Formation, Structure and Magnetic Properties of MFe2O4@SiO2 (M = Co, Mn, Zn, Ni, Cu) Nanocomposites

The formation, structure, and thermal and magnetic properties of MFe2O4@SiO2 (M = Co, Mn, Zn, Ni, Cu) (60% MFe2O4/40% SiO2) nanocomposites produced by a modified sol-gel method, followed by annealing at 300, 600, 900 and 1200 °C, were studied. The thermal analysis and Fourier transform infrared spectroscopy showed the formation of metal-glyoxylates below 210 °C and their decomposition into the corresponding ferrite around 300 °C. The evolution of crystalline phases and variation of crystallite sizes differs from ferrite to ferrite and depends on the annealing temperature. The magnetic measurements revealed the dependence of saturation and remanent magnetization, coercivity, and anisotropy on ferrite type, annealing temperature, and particle size. By annealing the nanocomposites (NCs) at 1200 °C paramagnetic MnFe2O4, CoFe2O4, NiFe2O4 and CuFe2O4 and antiferromagnetic ZnFe2O4 are obtained.


Introduction
The MFe 2 O 4 @SiO 2 (M = Co, Cu, Mn, Ni, Zn) spinel ferrites possess a cubic structure with tightly packed arrangement of oxygen atoms and metal ions occupying tetrahedral (A) or octahedral (B) sites [1]. Due to their distinctive structure and magnetic, optical, and electrical properties, the transitional divalent metal ferrites are promising functional materials for many applications [1][2][3][4][5][6][7][8][9][10]. Among different coating materials, SiO 2 is a notable surface modifier due to its exceptional stability, easy conjugation capacity with numerous functional groups, selective and specific coupling ability with biotargets, non-toxicity, and biocompatibility [11]. Mesoporous SiO 2 is a versatile nanocarrier candidate as it increases the biocompatibility of the incorporated nanoparticles, minimizes their agglomeration, and improves their stability [5]. Polymerized tetraethoxysilane network is often used as surface coating material for Fe oxide nanocrystals, preventing agglomeration and improving chemical stability [12]. The synthesis of SiO 2 particles with magnetic core by tailoring the silica shell thickness and surface properties was reported by Philipse et al. [11]. At low temperature, the magnetic behavior of SiO 2 -coated ZnFe 2 O 4 nanoparticles is governed by the core-shell interactions [6]. The saturation (M s ) and remanent (M R ) magnetizations are influenced by the silica coating, whereas the coercivity (H c ) of silica-coated and uncoated CoFe 2 O 4 are comparable, and that of MnFe 2 O 4 decreases by coating with silica [12]. The magnetic behavior of coated and uncoated nanoparticles is extensively studied both by experimental measurements and by numerical modeling, but the influence of interparticle interaction on the magnetic properties is not yet fully understood [13]. In the case of SiO 2coated CoFe 2 O 4 , the SiO 2 network shields the nanoparticles and diminishes the surface roughness and spin disorder leading to higher H c value of CoFe 2 O 4 @SiO 2 compared to that of CoFe 2 O 4 [9]. The low saturation magnetization (M S ) of MnFe 2 O 4 @SiO 2 nanocomposites is attributed to the presence of non-magnetic SiO 2 matrix, while the H c value of embedded in a SiO 2 matrix, obtained by sol-gel method followed by thermal treatment at 300, 600, 900, and 1200 • C. Thermal analysis and Fourier transform infrared (FT-IR) spectroscopy data were used to reveal the different formation and decomposition behaviors of metal-glyoxylates and to confirm the ferrite formation. The X-ray diffraction (XRD) analysis revealed the formation of main crystalline phase and allowed the calculation of crystallite size. The study brings valuable information on the changes of structure and magnetic properties of different transitional metal ferrites annealed at different temperatures.

Materials and Methods
Fe(NO 3 3 purchased from Merck (Darmstadt, Germany) were used for the synthesis. The purity of all reagents was higher than 98%.
The carboxylate-type precursors formation and decomposition were investigated by thermogravimetry (TG) and differential thermal analysis (DTA) in air, up to 1000 • C, at 10 • C·min −1 using alumina standards and Q600 SDT (TA Instruments, New Castle, DE, USA) analyzer. The crystalline phases were investigated by X-ray diffraction using a D8 Advance (Bruker, Karlsruhe, Germany) at ambient temperature with CuKα radiation (λ = 1.54060 Å) and LynxEye detector, operating at 40 kV and 40 mA. The vibration of chemical bonds from the ferrite structure and SiO 2 matrix were investigated on KBr pellets containing 1% sample using a Spectrum BX II Fourier transform infrared spectrometer (FT-IR, Perkin Elmer, Waltham, MA, USA). The shape and clustering of nanoparticles were studied on samples deposited and dried on carbon coated copper grids using a HD-2700 (Hitachi, Tokyo, Japan) transmission electron microscope (TEM). The magnetic measurements were performed using a 7400 vibrating-sample magnetometer (VSM, LakeShore Cryotronics, Westerville, OH, US). The hysteresis loops were recorded at room temperature in magnetic fields between −2 to 2 T, while the magnetization versus magnetic field measurements were performed to find M S up to 5 T on samples embedded in epoxy resin. The Co/Fe (CoFe 2 O 4 @SiO 2 ), Mn/Fe (MnFe 2 O 4 @SiO 2 ), Zn/Fe (ZnFe 2 O 4 @SiO 2 ), Ni/Fe (NiFe 2 O 4 @SiO 2 ), and Cu/Fe (CuFe 2 O 4 @SiO 2 ) molar ratio in the synthesized were confirmed by inductively coupled plasma optical emission spectrometry (ICP-OES) using an Optima 5300DV (Perkin Elmer, Norwalk, CT, USA) after microwave digestion of 50 mg sample with 21 mL aqua regia (7mL HNO 3 and 21 mL HCl) and dilution with 100 mL ultrapure water.   [2,[27][28][29]. In case of CuFO, a fourth process indicated by the exothermic effect at 464 • C is assigned to phase transformations in ferrite and SiO 2 matrix [2]. According to TG diagrams (Figure 1a), the lowest total mass loss was remarked in the case of ZCF (58.0%), while the highest total mass loss was found in the case of CuFO (69.4%).  [2,[27][28][29]. In case of CuFO, a fourth process indicated by the exothermic effect at 464 °C is assigned to phase transformations in ferrite and SiO2 matrix [2]. According to TG diagrams ( Figure  1a), the lowest total mass loss was remarked in the case of ZCF (58.0%), while the highest total mass loss was found in the case of CuFO (69.4%).

FT-IR Analysis
The FT-IR spectra of samples dried at 40 °C displayed an intense band at 1387 cm −1 attributed to the vibration of N-O bonds of nitrate groups ( Figure 2). This band disappeared for samples annealed at higher temperatures, confirming the conversion of nitrates [2]. In all cases, the formation of SiO2 matrix was indicated by its characteristic bands. The bands at 792-827 cm −1 were attributed to symmetric stretching/bending vibrations of Si-O chains in SiO4 tetrahedron, the bands at 1046-1103 cm −1 to the stretching/ bending vibrations of Si-O-Si bonds, while the shoulder at 1200-1237 cm −1 to the vibration of Si-O bonds in SiO2. In the cases of CuFO, CFO, and MFO, the bands at 1046-1103 cm −1 increased with the increase of annealing temperature, while the shoulder attributed to the vibration of Si-OH bonds (942-971 cm −1 ) disappeared at high temperatures (900 and 1200 °C). Dissimilarly, for NFO and ZFO annealed at 1200 °C this band shifted to 1138 cm −1 . In all cases, a possible explanation could be the different bond strength of metal and oxygen atoms that further leads to distinct bond length in SiO2 matrix, resulting in minor variations of the peak position for each ferrite. Ferrites can be called continuously bonded crystals with the atoms bonded to all nearest neighbors by equal forces [2,30,31]. Additionally, the Si-O-Si bond length variations under the influence of the neighboring atom could produce the peak wavenumber shift [2,31]. The band at 1643-1699 cm −1 corresponding to the stretching mode vibration of surface-adsorbed H-O-H molecules disappeared in the case of CFO and MFO annealed at 900 and 1200 °C [1,2,7]. The band at 563-623 cm −1 is specific to tetrahedral stretching vibration of  [8,19]. This slight shift towards low wavenumbers with the increase of annealing temperature suggests changes in crystallite size and M-O bond length in ferrite as a consequence of changes in the spinel structure [23].

FT-IR Analysis
The FT-IR spectra of samples dried at 40 • C displayed an intense band at 1387 cm −1 attributed to the vibration of N-O bonds of nitrate groups ( Figure 2). This band disappeared for samples annealed at higher temperatures, confirming the conversion of nitrates [2]. In all cases, the formation of SiO 2 matrix was indicated by its characteristic bands. The bands at 792-827 cm −1 were attributed to symmetric stretching/bending vibrations of Si-O chains in SiO 4 tetrahedron, the bands at 1046-1103 cm −1 to the stretching/ bending vibrations of Si-O-Si bonds, while the shoulder at 1200-1237 cm −1 to the vibration of Si-O bonds in SiO 2 . In the cases of CuFO, CFO, and MFO, the bands at 1046-1103 cm −1 increased with the increase of annealing temperature, while the shoulder attributed to the vibration of Si-OH bonds (942-971 cm −1 ) disappeared at high temperatures (900 and 1200 • C). Dissimilarly, for NFO and ZFO annealed at 1200 • C this band shifted to 1138 cm −1 . In all cases, a possible explanation could be the different bond strength of metal and oxygen atoms that further leads to distinct bond length in SiO 2 matrix, resulting in minor variations of the peak position for each ferrite. Ferrites can be called continuously bonded crystals with the atoms bonded to all nearest neighbors by equal forces [2,30,31]. Additionally, the Si-O-Si bond length variations under the influence of the neighboring atom could produce the peak wavenumber shift [2,31].  [8,19]. This slight shift towards low wavenumbers with the increase of annealing temperature suggests changes in crystallite size and M-O bond length in ferrite as a consequence of changes in the spinel structure [23].

Chemical Analysis
The M/Fe molar ratio calculated based on Co, Fe, Mn, Zn, Ni, and Cu concentrations measured by ICP-OES confirmed the theoretical elemental composition of the obtained

Chemical Analysis
The M/Fe molar ratio calculated based on Co, Fe, Mn, Zn, Ni, and Cu concentrations measured by ICP-OES confirmed the theoretical elemental composition of the obtained NCs (Table 1). The best fit of experimental and theoretical data was remarked for samples annealed at 1200 • C.

XRD Analysis
The XRD patterns show the crystallographic properties of transition metal ferrite nanostructures, providing significant information about the structure, crystal orientation, the average crystallite size, etc. [6]. The evolution of crystalline phases in CFO, MFO, ZFO, NFO, and CuFO is presented in Figure 3. At high annealing temperatures, the presence of well-defined and narrow peaks indicated well crystallized particles. By low temperature annealing only low crystalline phases were obtained [1]. The XRD pattern of CFO exhibited a broad diffraction halo in the range 15 • -30 • corresponding to the amorphous SiO 2 matrix and cubic spinel CoFe 2 O 4 crystalline single phase (JCPDS card no. 02-1045, [32])) indexed to (111), (220), (311), (222), (400), (422), (511), and (440) planes, belonging to Fd3m space group [2,3,33]; this confirms the good homogeneity and crystallization of the prepared compound [17]. The single CoFe 2 O 4 phase started to form at 300 • C due to the strong reducing atmosphere (CO) generated during decomposition, when amorphous, very reactive CoO and γ-Fe 2 O 3 were formed [34][35][36][37][38] [4,8]. A sharp diffraction peak overlaid on a broad base (such as 311 peak) indicated the presence of both well-and low crystalline phases due to the overlapping between the narrow, high-intensity and broad, low-intensity peaks [5]. In the case of samples annealed at 600 • C and 900 • C, a significant decrease in the intensity of the diffraction halo characteristic to the amorphous SiO 2 matrix compared to 300 • C was observed. In the cases of CFO and NFO, the formation of single, well-crystallized CoFe 2 [32]) and a prominent monoclinic CuO phase (JCPDS card no 89-5895) [8,32]. The presence of CuO could be a consequence of the larger complexation constant of Fe 3+ compared to that of Cu 2+ , leading to more stable Fe 3+ complexes than the corresponding Cu 2+ complexes [8]. and appearance of SiO2 crystalline phase. At high annealing temperatures, the intensity of diffraction peaks increased due to the higher crystallinity and inactive surface layer of the crystals, whereas the coalescence processes strengthened, facilitating the increase of grain size [1][2][3][4][5][6]. In this regard, at 1200 °C, a substantial agglomeration occurred without recrystallization, favoring the formation of single crystals instead of polycrystals [1,3,6]. Consequently, the annealing temperature plays a crucial role in governing the crystallinity and crystallite size [16]. 10   The crystallite size is an important parameter to tailor the magnetic and optical properties of ferrites [4][5][6]. The average crystallite size (DCS) was calculated using the most intense diffraction peak of (311) crystalline plane by Debye-Scherrer formula using Equation (1)   At 1200 • C, the NCs were well-crystallized, indicating the formation of single CoFe 2 O 4 and NiFe 2 O 4 phases for CFO and NFO, respectively. The peaks attributed to CoFe 2 O 4 were intense and sharp, indicating their high crystallinity [1]. In the case of MFO, beside the main MnFe 2 O 4 phase, two secondary phases of Fe 2 O 3 and SiO 2 (JCPDS card no. 89-3434 [2,32]) were observed. A possible explanation of this distinct formation of a mixed secondary phase of Fe 2 O 3 and SiO 2 could be the instability of Mn 2+ ions. The SiO 2 matrix produces steric repulsion between nanoparticles, preventing uncontrolled aggregation [27]. The oxidation-reduction reactions also depend on the oxygen partial pressure and the presence of air during the annealing process [17]. In the cases of ZFO and CuFO, beside the main ZnFe 2 O 4 and CuFe 2 O 4 phases, a secondary SiO 2 (α-cristobalite) phase was also present. Crystalline phases resulting following the interaction of ferrite with the SiO 2 matrix were not noticed. The width of the most intense peak (311) progressively decreased, and the intensity of diffraction lines increased with the increase of annealing temperature and appearance of SiO 2 crystalline phase. At high annealing temperatures, the intensity of diffraction peaks increased due to the higher crystallinity and inactive surface layer of the crystals, whereas the coalescence processes strengthened, facilitating the increase of grain size [1][2][3][4][5][6]. In this regard, at 1200 • C, a substantial agglomeration occurred without recrystallization, favoring the formation of single crystals instead of polycrystals [1,3,6]. Consequently, the annealing temperature plays a crucial role in governing the crystallinity and crystallite size [16]. The crystallite size is an important parameter to tailor the magnetic and optical properties of ferrites [4][5][6]. The average crystallite size (D CS ) was calculated using the most intense diffraction peak of (311) crystalline plane by Debye-Scherrer formula using Equation (1) [1,4,[6][7][8][15][16][17]20,23,[33][34][35][36][37][38]. The lattice parameter (a) calculated according to Equation (2) [6,7,16] is presented in Table 2.
where D CS is average crystallite diameter, β is the broadening of full width at half the maximum intensity (FWHM), θ is the Bragg angle, and λ is the X-ray wavelength.

Transmission Electron Microscopy
The TEM images and particle size distribution of studied ferrites are shown in Figure 4.

Transmission Electron Microscopy
The TEM images and particle size distribution of studied ferrites are shown in Figu 4.  At 1200 • C, the spherical particles formed irregular spongy aggregates with a large number of pores. The particles were randomly distributed and embedded in the silica matrix. The particle agglomeration was characteristic of chemically prepared NCs and was probably due to the assembling tendency of very small particles [29]. When the nucleation rate was higher than the growth rate, small and homogenously distributed particles were obtained. The histograms show a unimodal particle size distribution for all studied ferrites. The average particle size increased in the order of NiFe 2 O 4 @SiO 2 (25 nm) > CoFe 2 O 4 @SiO 2 (29 nm) > MnFe 2 O 4 @SiO 2 (48 nm) > ZnFe 2 O 4 @SiO 2 (50 nm) > CuFe 2 O 4 @SiO 2 (66 nm), The particle sizes estimated by XRD and TEM were comparable, the low differences appearing probably due to the influence of amorphous SiO 2 and large-size nanoparticles on the diffraction pattern.

Magnetic Behavior
The magnetic hysteresis loops of CFO, MFO, ZFO, NFO, and CuFO, annealed at 300, 600, 900, and 1200 • C, are presented in Figure 5. The hysteresis loops show that the samples became magnetically softer with increasing temperature [41]. The hysteresis loops had an S-shape at low magnetic fields and linear dependence at high fields, indicating the presence of small magnetic particles [42]. For particles smaller than the critical diameter, the spin-reversal energy was much lower than the thermal energy. In the absence of any magnetic field, the random orientation of the magnetic moments resulted in a zero average magnetic moment [29]. The low saturation magnetization (M S ) values of samples annealed at 300 (0.7-3.6 emu/g), 600 (1.3-11.7 emu/g), and 900 • C (6.1-19.4 emu/g), compared to those annealed at 1200 • C (10.8-28.5 emu/g), could be explained by the low crystallinity, occurrence of vacancies, interatomic spacing, low coordination number, and surface spin disorder [12,29,43]. The MFO, NFO, CFO, and CuFO had paramagnetic behavior between 900 and 1200 • C, and ferromagnetic behavior at lower temperatures, whereas the ZFO was antiferromagnetic at all annealing temperatures. The paramagnetic behavior was determined by the increase of magnetic moment orientation disorder in various sites when the surface/volume ratio increased [38,44]. This behavior resulted from the uncompensated spins of antiferromagnetic clusters, which created giant effective spins that interacted with the applied magnetic field. Since the antiferromagnetic interactions were present both in intra-and inter-cluster spins, the global magnetic behaviors of these samples were found to be different from that of conventional superparamagnetic systems [36]. Also, the presence of stable Mn-O bonds in the crystal system of MnFe 2 O 4 led to a stable magnetic phase [22]. The low values of MS for ZFO (0.7-10.8 emu/g) were determined by the lattice defects, particle size effects, core-shell interactions, spin canting, disordered distribution of cations, A-B super exchange interaction, and random spin orientation on the surface of nanoparticles. The magnetic transformation with the increasing annealing temperature in the case of ZFO may be the consequence of the Fe 3+ and Zn 2+ ions redistribution in the spinel structure. The different MS value for ZFO reported in the literature indicates that The low values of M S for ZFO (0.7-10.8 emu/g) were determined by the lattice defects, particle size effects, core-shell interactions, spin canting, disordered distribution of cations, A-B super exchange interaction, and random spin orientation on the surface of nanoparticles. The magnetic transformation with the increasing annealing temperature in the case of ZFO may be the consequence of the Fe 3+ and Zn 2+ ions redistribution in the spinel structure. The different M S value for ZFO reported in the literature indicates that M S value strongly depends on different factors, such as the synthesis route, precursors type, thermal treatments, etc. [23,38]. The M S value of NFO was lower than that of MFO and CFO 4 , but two times higher than that of ZFO, probably due to the increasing surface effects with decreasing particles size [18]. The magnetic performance was greatly influenced by the particle size, due to the formation of a magnetic domain in which all the magnetons are aligned in one direction by exchange force [16]. The M s increased with increasing crystallite size. The size and shape of nanoparticles changed with increasing annealing temperature, leading to surface effects, spin canting-induced surface disorder, pinning of the magnetic moment, and cation inversion in the spinel ferrite nanostructures. In all cases, the M s and remanent magnetization (M R ) increased with the increasing annealing temperature, the highest values being found for MFO (M s = 28.5 emu/g, M R = 14.83 emu/g) and the lowest for ZFO (M s = 10.8 emu/g, M R = 1.67 emu/g) ( Table 2). In accordance with Asghar [5] the very low and negligible values of M R suggest a superparamagnetic behavior of the NCs. The Ms and M R increased with the increase of annealing temperature [45]. The low remanent magnetization in the cases of ZFO, NFO, and CuFO could be explained by the low content of magnetic phases present in the nanocomposite [12]. The crystallite size and M s increased with the increase of the annealing temperature as follows: The surface dipole interactions, high surface energy and tension, together with the cation charge distribution within the nanocrystallite induced the reduction of the lattice, which further caused the decrease of the lattice parameter, and blocked the grain growth [1][2][3][4][5][6].
For nano-size particles with multi-axial orientation anisotropy, (K) was calculated from H c and M s using Equation (3) [36].
The coercivity (H c ) increased with increasing annealing temperature for CFO, MFO, and NFO, indicating a higher degree of disorder in the magnetic moment arrangement at higher annealing temperatures, as a result of higher spin disorder, especially at the surface layer, since H c was significantly depreciated in smaller size particle where the spin disorder is increased [23]. When the grain size attained the single domain state, the particles exhibited a paramagnetic behavior and the surface effects became dominant over magnetization [23]. The H c of the spinel ferrite nanoparticles was governed by the magnetocrystalline anisotropy, strain, inter-particle interaction, particle size, and morphology [34]. The H c also increased with the enhancement of surface potential barrier caused by the crystalline lattice defects in the surface layers. However, the dependence of H c on the particle size, internal strain, magnetic domain structure, shape, and magnetocrystalline K of the particles was not completely elucidated. The low H c at low annealing temperature in all samples suggested an enhanced coalescence of the crystallites that resulted in stronger magnetic coupling and higher magnetization [42,45]. The H c and magnetocrystalline K of CFO was determined by the Co 2+ ions located in octahedral sites that further induced frozen orbital angular momentum and strong spin-orbital coupling [16]. The anisotropy constant (K) increased with the increase of annealing temperature. Generally, the crystallographic orientations, the presence of defects or inhomogeneities caused the magnetic K of samples [46]. This behavior can be related to the influence of cationic stoichiometry and their distribution between the A and B sites, the decrease of magnetocrystalline anisotropy, the occupancy of magnetic cations sites, and the increase of random canting of the surface spins [36]. The modification of surface spin coordination created an important change in magnetic ordering due to the superexchange interaction mediated by oxygen ions and to the broken bonds when oxygen ions were missing from the surface [36,37]. The energy of a magnetic particle in an external field is proportional to the number of magnetic molecules in a single magnetic domain, and in consequence, to its size [36]. The magnetic behavior of the MFO and CFO nanocomposites at 1200 • C is characteristic for hard magnet type ferrites having large hysteresis cycles and high anisotropy [37].
Generally, the magnetic properties of spinel ferrites depend upon the composition, particle size, structure, and cation distribution between octahedral and tetrahedral sites [47]. The increase in Ms along the magnetic CoFe 2 O 4 nanoparticles, with annealing temperature as a consequence of the increase of crystallinity degree and particle size, was also remarked in our previous studies [48]. The Ms increased with the increase of the degree of crystallinity due to the surface effect determined by the disorder of surface spins [49]. The increase of Ms with the Co ferrite content and annealing temperature was also reported by Salunkhe et al. [44]. For CoFe 2 O 4 , Varna et al. [50] also stated the increase of Ms with the annealing temperature and particle size and the decrease of M R with the increase of particle size. Conversely, in our study, the increase of M R with the particle size was observed not only for CFO but also for the other studied ferrites. The high Ms and low H c of NiFe 2 O 4 was attributed to high crystallinity and uniform morphology by Majid et al. [20]. The H c variation with particle size could be the consequence of the domain structure, critical diameter, and anisotropy of the crystals [51]. The decrease of H c value could be associated with larger magneto crystalline anisotropy, while the increase of H c could be associated with the scattering in the anisotropy field directions and with inhomogeneous broadening [52]. The magnetic properties of CuFe 2 O 4 were also reported to be strongly dependent on particle size, shape, and purity [53]. At temperatures below the Neel temperature, ZnFe 2 O 4 is antiferromagnetic but converts into diamagnetic, superparamagnetic, or ferromagnetic material when the size of ZnFe 2 O 4 reaches nanometer level [48,54]. The superparamagnetic behavior of the nanocrystalline ZnFe 2 O 4 is determined by the increase of the magnetic moment orientation disorder when the ratio surface/volume increases [38].

Conclusions
In this work, we performed a comparative study of structural, morphological, and magnetic properties of MFe 2 O 4 @SiO 2 (M = Co, Mn, Zn, Ni, Cu) obtained by a modified sol-gel method, followed by thermal treatment in the range of 300-1200 • C. Additionally, the relationship between crystallite size, lattice parameter, saturation, and remanent magnetizations and magnetic anisotropy was investigated. Thermal analysis indicated interesting, different formation and decomposition behaviors of metal-glyoxylates and, along with Fourier transform infrared (FT-IR) spectroscopy, confirmed the ferrite formation. Also, the study brought valuable information on the changes of structural and magnetic properties of ferrites annealed at different temperatures. Compared to the classical sol-gel synthesis approach, the modified sol-gel method offers important benefits such as reduced time, versatility, and simplicity to obtain pure and homogeneous nanoparticles; though, in the cases of CuFe 2 O 4 , ZnFe 2 O 4 , and MnFe 2 O 4 , secondary crystalline phases at high annealing temperatures and amorphous phases at low annealing temperatures were also noted. The thermal analysis showed the formation of glyoxylate precursors up to 210 • C and their decomposition into ferrites around 300 • C. The Mn and Ni ferrites were obtained by decomposition of the glyoxylate precursors in a single stage, while Co, Cu, and Zn ferrites by the decomposition of glyoxylates in two stages. XRD indicated the formation of single crystalline phases in the cases of Co and Ni ferrites at all annealing temperatures, of poorly crystallized Zn and Cu ferrites at low temperatures, and of crystallized Zn and Cu ferrites accompanied by crystalline ZnO and CuO at 600 and 900 • C and by crystalline SiO 2 at 1200 • C. In the case of Mn ferrite, the crystalline phase was unpurified by crystalline Fe 2 O 3 at 600-900 • C and by SiO 2 at 1200 • C. The crystallite sizes increased with increasing annealing temperature ranging between 9. annealing at 1200 • C, the average crystallite size estimated both from XRD and TEM data was comparable. The average particle size increased in the order of: NiFe 2 O 4 @SiO 2 > CoFe 2 O 4 @SiO 2 > MnFe 2 O 4 @SiO 2 > ZnFe 2 O 4 @SiO 2 > CuFe 2 O 4 @SiO 2 . The M S and M R increased with increasing annealing temperature, the highest values being measured for MnFe 2 O 4 and the lowest values for ZnFe 2 O 4 in spite of having the largest crystallite sizes. Mn ferrites, Ni ferrites, Cu ferrites, and Co ferrites had paramagnetic behavior above 900 • C and ferromagnetic behavior at lower temperatures, whereas Zn ferrite was antiferromagnetic at all temperatures. The H C increased with increasing annealing temperature in the cases of CoFe 2 O 4 , MnFe 2 O 4 and NiFe 2 O 4 , while the magnetic K increased with temperature in the case of all ferrites.