZnFe2O4 Cores. Magnetochemistry, 11(1), 2. https://doi.org/10.3390/magnetochemistry11010002

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