Effect of Transition Metal Doping on the Structural, Morphological, and Magnetic Properties of NiFe2O4

Sol-gel route followed by thermal treatment was used to produce NiFe2O4 doped with transition metal ions (Zn2+, Mn2+, Co2+). The structural, morphological, and magnetic properties of the doped NiFe2O4 were compared with those of virgin NiFe2O4. The metal-glyoxylates’ formation and decomposition as well as the thermal stability of the doped and virgin ferrites were assessed by thermal analysis. The functional groups identified by Fourier-transform infrared spectroscopy confirmed the decomposition of metal nitrates, the formation and decomposition of precursors, and the formation of the SiO2 matrix. The X-ray diffraction indicated that the sol-gel synthesis produced single-phase crystalline ferrites in case of virgin, Zn2+ and Co2+-doped Ni-ferrites. By doping with Mn2+, several secondary phases derived from the SiO2 matrix accompanied the crystalline spinel ferrite. The crystallite sizes depended on the annealing temperature and type of doping ion. The gradual increase of lattice parameters suggested the uniform distribution of doping metal ions in the NiFe2O4 lattice. The saturation magnetization, remanent magnetizations, coercivity, and anisotropy were found to depend on the doping ion, annealing temperature, and particle size. The high saturation magnetization values of the obtained nanocomposites make them suitable for a wide range of applications in the field of sensors development and construction.


Introduction
Spinel ferrites are the topic of numerous studies due to their magnetic nature and crystalline structure. Small changes of the particle size, composition or presence of surface effects give them unique magnetic features [1]. Nanosized spinel ferrites received a huge amount of interest due to their low cost, excellent chemical stability, moderate saturation magnetization, high surface area, high wear resistance, low density, low thermal expansion coefficient, and low toxicity to both human health and environment [2][3][4]. These ferrites are promising candidates for a broad range of applications in the industry (magnetic recording media, photoelectric devices, sensors, magnetic pigments, photocatalysts in dye degradation, controlled signal transformation, storage devices, batteries, solar cells) and biomedicine (controlled drug delivery, tumor treatment, magnetic resonance imaging, biomagnetic separation, cellular therapy, tissue repair, cell separation, and biosensing) [1][2][3][4][5][6][7][8].
The ferrite structure and properties are sensitive to the synthesis method, additive substitution, and calcination process [12][13][14]. Doping with transition metal ions, such as Co 2+ , Ni 2+ , or Zn 2+ , is an effective way to improve and control the structure and consequently the optical, electrical, dielectric, and magnetic properties of nanosized NiFe 2 O 4 [1,7,8,[15][16][17]. The doping with transition metal ions into spinel ferrite structure changes the cations' distribution between the tetrahedral (A) and octahedral (B) sites, leading to different magnetic properties. The dopant ion may also change the energy of the grain boundaries, acting as a driving force of the grain growth [1]. The electrical resistivity can also be improved by doping the host matrices with smaller divalent cations or by controlling their microstructures [8]. Zn 2+ doping disturbs the cation distribution, enhances the dielectric and magnetic properties [18]. The substitution of NiFe 2 O 4 with magnetic divalent transition metal ions like Mn 2+ led to appealing magnetic and electrical features [2,12,14,19,20]. By adjusting the Mn-to-Ni ratio in the ferrite, the magnetic properties of the ferrite can be controlled [2]. The Ni 2+ ions' addition overcomes the grain formation, leading to low surface roughness [13,21].
The physico-chemical properties of nanosized ferrites are highly influenced by the synthesis route, dopant ion nature and amount, as well as the presence of structural order-disorder effects [1,5]. The annealing temperature influences the grain boundary migration and grain boundary diffusion, which further determines the grain shape, grain size, core density, and microstructure [4]. The synthesis route is a key factor to obtain high-purity nanoferrites [1]. Several methods for producing nanoferrites, such as sol-gel, co-precipitation, refluxing, hydrothermal, mechano-chemical, solid-state, precursor, auto combustion, microwave plasma, microemulsion, mechanical alloying, etc. are described in the literature [1,5,7]. Among these, to produce ferrite nanocomposites, the sol-gel method is one of the most-used approaches due to its simplicity, low cost, low processing temperature, and good control over the structure, physico-chemical properties, surface properties, and magnetic behavior [22]. To obtain spinel ferrites by the sol-gel method, nitrate salts are frequently used, as they act as water-soluble, low-temperature oxidizing agents [23]. Solvothermal synthesis allows the large-scale production of ferrites with controlled size and shape by choosing the appropriate aqueous or non-aqueous solvent mixture, by varying the synthesis temperature, pressure, and reaction time [3]. The microwave-assisted synthesis of ferrites has a lower yield than hydrothermal or thermal-decomposition methods [3]. The co-precipitation method is another frequently used method to produce nanoparticles with a specific shape and size [22]. The major disadvantage associated with the ferrite production by co-precipitation is the poor crystallinity of the resulting NPs, that may be enhanced by subsequent heat treatment [3]. Auto-combustion is a simple and lowcost process that requires a short reaction time and low energy consumption [4,24]. The ferrites prepared by this method have homogeneous chemical composition, high-purity, and good sinterability [24]. In the modified sol-gel method, the reactants are mixed with tetraethyl orthosilicate (TEOS), the sol is exposed to air until the gelation of the silica (SiO 2 ) network, the gels are thermally treated to obtain carboxylate precursors that are further thermally decomposed into the oxidic systems. This method is versatile, simple, and effective in producing pure nanoparticles, but has the drawback of having the presence of amorphous phases at low annealing temperatures and of secondary crystalline phases at high annealing temperatures [25]. Among different coating materials, mesoporous SiO 2 is non-toxic and biocompatible, allows the control of the particle growth, minimizes the nanoparticles agglomeration, improves their stability, enhances the magnetic guidability and bio-compatibility, and favors the conjugation with functional groups, [26][27][28][29].
The paper aims to investigate the structural, morphological and magnetic properties of virgin ) embedded in a SiO 2 matrix produced by sol-gel route, followed by thermal treatment at various temperatures. This study is of particular interest due to the lack of information on the effect of dopant nature (Zn 2+ , Mn 2+ and Co 2+ ) on the size and magnetic properties of mixed M 0.15 Ni 0.85 Fe 2 O 4 (M=Co, Mn and Zn) type ferrites embedded in SiO 2 matrix. Because the oxidic phases at low temperatures are poorly crystalline or even amorphous, the desired surface properties and crystallinity can be achieved by using specific annealing conditions. Besides, the reactivity of the amorphous phases allows their participation in a variety of chemical transformations. In this regard, the X-ray diffraction (XRD) parameters were compared for different annealing temperatures to get important structural information. The thermal (TG-DTA) analysis and Fourier transform infrared (FT-IR) spectroscopy depicted the formation and decomposition of metallic glyoxylate precursors, the stability of the produced ferrites and formation SiO 2 matrix. A special emphasis was given to the evolution of magnetic properties (saturation magnetization (M S ), remanent magnetization (M R ), coercivity (H C ), and anisotropy (K)) with the increase of annealing temperature and the type of doping ion.   Figure 1. To prepare the sols, the metal nitrates were mixed with 1,2-ED, TEOS and ethanol by using a NO 3 − /ED/TEOS molar ratio of 1/1/0.50. The resulting sols were stirred continuously for 30 min and maintained in open air, at room temperature until gelation occurs. The formed gel embedded a homogenous mixture of metal nitrates and 1,2-ED. As the production of high-purity gels with high crystallites size is favored by a thermal pretreatment before annealing [5], the obtained gels were grinded, dried at 40 and 200 • C, and annealed at 400 • C (5 h), 700 • C (5 h) and 1000 • C (5 h), respectively, by using a LT9 muffle furnace (Nabertherm, Lilienthal, Germany). By heating the gels at 200 • C the redox reactions between the nitrates and 1,2-ED take place in the pores of the SiO 2 matrix resulting a mixture of Fe(III), Ni(II), and M(II) glyoxylates. The mixtures of glyoxylates around 300 • C decompose into metal oxides that reacts at temperatures above 300 • C and forms the ferrites. SiO 2 has de role of a spacer between the nanoparticles, reducing the particle agglomeration [14,29].

Characterization
The thermal behavior was investigated by thermogravimetric (TG) and differential thermal analysis (DTA) by using a Q600 SDT (TA Instruments, Newcastle, DE, USA) thermal analyzer, in air up to 1000 • C, at 5 • C/min. The FT-IR spectra were recorded by using a Spectrum BX II (Perkin Elmer, Waltham, MA, USA) Fourier-transform infrared spectrometer on pellets containing 1% (w/w) sample in KBr. The X-ray diffraction patterns were recorded by using a D8 Advance (Bruker, Karlsruhe, Germany) diffractometer, operating at room temperature, 40 kV, and 40 mA with CuKα radiation (λ = 1.54060 Å). The Co/Ni/Fe (Co 0.15 Ni 0.85 Fe 2 O 4 @SiO 2 ), Mn/Ni/Fe (Mn 0.15 Ni 0.85 Fe 2 O 4 @SiO 2 ), and Zn/Ni/Fe (Zn 0.15 Ni 0.85 Fe 2 O 4 @SiO 2 ) molar ratios were verified by inductively coupled plasma optical emission spectrometry (ICP-OES) by using a Perkin Elmer Optima 5300 DV (Norwalk, CT, USA) spectrometer, after microwave digestion with aqua regia. The nanoparticles morphology was studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) on samples deposited from suspension onto carboncoated copper grids by using an HD-2700 (Hitachi, Tokyo, Japan) transmission electron microscope and a SU8230 (Hitachi, Tokyo, Japan) scanning electron microscope. A cryogenfree vibrating-sample magnetometer (Cryogenic Limited, London, UK) was used for the magnetic measurements.

Thermal Analysis
The TG/DTA curves of virgin and doped NiFe 2 O 4 samples dried at 40 • C are presented in Figure 2. The DTA curve shows three processes: (I) loss of moisture and physically adsorbed water suggested by the endothermic effects at 64-95 • C, (II) formation of metal-glyoxylate precursors shown by the exothermic effects at 116-182 • C and (III) decomposition of glyoxylate precursors into ferrites as indicated by the exothermic effect at 260-315 • C.   [7,8]. The TG curve indicate a total mass loss of 61.7%. Thus, between 260 and 277 • C, the virgin Ni-ferrite, as well as the Zn-and Mn-doped ferrites are formed, whereas the Co-doped ferrite is formed at 315 • C. The mass losses are comparable, the highest mass loss being recorded for the virgin Ni-ferrite and the lowest mass loss for Co doped Ni-ferrite.

Fourier-Transform Infrared Spectroscopy
The FT-IR spectra offers data on the presence of different functional groups, molecular geometry and inter-molecular interactions [1]. In samples heated at 40 • C, the FT-IR spectra ( Figure 3) display an intense band at 1384 cm −1 specific to nitrate groups [25,30], which disappears for samples heated at 200 • C, indicating the nitrates decomposition.  [1,6,25,30], indicating the formation of ferrites with cubic structure [1]. The doping of NiFe 2 O 4 with larger size and higher atomic weight divalent ions forces the migration of Fe 3+ ions to the octahedral (B) sites leading to a decrease of the tetrahedral vibration frequency and an increase of the octahedral vibration frequency [9].
The variation of the oxygen atoms' positions results in structural distortion of the FeO 6 , FeO 4 , and NiO 6 complexes that highly disturb the NiFe 2 O 4 lattice, leading to structural changes with high impact on the physico-chemical properties [5]. In case of doping with Mn 2+ ions, the diffraction peak situated near 2θ = 35 • are slightly shifted. Some possible explanations could be the Mn 2+ ions that enter in the octahedral (B) sites as well as the larger radius of Mn 2+ (0.80 Å) than of Ni 2+ (0.72 Å) [15]. The crystallite size (D) calculated from the most intense diffraction peaks (311), lattice constant (a), unit cell volume (V), bulk density (d p ), X-ray density (d XRD ), porosity (P), and hopping length in tetrahedral (L A ) and octahedral (L B ) sites [6,8,[33][34][35] are shown in Table 1. XRD parameters are influenced not only by the crystallite size, lattice strain and defects, but also by the annealing temperature and doping ions [6]. The sharpening and narrowing of the diffraction peaks suggest the crystallite size become more obvious with the annealing temperature [16]. At high annealing temperatures (1000 • C), a significant agglomeration takes place without subsequent recrystallization, supporting the formation of a single crystal instead of a polycrystal structure [5,36].   The lattice constant (a) increases, whereas the X-ray density (d XRD ) decreases with increasing crystallite size. Some possible explanations could be the surface tension decrease caused by the size effect and the expansion of unit cell by replacing Ni 2+ with Zn 2+ , Co 2+ , and Mn 2+ ions [6,17,33]. Considering the small difference between the atomic weight of Ni 2+ and Mn 2+ ions, the d XRD variation may be attributed to the changes of the lattice constant (a) [37]. The lattice constant (a) shows a linear behavior and it follows Vegard's law. The differences between the lattice parameter of investigated samples were attributed to the different ionic radii of Fe 3+ (tetra: 0.49; octa: 0.64 Å), Zn 2+ (tetra: 0.60; octa: 0.74 Å), Ni 2+ (tetra: 0.54; octa: 0.78 Å), Mn 2+ (tetra: 0.58; octa: 0.69 Å), and Co 2+ (tetra: 0.58; octa: 0.74 Å) [17,33,35]. The decrease of porosity (P) with the increase of annealing temperature may be a consequence of the rapid densification during the annealing process [6,17,33].

Chemical Analysis
The M/Ni/Fe molar ratio calculated based on Co, Mn, Zn, Ni and Fe concentrations measured by ICP-OES confirmed the theoretical elemental composition of the obtained NCs (Table 1). In all cases, the best fit of experimental and theoretical data was remarked for samples annealed at 1000 • C. In case of Mn-dopped NiFe 2 O 4 annealed at 700 and 1000 • C, the Mn/Ni/Fe molar ratio could not be calculated based on the metal concentrations, due to the presence of Fe 2 SiO 4 as secondary phase.

Transmission and Scanning Electron Microscopy
The TEM images ( Figure 5) reveal irregularly shaped particles that form agglomerates. As a result of the doping with Zn 2+ and Co 2+ ions, a decrease of the particle size from 29 nm (NiFe 2 O 4 ) to 10 nm (Zn-NiFe 2 O 4 ) and 21 nm (Co-NiFe 2 O 4 ) was observed, whereas by doping with Mn 2+ ion, the particle size increases to 43 nm (Mn-NiFe 2 O 4 ).
The variation of particle size by doping may be determined by the different kinetics of metal oxides' formation reaction, the different particle growth rate or the presence of structural disorder and strain in the lattice due to different ionic radii [14,37]. The different particle arrangement could be attributed to the formation of well-delimited grains that form solid boundaries. The particle agglomeration is frequently observed in case of NCs synthesized by chemical routes and is caused most probably by the assembling tendency of small particles, magnetic nature, and weak surface interaction due to Van der Waals forces [8,9,25,33]. The internal heat energy produced during the annealing may also lead to the agglomeration of particles due to interfacial surface tensions [8,25]. The differences obtained between particle and crystallite size result most probably due to the interference of amorphous matrix and of large-size nanoparticles that highly influence the diffraction patterns, by the large fraction of the total number of atoms contained [8]. The crystal-growth rate increase could be attributed to volume expansion and supersaturation reduction of the system at high annealing temperatures, which further leads to increase of the amorphous Fe oxides solubility and crystallization of M 0.15 Ni 0.85 Fe 2 O 4 when Mn, Zn, and Co diffuse into the crystal structure of NiFe 2 O 4 . When the nucleation rate exceeds the growth rate, small and homogenously distributed particles are obtained. At high annealing temperatures, these particles may join together due to coalescence, formation of crystalline clusters, and joint cementation [8,14,25,37].
The SEM images ( Figure 6) indicates agglomerations of homogenous, clearly delimited particles typical of ferrite materials containing magnetic elements [25]. The particles in Zn and Co dopped NiFe 2 O 4 have a homogenous microstructure with closely packed, irregularly shaped small particles, whereas those in Mn dopped NiFe 2 O 4 are bigger and more loosely packed.

Magnetic Properties
All samples display superparamagnetic behavior with well-defined hysteresis loops (Figure 7), but important differences in the magnetic parameters are induced by the doping ions. Small particles contain fewer domain walls and require higher demagnetization force, whereas large particles have a higher probability of domain formation [9].  Table 2. The doping of NiFe 2 O 4 with Zn 2+ , Mn 2+ , or Co 2+ ions lead to a decrease of the M S and M R after annealing at 700 and 1000 • C. Above the single-domain critical size, the competition between the increasing magnetostatic energy and the domain-wall energy favors the domain-wall formation and the single-domain particle splits into multi-domain [9]. The low magnetization value of Co-NiFe 2 O 4 is due to the incomplete crystallization and small-sized crystallites, which generate structural disorder on the nanoparticles surface. As the particles surface behaves as an inactive layer, its magnetization become negligible [5]. Some possible explanations for the variation of Ms in case of doped NiFe 2 O 4 could be: (i) occupation of the octahedral sites by Zn 2+ ions, (ii) random incomplete A-O-B linkages resulting in the replacement of non-magnetic ions by magnetic ions in the spinel, and (iii) the presence of non-collinear magnetic structures [16]. The magnetization caused by domain wall movement needs less energy than the domain rotation. The number of domain walls increases with increasing particle size. In case of Zn 2+ doping, the wall movement contribution to magnetization is higher than that of the domain rotation [33]. Moreover, the presence of impurity phases with antiparallel magnetic ordering to the ferrite ordering reduces the M S . The doping with Co 2+ ions having higher magnetic moment than Ni 2+ ions result in a decrease of M S , as Ni 2+ ion may occupy both the tetrahedral (A) and octahedral (B) sites [36].
By doping NiFe 2 O 4 , the H C decreases at 700 • C and increases at 1000 • C as a consequence of increased spin disorder in the surface layer and smaller particle size [1,32]. The H C value of 48 Oe of virgin NiFe 2 O 4 decreases to 11 Oe in case of Co doping and annealing at 700 • C, most probably due to agglomerates' formation which leads to the increase of average particles size above the critical single domain of NiFe 2 O 4 particles and further leads to a multidomain magnetic structure [5]. Moreover, by annealing at low temperatures, the grain growth occurs, weakening the domain wall pinning effects at the grain boundary [5].
To calculate the magnetic anisotropy constant (K) of the samples, we assumed that the spinel ferrite particles have spherical shape. The K value of virgin NiFe 2 O 4 is larger than that of doped NiFe 2 O 4 . The magnetic anisotropy of particles behaves as energy barrier and stops the switching of the magnetization's direction to the easy axis [38,39]. At a certain temperature, the thermal activation overcomes the magnetic anisotropy energy barrier and the magnetization direction of the particles change, indicating a super-paramagnetic behavior [38,39]. A conceivable explanation could be the presence of a magnetically disordered surface layer, where a competition of exchange interactions between surface spins exists. Moreover, the magnetic disorder may originate in uneven magnetic interactions of the surface spins, arbitrarily oriented grains of different sizes and disordered vacancies [38,39].

Conclusions
The structural, morphological and magnetic properties of virgin and Zn-, Mn-, and Co-doped NiFe 2 O 4 embedded in SiO 2 matrix obtained through a modified sol-gel route and thermal treatment were investigated. The FT-IR spectra evidenced the formation of metallic precursors and of SiO 2 matrix. The TG/DTA curves of samples dried at 40 • C indicated the formation and decomposition of metallic precursors to ferrites in single or two stages, with comparable mass losses. The XRD analysis revealed single-phase ferrites for virgin, Zn-and Co-doped NiFe 2 O 4 , and the presence of secondary crystalline phases derived from the SiO 2 matrix (cristobalite, quartz, and Fe 2 SiO 4 ) in case of Mn-doped NiFe 2 O 4 . XRD parameters were influenced not only by the crystallite size, lattice strain, and defects, but also by the annealing temperature and doping ions. The lattice constant and unit cell volume increase by doping with Mn 2+ ion and decrease by doping with Zn 2+ and Co 2+ ions. By contrast, X-ray and bulk densities, and porosity decrease by doping with Mn 2+ and increase with doping Zn 2+ and Co 2+ ions. The NiFe 2 O 4 particle size increases by doping with Mn 2+ and decrease by doping with Zn 2+ and Co 2+ ions, respectively. The doping of NiFe 2 O 4 with Zn 2+ , Mn 2+ and Co 2+ leads to a decrease of the saturation magnetization and remanent magnetization, whereas the coercivity decreases at 700 • C and increases at 1000 • C. The obtained magnetic transition metal dopped-Ni ferrite nanoparticles are possible candidates for various medical applications like controlled drug delivery, cancer therapy, biosensing, and magnetic resonance imaging.