Sol-Gel Synthesis, Structure, Morphology and Magnetic Properties of Ni0.6Mn0.4Fe2O4 Nanoparticles Embedded in SiO2 Matrix

The structure, morphology and magnetic properties of (Ni0.6Mn0.4Fe2O4)α(SiO2)100−α (α = 0–100%) nanocomposites (NCs) produced by sol-gel synthesis were investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and vibrating sample magnetometry (VSM). At low calcination temperatures (300 °C), poorly crystallized Ni0.6Mn0.4Fe2O4, while at high calcination temperatures, well-crystallized Ni0.6Mn0.4Fe2O4 was obtained along with α-Fe2O3, quartz, cristobalite or iron silicate secondary phase, depending on the Ni0.6Mn0.4Fe2O4 content in the NCs. The average crystallite size increases from 2.6 to 74.5 nm with the increase of calcination temperature and ferrite content embedded in the SiO2 matrix. The saturation magnetization (Ms) enhances from 2.5 to 80.5 emu/g, the remanent magnetization (MR) from 0.68 to 12.6 emu/g and the coercive field (HC) from 126 to 260 Oe with increasing of Ni0.6Mn0.4Fe2O4 content in the NCs. The SiO2 matrix has a diamagnetic behavior with a minor ferromagnetic fraction, Ni0.6Mn0.4Fe2O4 embedded in SiO2 matrix displays superparamagnetic behavior, while unembedded Ni0.6Mn0.4Fe2O4 has a high-quality ferromagnetic behavior.


Introduction
Nanosized mixed metal oxides with high surface area and small particle size display unique properties [1]. MFe 2 O 4 (M = Zn, Co, Mn, Ni, etc.) type magnetic spinel ferrites with the general formula have numerous applications due to their high reactivity, chemical stability, optical, electrical and catalytic/ photocatalytic behaviors. Additionally, this type of magnetic nanoparticle is easily separated and recycled without important loss of their chemical activity [1,2].
Nickel ferrite (NiFe 2 O 4 ) has an inverse spinel structure with Ni 2+ ions occupying octahedral (B) sites and Fe 3+ ions occupying tetrahedral (A) as well as octahedral (B) sites. It presents high saturation magnetization (M S ), resistivity and low losses over a large frequency range, that resulted in applications in diverse fields [3,4]. Manganese ferrite (MnFe 2 O 4 ) is of great interest due to its good biocompatibility, coloristic properties, tunable magnetic properties, guidability in a magnetic field and excellent chemical stability. MnFe 2 O 4 nanoparticles are also recognized as efficient agents for magnetic hyperthermia and magnetic resonance imaging [1][2][3][4][5]. MnFe 2 O 4 has a spinel crystal structure with Fe 3+
The structure of NCs was investigated by XRD using a D8 Advance (Bruker, Karlsruhe, Germany) diffractometer, operating at 40 kV and 40 mA and employing CuKα radiation with λ = 1.54060 Å, at room temperature. The formation of the ferrite and SiO 2 matrix were investigated using a Spectrum BX II (Perkin Elmer, Waltham, MA, USA) Fouriertransform infrared spectrometer in the range of 400-4000 cm −1 . AFM was carried-out using a JSPM 4210 (JEOL, Tokio, Japan) scanning probe microscope using NSC15 silicon nitride cantilevers with resonant frequency of 325 kHz and force constant of 40 N/m, in tapping mode. The NCs were dispersed into ultrapure water, transferred on glass slides by vertical adsorption for 30 s and dried in air. Several areas of variable size (2.5 µm × 2.5 µm to 1 µm × 1 µm) of the dried glass slides were scanned. A cryogenic VSM magnetometer (Cryogenic Ltd., London, UK) was used for the magnetic measurements. The M S was determined in high magnetic field up to 10 T, whereas the magnetic hysteresis loops were conducted on samples incorporated in an epoxy resin to avoid any particle movement, between −2 to 2 T, at 300 K.
As the ferrite content decreases, in NCs with α = 25 and 50%, besides Ni0.6Mn04Fe2O4, the presence of α-Fe2O3 and Fe2SiO4 (JCPDS card no. 87-0315 [12]) secondary phases is also noticed. We assume that the formation of Fe2SiO4 could be related to difficulty of oxygen diffusion from the pores of SiO2 matrix and partial reduction of Fe 3+ into Fe 2+ , which reacts with the SiO2 matrix and forms Fe2SiO4 under the reducing condition produced by the decomposition of carboxylate precursors.
At 1100 °C, in case of NCs with α = 75-100% the well-crystallized Ni0.6Mn0.4Fe2O4 At 700 • C, in case of NC with α = 75%, the single and well-crystallized Ni 0.6 Mn 04 Fe 2 O 4 is observed, while in the case of NC with α = 100%, the α-Fe 2 O 3 (JCPDS card no. 89-0599 [12]) secondary phase is also present. The presence of α-Fe 2 O 3 might be attributed to partially embedding of the ferrite in the SiO 2 matrix, due to the low content or lack of SiO 2 and the short time or calcination temperature required to produce pure crystalline Ni 0.6 Mn 04 Fe 2 O 4 phase [5].
As the ferrite content decreases, in NCs with α = 25 and 50%, besides Ni 0.6 Mn 04 Fe 2 O 4 , the presence of α-Fe 2 O 3 and Fe 2 SiO 4 (JCPDS card no. 87-0315 [12]) secondary phases is also noticed. We assume that the formation of Fe 2 SiO 4 could be related to difficulty of oxygen diffusion from the pores of SiO 2 matrix and partial reduction of Fe 3+ into Fe 2+ , which reacts with the SiO 2 matrix and forms Fe 2 SiO 4 under the reducing condition produced by the decomposition of carboxylate precursors.
At 1100 • C, in case of NCs with α = 75-100% the well-crystallized Ni 0.6 Mn 0.4 Fe 2 O 4 phase together with traces of α-Fe 2 O 3 secondary phase are observed. In NCs with α = 50% containing ferrite and SiO 2 matrix in 1:1 molar ratio, besides the main phase of Ni 0.6 Mn 04 Fe 2 O 4 , the secondary phases of crystallized SiO 2 matrix are also noticed (cristobalite, JCPDS card no. 89-8936 and quartz, JCPDS card no. 89-8936 [12]), while in NC with α = 25%, Fe 2 SiO 4 is also obtained. Although it was reported that the thermal treatment may induce polymorphous transitions in Fe 2 O 3 , especially in the case of nanosized powders or nanoparticles embedded in amorphous and porous SiO 2 matrix, in our case only α-Fe 2 O 3 was observed [14]. The peaks corresponding to ferrite become more intense at 1100 • C, indicating high degree of crystallinity, crystallite size (due to the crystal coalescence process), nucleation rate and low effect of the inert surface layer [5]. Also, the highest peak shifts to higher angles with increasing Ni 0.6 Mn 04 Fe 2 O 4 content embedded in the SiO 2 matrix.
Among the available methods to estimate the crystallite size, those using the diffraction profile analysis, namely Williamson-Hall and Warren-Averbach procedures, require several diffraction profiles [15,16]. Considering that in our case, especially at low calcination temperatures, we have only few diffraction peaks, we estimated the average crystallite using the Scherrer method, which requires the full the width at half maximum (FWHM) for a single diffraction line [9]. Though the X-ray profile analysis is an average method, it is still a reliable method for measuring the crystallite size, apart from transmission electron microscopy (TEM). The average crystallite size of NCs calculated using the Debye-Scherrer formula [3,17] are presented in Table 1. The low ferrite content embedded in the amorphous SiO 2 matrix retards the expansion of the crystallite size, whereas high ferrite content favors both nucleation and growth of crystallite size at the nucleation centers, leading to higher crystallite size [1]. By increasing the calcination temperature, the Ni 2+ and Fe 3+ ions tend to occupy specific positions in the crystal lattice of the ferrite [18,19]. The crystallites were more compact at low ferrite content embedding in SiO 2 , since the smaller Ni 2+ ion can dissolve in the spinel lattice, while high ferrite content embedding in SiO 2 matrix causes the increase of the porosity leading to higher crystallite size [18]. During the calcination process, coalescence occurs, the smaller crystallites being merged together to form the large crystallites [7].
At all temperatures, the FT-IR spectra ( [3]. Generally, the Ni 2+ ions occupy the octahedral (B) sites, whereas Mn 2+/ Fe 3+ ions prefer both octahedral (B) and tetrahedral (A) sites [17]. The absorption bands shifting to lower values is accredited to the movement of Fe 3+ , Mn 2+ and Ni 2+ ions corresponding to the O 2− ions in the octahedral (B) and tetrahedral (A) sites, and consequently the change of the Fe 3+ -O 2− (M 3+ -O 2− ) and M 2+ -O 2− bond length, respectively [4]. The intensity of the vibrational band at 568-596 cm −1 increases with the increasing calcination temperature, due to the increasing ferrite crystallinity, since the ferrites consist of crystals bonded to all adjacent neighbors through ionic, covalent or van der Waals forces [5,11,20,21].
The small shift of the vibrational band originates from the movement of ions among the tetrahedral (A) and octahedral (A) sites as a result of the increasing calcination temperature [5,11,21]. The characteristic bands of the SiO 2 matrix were detected in the FT-IR spectra of NCs with α = 0-75%, as follows: 1068-1106 cm −1 with a shoulder at about 1200 cm −1 related to vibration of Si-O-Si chains, 788-805 cm −1 related to the vibrations of SiO 4 tetrahedron and 446-476 cm −1 related to the Si-O bond vibration and overlapping the band of Fe-O vibration [5,11]. The high intensity of these bands indicates a low polycondensation degree of the SiO 2 network [5]. The broad peaks observed at 3298-3313 cm −1 and at 1606-1626 cm −1 are ascribed to the vibrations of the -OH group and hydrogen bonds from adsorbed water molecules [1].
AFM was previously used to study the temperature effect on Ni and Mn ferrite nanoparticles transferred as thin film onto solid substrate. Ashiq et al. evidenced by AFM that Ni ferrite nanoparticles dispersion in liquid environment is proper to obtain well-structured thin films [22]. Moreover, Tong et al. reported particle diameters of 25 nm at 400 • C; 44 nm at 500 • C and 65 nm at 700 • C, and surface roughness depending on the nanoparticle disposal in the topography [23].
The use of Mn ferrite nanoparticles as dispersed matter into the liquid environment as magnetic ink was also reported [24]. The printed thin film investigated with AFM revealed Mn ferrite nanoparticles of about 95 nm and the surface roughness depending on the particle diameter and on the observed agglomeration tendency [24]. The AFM topographic images are presented in Figure 2a-o. A dependence of nanoparticle diameter on the calcination temperature was observed for pure Ni 0.6 Mn 04 Fe 2 O 4 (Figure 2a-c). The diameter of the round shape particles increases from about 18 nm at 300 • C to 52 nm at 700 • C, and 75 nm at 1100 • C, respectively. The crystallite size increase with the temperature increase was also observed based on the XRD data. The particle size revealed by AFM correlation with XRD crystallite size of pure Ni-Mn ferrite indicates a polycrystalline state at low temperatures (crystallite size is considerably smaller than particle size) and monocrystalline state (crystallite size is very close to the particle diameter) at 1100 • C. Establishing a certain number of crystallites per particle requires a more enhanced investigation based on scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) analysis [16].
XRD patterns show that the SiO 2 matrix is amorphous at all calcination temperatures. However, the particle size and shape evolution with increasing temperature may be observed using AFM. Figure 2m reveals small round shape nanoparticles and a diameter increasing with the calcination temperature, i.e., about 12 nm at 300 • C, 28 nm at 700 • C and 35 nm at 1100 • C (Figure 2n,o). Previous studies confirm the shape and sizes of the silica nanoparticles observed by AFM [25,26].
The NCs with α = 25-75% combine the morphological and structural features of both Ni-Mn ferrite and amorphous SiO 2 nanoparticles. The diameter of the round-shape nanoparticles is strongly influenced by the calcination temperature and composition (Figure 2d-l). The lowest size particles were observed at 300 • C and the bigger ones at 1100 • C ( Table 1). The amorphous SiO 2 matrix increases the particle size compared to the ferrite crystallites due to the embedding effect. This effect is more visible at 300 • C than at 1100 • C. At higher calcination temperatures, the ferrite crystallite is well covered by an amorphous SiO 2 layer which forms the composite nanoparticle. The insulating behavior of the amorphous SiO 2 matrix prevents the overgrowth of magnetic domains and guarantees the nano-structural stability. A slight decrease of the nanoparticle size occurs by increasing the amorphous SiO 2 content. This decrease is most obvious at 1100 • C (Figure 2c,f,i,l), where the amorphous SiO 2 matrix inhibits the development of bigger ferrite crystallites (Table 1). A similar behavior was reported for other ferrite systems [5,11].
The powder dispersion in an aqueous environment facilitates the nanoparticle arrangement, assuring a uniform adsorption onto the solid substrate creating well-structured thin films [27], as observed in Figure 3a-o. The film roughness depends on the nanoparticle diameter and their disposal on the substrate surface. Thus, the lower roughness values are obtained at 200 • C (Figure 3a,d,g,j,m) due to the uniform adsorption of fine nanoparticles. The particle diameter increases with the calcination temperature, while the adsorbed film uniformity depends on the local heights formed by bigger nanoparticles (Figure 3c,f,i,l).   NCs, α100% (a,b,c); α = 25% (d,e,f); α = 50% (g,h,i); α = 75% (j,k,l) and α = 100% (m,n,o) calcined at 300, 700 and 1100 • C.  The morphological aspects of the nanoparticle thin films revealed by AFM correlated with the magnetic properties allow the design of functionalized surfaces for various applications where thermal deposition at high temperatures it is not possible, i.e., such as polymer coating.    The peaks' broadening indicates a larger distribution of the particle sizes. For the NCs with α = 25-100%, dM/d(µ 0 H) vs. H curves have a single and sharp peak. The morphology and the phase purity of NCs, as well as their magnetic properties, are strongly affected by the calcination temperature [3,5]. The SiO 2 matrix has diamagnetic behavior for both 700 and 1100 • C calcination temperatures. For the NCs with α = 100% (Ni 0.6 Mn 0.4 Fe 2 O 4 ), typical hysteresis loops for ferromagnetic materials were obtained, for all the calcination temperatures, due to the presence of larger size crystallites and particles as found in XRD and AFM analyses. The unembedded Ni 0.6 Mn 0.4 Fe 2 O 4 (α = 100%) has a much higher M S , especially when it is calcined at 1100 • C, than the ferrites embedded in the SiO 2 matrix (α = 25-75%), with pretty narrow hysteresis loops, close to a superparamagnetic behavior.
behavior for both 700 and 1100 °C calcination temperatures. For the NCs with α = 100% (Ni0.6Mn0.4Fe2O4), typical hysteresis loops for ferromagnetic materials were obtained, for all the calcination temperatures, due to the presence of larger size crystallites and particles as found in XRD and AFM analyses. The unembedded Ni0.6Mn0.4Fe2O4 (α = 100%) has a much higher MS, especially when it is calcined at 1100 °C, than the ferrites embedded in the SiO2 matrix (α = 25-75%), with pretty narrow hysteresis loops, close to a superparamagnetic behavior.  The superparamagnetic-like behavior of the NCs is a consequence of the low sizes of the crystallites and of their low magnetic anisotropy which allow their easily thermal activation [3,4]. The increase of the calcination temperature can lead to the improvement of M S and M R , as a result of a better crystallinity of the ferrite, of proper interatomic lengths changing of the atomic coordination number, etc. The M S values of NCs with high content of Ni 0.6 Mn 0.4 Fe 2 O 4 embedded in the SiO 2 matrix are larger due to larger particles sizes which show reduced spin canting and other surface effects which are usually present in small size particles. The main mechanisms of the magnetization process are the domain wall motions and the magnetic moment rotations [3]. The spin disorder on the nanoparticle surface can also strongly affect the M S value. Moreover, the lattice defects can weaken the magnetic super-exchange interaction between the tetrahedral (A) and the octahedral (B) sites [3]. The involved magnetic Fe 3+ , Ni 2+ and Mn 2+ ions have magnetic moments with the following values: 5, 2 and 5 µ B respectively [20]. The distribution of cations between tetrahedral (A) and octahedral (B) sites of the spinel decides the magnetic moment per formula unit. The addition of Mn 2+ ions in the Ni ferrite can induce a migration of the Fe 3+ ions from the tetrahedral (A) to the octahedral (B) sites leading to a spin imbalance between the two sites, resulting in an increase of the magnetization at the octahedral (B) sites [20]. The surface energy of nanosized particles is large and can modify the typical cation distribution between the A and B sites [3,5]. The SiO 2 matrix can partially dilute the magnetic matrix of the cations and it can create disorder at the surface of the particles and increase the number of defects, broken bonds, canted spins, and pinning of the magnetic field lines [2,5]. The nanoparticles calcined at 700 • C have rather low values of M S since they show lower crystallinity, large defect concentration, reduced coordination number and increased interatomic spacing [5]. The M S values of NCs calcined at 700 • C increase with increasing N i0 . 6 Mn 0.4 Fe 2 O 4 content, not far from a linear dependence, from 2.5 emu/g (α = 25%) to 31.5 emu/g (α = 100%). This behavior indicates that the main contribution to magnetization is given by the ferrite content in the samples. A possible explanation of the deviation from the linear dependence can be the disorder of magnetic moments on the surface of particles, mainly for the small size particles which have a higher surface-to-volume ratio [2,5]. The increase of M S with increasing particle sizes is typical for nano-sized ferrites [28]. Excepting the sample with α = 25%, there is a very good proportionality between particle and the crystallite sizes. The crystallite sizes also increase continuously with the ferrite content. This behavior suggests that the SiO 2 content has a negligible effect on the interaction between the magnetic moments of the cations from tetrahedral (A) and octahedral (B) sites, i.e., the magnetic order is not significantly changed by the SiO 2 matrix. The H C decreases with increasing ferrite content, or with growing of the crystallite sizes as expected for multi-domain nanoparticles [28,29]. The H C decreases from 185 Oe (α = 25%) to 126 Oe (α = 100%). The M R decreases from 4.5 emu/g (α = 100%) to 0.68 emu/g (α = 25%) mainly due to the increasing disorder of the magnetic moments in the outer shell of the smaller sized particles [2,5]. The magnetic properties of these NCs are also affected by their bulk densities and by their grain sizes and grain size distributions. The strain released by the larger particles is higher than those of the smaller ones, resulting in lattice expansion. The pores can also contribute to the magnetic properties of the NCs, acting as pinning centers for the domain walls and for the magnetic moments of the cations [5]. The observed M S values are in good agreement with the cation distribution theory and Neel's molecular field model [1]. The lower values of M S for some of the NCs can be explained by the effect of the spin canting in the frame of the non-colinear Yafet-Kittel model in the presence of Jahn-Teller cations [8].
The coercive field, H C , is given mainly by the magneto-crystalline anisotropy, but also by the exchange anisotropy due to the magnetic moment's interaction from the particle surface [15]. Generally, the M-H curves do not reach complete magnetic saturation, even in 10 T. For these cases, the M S was estimated by using the law of approach to magnetic saturation [30,31]. The absence of complete saturation in ferromagnetic nanoparticles is generally related to the magnetic moments' disorder in the surface layers of the particles which needs a larger magnetic field for saturation, in association with the lower anisotropy of the smaller sized particles [10]. The H C values are rather low, in the range from 126 to 260 Oe. As can be seen, the M S increases for the NCs with lower SiO 2 matrix content. This behavior can be related to the decrease of the particle sizes with SiO 2 content increase and the associated micro-strains, and probably, the magnetic particles morphology and magnetic domain sizes [3]. The H C decreases nearly linearly with increasing SiO 2 content due to a continuous decrease of the crystallite sizes in the single-domain range under the influence of the SiO 2 matrix [9]. The larger sized nanoparticles are composed of multi-domains, where the H C decreases due to the formation of domain walls in the nanoparticles [7]. The measured M S values of our previously reported (Zn 0.6 Mn 0.4 Fe 2 O 4 ) α (SiO 2 ) 100−α (α = 100%) NCs [5] are similar to those belonging to the (N i0 . 6 Mn 0.4 Fe 2 O 4 ) α (SiO 2 ) 100−α (α = 100%) NCs. The M S of both systems keeps the same trend, decreasing with increasing SiO 2 matrix content, which results in decreasing particle sizes. The NCs calcined at 1100 • C from the both series behave similarly showing the enhancement of the H C with increasing SiO 2 matrix content, in spite of the much larger values of H C for the Zn 0.6 Mn 0.4 Fe 2 O 4 ) α (SiO 2 ) 100−α nanoparticles. These behaviors are typical for particle sizes belonging to the multi-domain range [5,28,29]. The Ni-Mn ferrites calcined at 700 • C also belong to this category, while the previous Zn-Mn ferrites calcined at 700 • C (with smaller particle sizes) behave differently, with a H C which depreciates with decreasing SiO 2 matrix content (or with increasing particle sizes), suggesting that most of the particles have sizes belonging to single-domain range.

Conclusions
Sol-gel route followed by calcination was used to synthesize (Ni 0.6 Mn 0.4 Fe 2 O 4 ) α (SiO 2 ) 100−α (α = 0, 25, 50, 75, 100%) NCs. In the absence of an SiO 2 matrix (α = 100%), single-phase crystalline Ni 0.6 Mn 0.4 Fe 2 O 4 was obtained at 300 • C, while at 700 and 1100 • C, ferrite is accompanied by an α-Fe 2 O 3 secondary phase. By embedding high ferrite contents in the SiO 2 matrix (α = 75%), a single phase of Ni 0.6 Mn 0.4 Fe 2 O 4 was obtained at 300 and 700 • C, but at 1100 • C, besides the crystalline ferrite, α-Fe 2 O 3 is also present. By embedding the ferrite in equal content with the SiO 2 matrix (α = 50%), poorly crystallized single-phase Ni 0.6 Mn 0.4 Fe 2 O 4 is formed at 300 • C, α-Fe 2 O 3 and Fe 2 SiO 4 secondary phases accompany the Ni 0.6 Mn 0.4 Fe 2 O 4 at 700 • C, while at 1100 • C Ni 0.6 Mn 0.4 Fe 2 O 4 is accompanied by quartz and cristobalite. The embedding of low ferrite content (α = 25%) in the SiO 2 matrix results in similar crystalline phases as in the case of NCs with α = 50% except that Fe 2 SiO 4 secondary phase is also formed at 1100 • C. The increase of the calcination temperature and ferrite content embedded in the SiO 2 matrix led to an increase of the average crystallites size: 2.6-4.6 nm (300 • C), 16.5-50.1 nm (700 • C) and 30.3-74.5 nm (1100 • C). AFM investigation revealed that the average particle diameter increases with increasing calcination temperature, while the amorphous SiO 2 acts as an insulator among magnetic crystallites and prevents their overgrowth, especially at 1100 • C. The magnetic parameters enhance with increasing Ni 0.6 Mn 0.4 Fe 2 O 4 content embedded in the SiO 2 matrix: M S from 2.5 to 31.5 emu/g (700 • C) and from 4.5 to 80.5 emu/g (1100 • C), M R from 0.68 to 4.5 emu/g (700 • C) and from 1.1 to 12.6 emu/g (1100 • C), H C from 126 to 186 Oe (700 • C) and from 150 to 260 Oe (1100 • C). The embedding of ferrite in the SiO 2 matrix led to the particle sizes decreasing in the nano-range, but also to the alteration of the magnetic parameters. As expected, unembedded Ni 0.6 Mn 0.4 Fe 2 O 4 (α = 100%) is ferromagnetic, the SiO 2 matrix (α = 0%) is diamagnetic with a small ferromagnetic fraction, while the Ni 0.6 Mn 0.4 Fe 2 O 4 embedded in SiO 2 is superparamagnetic. The obtained NCs can be further developed to obtain soft and thin magnetic films on various solid substrates with tailored properties by varying the ferrite-to-matrix ratio and by a proper management of adsorption process.