1 INTRODUCTION

MgFe2O4 is a binary spinel ferrite that belongs to the family of spinel structures with the general formula MFe2O4 (M = Zn2+, Co2+, Cu2+, Ni2+, Mg2+, Fe2+, Mn2+) or (AB2O4) where A and B are the tetrahedral and octahedral sites of the cations, respectively. The structure is cubic with space group Fd3m and is considered a soft magnetic material. The distribution of the cations between the tetrahedral and octahedral sites in the spinel can change the optical, magnetic, and electrical response [1]. MgFe2O4 is an inverse spinel; when having a nanoscale particle size, it presents an inversion degree [2, 3]. This material has shown a variety of potential applications in the areas of science and engineering, such as supercapacitor manufacturing, microwave devices, humidity sensors, electromagnets, hyperthermia, and antimicrobial response [4–9]. Normally these applications have a direct relationship with the morphology and characteristic structure of the material, i.e., when a few microns or nano-size range particles of MgFe2O4 are obtained, the magnetic behavior increases considerably; this can be related to a superparamagnetic behavior of each particle generating a fast response to applied magnetic fields with negligible coercivity (the field required to bring the magnetization to zero) and remanence (residual magnetism). These features make superparamagnetic nanoparticles very attractive for a broad range of biomedical applications such as hyperthermia [10, 11]. The key to changing or controlling these properties is the synthesis path. Various methods of synthesis, each one with its advantages and disadvantages, have been used to obtain magnetic materials, such as solvothermal and hydrothermal routes [12, 13], coprecipitation routes [14, 15], solution combustion [16, 17] and specific methods such as Stöber method [18] among others. The selection of the most suitable synthesis route depends on the requirements for synthesized materials. In the last decades, the solution–combustion route has been used to obtain nanomaterials of different oxides [19–21]. This route is mainly a highly exothermic oxidation–reduction reaction in which oxidizing agents (precursors of the oxide to be obtained generally nitrates) and one or more fuels are used [21]. Some authors report a relationship between the fuel used and the particle size obtained. Specifically, for the synthesis of MgFe2O4 nanoparticles, sizes between 50 and 100 nm were obtained using urea [22], sizes between 25 to 41 nm using citric acid [23], 60 nm using maleic anhydride as well as using absolute alcohol, 31 nm with EDTA [24], and 57 nm with glycine [16]. Likewise, diverse morphologies highlighting amorphous and cavernous structures and spheres joined by necks have been shown. In term of synthesis of MgFe2O4, the fuel type directly alters the reaction temperature, which has a very important effect on how Mg+2 ions are located in the ferrite’s internal structure, thus affecting the material’s magnetic response [1]. Using 6-aminohexanoic acid as fuel has the advantage of minimizing the reaction temperature and avoiding steps in the processing. The main attraction of this process is its ability to obtain nano- and micro-structured materials through simple experimental assemblies [3]. One of the unique characteristic properties of the materials obtained by this route is the particle size, which is usually found in micro and nanometric scales beside their high state of aggregation due to high temperatures and release of gases like CO2, N2, H2O vapor, generating necks between particles and porous structures [25]. However, for some applications, fine and stable particles with narrow size distribution are necessary. Thus, it is important to modify the synthesis route in such a way that dispersed and smaller particles are obtained. To achieve this objective, the use of salts can be an alternative, as it has been reported in the formation of dispersed powders of Ce2O [26], LaMnO3 [27], ZnFe2O4 [28], YFeO3 [29], MnFe2O4 [30], hydroxyapatite and chlorapatite [31], MgAl2O4 [32], Ni/NiO [33], ZnFe2O4/ZnS [34], Fe2O3 [35], and NiFe2O4 [36]. These salts cover the surface of all the particles and prevent the formation of necks between them, thus avoiding the generation of cavities and modified particles with improved properties.

This paper evaluates the influence of the concentration of NaCl on the magnetic properties of MgFe2O4 nanostructures obtained in one step using solution combustion synthesis. We characterize the synthesized materials microscopically and structurally and evaluate the superparamagnetic properties and the agglomeration reduction typical in traditional combustion reactions.

2 EXPERIMENTAL

2.1 Synthesis of MgFe2O4 Nanoparticles

The starting reagents used for the synthesis of magnesium ferrites by the solution combustion route assisted by molten salts were iron nitrate nonahydrate 98% (Fe(NO3)3·9H2O, PanReac), magnesium nitrate hexahydrate 98% (Mg(NO3)2·6H2O, Alfa Aesar) as oxidants, 6-aminohexanoic acid 99% (C6H13NO2, Merck) as fuel, sodium chloride 99.5% (NaCl, Carlo Erba). The fuel/oxidant ratio used in the synthesis (parameter Φ) was set to 0.8 [37]. Equation 1 shows the combustion reaction as a function of the content of salt σ (σ = 0, 1.7, and 3.4 mol).

$$\begin{gathered} {\text{Mg}}{{\left( {{\text{N}}{{{\text{O}}}_{{\text{3}}}}} \right)}_{2}}\cdot6{{{\text{H}}}_{{\text{2}}}}{\text{O + 2Fe}}{{\left( {{\text{N}}{{{\text{O}}}_{{\text{3}}}}} \right)}_{3}}\cdot9{{{\text{H}}}_{{\text{2}}}}{\text{O }} \\ {\text{ + 1}}{\text{.5}}{{{\text{C}}}_{{\text{6}}}}{{{\text{H}}}_{{{\text{13}}}}}{\text{N}}{{{\text{O}}}_{{\text{2}}}}{\text{ + 2}}{\text{.5}}{{{\text{O}}}_{{\text{2}}}} + {{\sigma \text{salt}}} \to {\text{MgF}}{{{\text{e}}}_{{\text{2}}}}{{{\text{O}}}_{{\text{4}}}} \\ + \,\,9.1{\text{C}}{{{\text{O}}}_{{\text{2}}}}\left( {\text{g}} \right) + \,\,4.8{{{\text{N}}}_{{\text{2}}}}\left( {\text{g}} \right) + 33.9{{{\text{H}}}_{{\text{2}}}}{\text{O}}\left( {\text{g}} \right) + \sigma {\text{ salt,}} \\ \end{gathered}$$
(1)

Initially, the precursors of magnesium and iron were mixed in an aqueous solution using the minimum amount of water needed to dilute the nitrates. Subsequently, 6-aminohexanoic acid and sodium chloride was added to the solution to achieve a final NaCl concentration of 0, 1.7, and 3.4 mol, respectively. Once the system was solubilized, it was subjected to constant agitation at 350 rpm and a temperature of 180°C until the formation of a gel. At this point, the magnetic agitation was interrupted, and the temperature increased to approximately 250°C followed by the combustion reaction, as shown in Fig. 1. Finally, the powders were recovered and macerated before a final step to remove excess salt used in the reaction by several powder washes, including filtration using a vacuum pump and filter paper.

Fig. 1.
figure 1

Photographs of the combustion reaction in the present of different concentrations of NaCl: (a) 0 mol of salt, (b) 1.7 mol of salt, and (c) 3.4 mol of salt. The presence of salt affects the generation of flame and gases during the combustion reaction.

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2.2 Characterization

The crystalline phases of the powders obtained were evaluated by X-ray diffraction (XRD) analysis using a D8 Advance Eco Bruker model diffractometer over the 2θ range between 20° and 80°, step size of 0.02° (2θ) and an accumulated counting time of 0.2 s (Cu Kα radiation, λ = 0.1541 nm). Raman spectra were obtained at room temperature using a JASCO NRS-3300 spectrometer. A 532 nm Ar ion laser was used as the excitation source, and its power was kept at 10.5 mW. The scanning was done from 100 to 850 cm–1. The morphology and dispersion grade were analyzed by scanning electron microscopy (SEM, EVO MA10 Carl Zeiss microscope) and transmission electron microscopy (TEM, Tecnai F20 Super Twin TMP de FEI). To confirm the superparamagnetic behavior at temperatures above 300 K, hysteresis loops were recorded using a vibrating sample magnetometer (7400-S LakeShore 7400-S Series). The respective parameters such as the saturation magnetization Ms, remanence Mr, coercivity Hc, and squareness ratio SQR were extracted from the hysteresis loops.

3 RESULTS AND DISCUSSION

An ultrafine powder of reddish-brown color with a clear magnetic response was obtained by solution combustion synthesis. Figure 1 shows the moment when the combustion process occurs.

As the salt ratio increases, the general behavior of the reaction changes. Without salt, a flame of high intensity is formed with the minor release of gases. On the other hand, as the proportion of salt increases, the flame’s intensity decreases, which could be associated with the change in the released gases that modifies the atmosphere during the reaction, i.e., the gaseous decomposition products are a mixture of nitrogen oxides, NH3, and HNCO. These gases are known to be hypergolic in contact with each other once they attain a critical density and the required temperature, they burn with a flame [25, 38]. Furthermore, the flame temperature of the reaction can be estimated, for which a theoretical analysis can be carried out. The theoretical calculations based on thermodynamic consideration help to estimate flame temperature. The adiabatic temperature Tad of the reactions as a function of salt content is calculated by Eq. (2) [25, 28]:

$$Q = - \Delta {{H}^{0}} = {{n}_{{{\text{salt}}}}}\Delta {{H}_{{\text{m}}}} + \mathop \smallint \limits_{298}^T \sum {{\left( {n{{C}_{{\text{p}}}}} \right)}_{{{\text{products + salt}}}}}dT,$$
(2)

where Q is the heat absorbed by the products and the added salt under adiabatic conditions; nsalt and ΔHm are the molar number and the melting enthalpy of the added salt, respectively; T is the adiabatic flame temperature of the combustion reaction system; Cp is the heat capacity of the products and salt at constant pressure; and ΔH° is the standard enthalpy of formation expressed by Eq. (3):

$$\Delta H_{{298}}^{0} = \sum \Delta H_{{298}}^{0}\left( {{\text{products}}} \right) - \sum \Delta H_{{298}}^{0}\left( {{\text{reagents}}} \right).$$
(3)

Using the thermodynamics data provided in Table 1 and obtained from Eqs. (1)–(3), the adiabatic temperatures for the salt variation σ (0, 1.7, and 3.4 mol) were calculated as 1991, 1889, and 1797 K, respectively. This confirms that as the salt content increases, the combustion temperature decreases. This effect can be explained by the fact that the salt absorbs large reaction of the released heat [35]. Even so, the estimated temperatures may be sufficient to obtain MgFe2O4 in one step [39, 40].

Table 1.   Thermodynamic data for the materials in the reaction process
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Figure 2 shows the X-ray diffractograms for the powders synthesized with the different molar relations of NaCl (0, 1.7, and 3.4 mol). In the combustion process without salt, we detected the two main phases: magnesium ferrite, indexing with the JCPDS card no. 00-036-0398, and a non-stoichiometric phase of magnesium ferrite, indexing with the JCPDS card no. 01-077-2366. When the content of salt increases, the non-stoichiometric phase vanishes and remains in the magnesium ferrite phase. The increment in the salt proportion affects the broadening of peaks and their width, indicating the decrease in the particle size [41]. Using the Scherrer equation to provide approximate results, the crystallite size of the powder formed by 0, 1.7, and 3.4 mol of NaCl salt was estimated at 21.07, 5.90, and 6.48 nm, respectively, indicating an apparent effect on the size of the particles obtained. This can be associated with the presence of salt. Similar effect was found in synthesis of ZnFe2O4 using NaCl and KCl salts: increase in salt concentration resulted in decrease in the temperature of the combustion reaction [42]. These results corroborated that the temperatures reached during the salt-assisted combustion allowed the formation of the magnesium ferrite structures in a one-step reaction.

Fig. 2.
figure 2

X-ray diffractograms of MgFe2O4 obtained by the molten salt-assisted combustion method: (a) 0 mol of salt, (b) 1.7 mol of salt, and (c) 3.4 mol of salt.

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The spinel phase formation in the nanocrystalline MgFe2O4 samples is supported by the Raman spectra recorded at room temperature (Fig. 3). Five Raman active modes are expected for the MgFe2O4 (A1g + Eg + 3F2g). The peak around 660–720 cm–1 is attributed to A1g due to symmetric stretching of oxygen atoms and metal–oxygen bonds in the tetrahedral coordination AO4. In comparison, modes below 600 cm–1 belong to the motion of the oxygen atoms in octahedral BO6. It is possible to observe splitting of the peak corresponding to A1g mode; some authors correlate this behavior to the order–disorder effect of two metal ions (Fe3+ and Mg2+) over the octahedral and tetrahedral sites that exist in the spinel [7, 43]. However, other authors suggest that this effect is related to the substitution of Fe3+ ions by Mg2+ ions in the tetrahedral sites and the difference between the mass of Fe3+ and Mg2+ splits the A1g mode into two branches. The bands observed in the region of 460–640 cm−1 were reported to be caused by the predominant stretching vibration associated with the octahedral Fe3+O6 sublattice [44, 45]. Despite this inconsistency, all authors agree that the highest-frequency Raman modes are associated with the tetrahedral sublattice [45]. In the present study we observed that the inclusion of salt in the combustion reaction considerably affects either the order–disorder effect or the degree of substitution of Mg ions.

Fig. 3.
figure 3

Raman scattering spectra of MgFe2O4 obtained by the molten salt-assisted combustion method: (a) 0 mol of salt, (b) 1.7 mol of salt, and (c) 3.4 mol of salt.

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Scanning electron microscopy (SEM) micrographs were performed to obtain information on the morphology of MgFe2O4 powders synthesized by the solution combustion route with variation in the salt content. In the structures formed by the combustion reaction in the absence of NaCl (Figs. 4a–4d), the formation of highly aggregated particles representing necks, pores, and cavities was observed. This is mainly due to the accumulation and posterior liberation of typical gases at high temperatures. Once the salt is added, a drastic change in morphology is observed in the synthesized material. Even with only a 1.7 mol salt concentration (Figs. 4e–4h), the absence of pores and cavities can be observed in the structure of the material. Similar behavior was revealed in [27, 30, 32]: gases generated by combustion cannot be lodged internally in the internal structure of the materials present, which forces them to leave the combustion chamber. This agrees with Fig. 1c, in which it is observed that as more salt, less flame is generated, and more gases release. Nevertheless, we observed certain degree of aggregation, which are composed of smaller solid particles. This effect can be enhanced by increasing salt concentration to 3.4 mol (Figs. 4i–4l). In this case, low aggregation of the particles and their decrease in size was detected. In addition, individual particles can form more extensive complexes. Different authors proposed the possible formation mechanism of well-dispersed nanoparticles produced using the solution combustion approach. Zhang et al. [46] suggested that this process of good dispersion can be associated with the evaporation of the solvent and subsequent precipitation of the salt before combustion protecting the particle formation inside the solvent of the surroundings in such a way that a crust or salt matrix is formed that covers the complex or crystallite that later in the process of combustion does not have contact with other crystallites of the material avoiding the formation of pre-sintering necks typical of combustion processes. Along with a similar mechanism reported by Abbasian et al. [36], the authors found out that once the combustion reaction ends, the salt-coated MgFe2O4 nanoparticles are trapped into the salt matrix, which prevents the re-agglomeration of the newly formed crystallites and stabilizes the derived nanoparticles. By dissolving the salt and applying washing procedures the agglomerates can be fragmented into smaller particles.

Fig. 4.
figure 4

SEM images of MgFe2O4 materials at different scales. Each column represents salt content: (a)–(d) without salt; (e)–(h) 1.7 mol; and (i)–(l) 3.4 mol.

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Figure 5 shows TEM micrographics of MgFe2O4. As shown in Figs. 5a and 5b, inhomogeneous nanoparticles with an irregular shape and agglomerates can be observed in the processes without salt. When the salt (1.7 mol) was added (Figs. 5c–5f), the morphology of particles changed noticeably to quasi-spherical and well–dispersed. These results coincide with the decrease in crystallite observed in the X-ray diffraction and SEM analysis. During combustion, the melted salts can deposit between the crystallites, preventing the sintering process and avoiding the formation of the agglomerate.

Fig. 5.
figure 5

TEM images of MgFe2O4 at different scales. Each row represents salt content: (a, b) without salt; (c, d) 1.7 mol; and (e, f) 3.4 mol.

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Magnetization curves of the samples obtained as a result of VSM measurements are shown in Fig. 6. The measured hysteresis loop is typical of soft magnetic material. As can be seen, the addition of salt affects the magnetic properties of synthesized material. The respective magnetization, remanence, and coercivity values were derived from Fig. 6 and shown in Fig. 7. These parameters decrease with increasing salt content, thus suggesting superparamagnetic behavior of nanostructures and single-domain crystals. The value of magnetization for MgFe2O4 ferrite nanocrystals obtained from combustion synthesis without salt was 23.83 emu/g which is similar to those obtained by other authors using other chemical syntheses [47]. With increasing the salt content in the synthesis from 1.7 to 3.4 mol, the value of magnetization decreased from 9.55 to 6.68 emu/g. This decrease in magnetization could be associated with different mechanisms such as a magnetically dead layer on the surface of the particles, spin glass or grain growth, and A–B exchange interactions [44].

Fig. 6.
figure 6

Magnetic hysteresis loops for salt-assisted synthesized powders in (a) 0, (b) 1.7, and (c) 3.4 mol of salt.

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Fig. 7.
figure 7

Effect of NaCl concentration on magnetization, remanence, and coercivity of MgFe2O4 nanomaterials.

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The respective squareness ratio Mr/Ms for the as-prepared powders was calculated based on Fig. 7. This relation allows one to measure the ease with which the direction of magnetization is reoriented towards the nearest axis after removing the field. The values obtained for all powders are 0.27 without salt, 0.27 with 1.7 mol of salt, and 0.07 with 3.4 mol of salt, which indicates that they are soft magnets with superparamagnetism when mol of salt is 3.4 with single magnetic domain particles.

CONCLUSIONS

In this work, we successfully synthesized nanocrystalline MgFe2O4 powder by the salt-assisted solution combustion method. We revealed the effect of increasing concentration of sodium chloride on morphological and structural properties, as well as magnetic behavior of the synthesized materials. Thermodynamic considerations show that calculated values of adiabatic flame decrease with increase in salt content. XRD results revealed that the salt content influences the formation of single-phase MgFe2O4 beside the size particle of nanocrystalline. Analysis of Raman spectra confirmed the formation of the spinel structure. In turn, SEM and TEM images revealed that the aggregation grade of magnetic ferrite particles diminishes with increasing NaCl content. Hysteresis curves obtained from magnetic measurements indicated the soft nature of the prepared nanoferrites. The increase in the salt concentration decreases the saturation magnetization, coercivity field, and magnetization remanence. This suggests that a superparamagnetic-like behavior was induced, perhaps driven by the nanoparticle morphology changes. Moreover, we demonstrated that the salt content is a crucial parameter for controlling the properties of the magnetic nanoparticles obtained by the solution combustion route.