1 Introduction

Spinel ferrites (MIIFe2O4, MII = Co, Fe, Ni, etc.) are a class of mixed metal oxides that offer a great variety of applications. Among them, cobalt ferrite (CoFe2O4) occupies a representative place. CoFe2O4 is a hard magnetic material with remarkable properties such as: high magnetic coercivity, moderate saturation magnetization, a high Curie temperature (793 K), strong magnetocrystalline anisotropy, high chemical and structural stability. Cobalt ferrite nanoparticles are used in electronic devices (energy storage, microwave devices, sensors and biosensors, recording media), for the removal of toxic pollutants from water, in biomedical applications for drug delivery and hyperthermia [1,2,3]. Recent studies revealed the antibacterial properties of the cobalt ferrite nanoparticles on multidrug resistant bacterial strains [4]. For biomedical applications, it is required to modify the surface of the cobalt ferrite nanoparticles by various bioactive and biocompatible molecules [5]. These biomolecules increase the specificity toward cellular targets, prevent the agglomeration of the particles and also decrease the cytotoxicity of these particles [6]. The most commonly used biomolecules are the natural and synthetic polymers [7,8,9], the aminoacids [10], the polyhydroxycarboxylic acids and the bile acids [11].

Nanostructured cobalt ferrite has been the subject of countless papers for the last few decades. The study of their synthesis methods, properties and their practical applications is of great interest. Several factors such as particle size, shape and porosity that are closely related to the synthesis method, have significant impact on the cobalt ferrite properties [12]. Among the chemical methods, special attention is given to coprecipitation and Pechini methods [13, 14], sol–gel process [15, 16], hydrothermal/solvothermal technique [1, 17,18,19], precursor method [20], combustion method and microwave-assisted combustion method [21,22,23,24], microemulsion method [7] and polyol process [25]. Das et al. [17] reported a significant modification in the size of the particles and magnetic properties when adding the CTAB surfactant in the synthesis of CoFe2O4 nanoparticles through the hydrothermal method. Karakas [22] reported that the type of fuel (urea, glycine and citric acid) has notable influence on the structural, morphological and magnetic properties of the synthesized CoFe2O4 nanoparticles through the microwave-assisted combustion method.

Generally, the chemical methods are associated with eco-toxicity, high energy and time consumption, and high cost. One of the major challenge facing chemists and engineers is developing processing routes that use and generate nontoxic safe reagents that are eco-friendly and biosafe, rather than hazardous substances. In the last years, there is an increased interest in magnetic nanoparticles preparation via green synthesis routes. These green approaches for the synthesis of spinel ferrite nanoparticles are cost-effective and environmental-friendly alternatives to the chemical procedures.

The biological materials (e.g., bacteria, fungi, algae and plant extracts) are essential for this purpose [26]. Plants are very good candidates as biological agents. The use of plant extracts from leaves, flowers, seeds, bark, roots and fruits is efficient for the synthesis of metal/metal oxide nanoparticles [27, 28]. The bioactive compounds (carbohydrates, polysaccharides, aminoacids, phenols, flavonoids, etc.) contained in plant extracts can act as capping agents, reducing agents, stabilizing and chelating agents for “capturing” the metal ions. The literature shows that various synthesis techniques make use of plant extracts, such as Aloe vera leaves [29], Hibiscus rosa-sinensis leaves/flowers [30], sesame seeds [31], ginger root and cardamon seeds [32], tulsi seeds (Ocimum sanctum) and garlic cloves (Allium sativum) [33], cashew gum [34], to obtain nanocrystalline spinel ferrites. The rosemary-assisted synthesis of cobalt ferrite nanoparticles has been reported in a recent study [3]. Several approaches using natural sources like grape, lemon and turmeric extracts have been employed for the production of different nanostructured compounds [27, 28]. Ansari et al. [16] used mono-, di- and polisacharides in the synthesis of the cobalt ferrite nanocomposites via a new sol–gel autocombustion technique. Satheeshkumar et al. reported a green synthesis route of Ag substituted cobalt ferrite nanoparticles using honey, a natural source of glucose and fructose [35]. The literature also reported the impact of the operational parameters (extract type, concentration and volume), as well as the synthesis methodology on properties of cobalt ferrite nanoparticles.

In our previous works [20], the magnetic ferrite nanoparticles have been obtained via two synthesis techniques: the precursor method and the wet ferritization method, both methods belonging to the soft/wet chemistry methods. These two procedures are based on molecular precursors—multimetallic components that lead to a homogeneous distribution of chemical species at molecular level. To became environmentally responsive, our group focused its research on substituting some of the chemical components by natural sources in the soft chemistry methods [30]. In this study, an aqueous extract of tamarind fruit has been used as chelating agent.

Tamarind is a hardwood tree known scientifically as Tamarindus indica and belongs to the family Leguminosae (Fabaceae). The tamarind fruit is remedy in the treatment of a large number of ailments. It has antifungal and antibacterial properties [36]. The chemical composition of tamarind fruits depends on the geographical area where it is grown, but generally consists of high level of proteins, carbohydrates, minerals and B vitamins (riboflavin, niacin and thiamine) [36]. The main feature of tamarind fruit is its sweet acidic taste, due to the presence of tartaric acid and reducing sugars. Tamarind fruits are the richest natural source of tartaric acid (12.2–23.8%). Besides tartaric acid, the tamarind pulp contains citric, succinic and malic acid, many essential aminoacids and reducing sugars (glucose and fructose). These biomolecules act as chelating agents for the metal ions. The tamarind pulp is also rich in minerals: K, P, Ca, Mg [37, 38]. The use of the tamarind fruit extract in the synthesis of Ag, CoAl2O4 and CuO nanoparticles was reported in the literature [39,40,41].

In order to replace the chemical components—tartaric and malic acids—in their usage as chelating agents for tartarates/malates compounds as precursors for mixed oxides [43, 44], the green alternative chelating agents must possess high polyhydroxycarboxylic acids content and have a strong ability to form multimetallic complexes [40]. As described above, the tamarind fruit extract has these properties.

The major objective of this work was the synthesis of CoFe2O4 and tamarind/CoFe2O4 nanoparticles. The synthesis of these ferrites used the precursor method and the two-step wet chemical process, both methods belonging to soft chemistry. This work emphasized the role of the tamarind fruit as a biorenewable and sustainable resource for the production of ferrite nanoparticles. The structure, morphology and magnetic properties of the cobalt ferrite samples were investigated.

2 Experimental

Quality reagents (Merck): iron nitrate (Fe(NO3)3·9H2O) and cobalt nitrate (Co(NO3)2·6H2O) were used in the synthesis of cobalt ferrite The tamarind fruits were purchased from Golden Harvest Daily, Karnataka, India, and the ammonia solution 25% from Chimreactiv, Romania.

2.1 Tamarind fruit extract preparation

The extract was prepared according to a previously described recipe [40]: 100 ml distilled water was added to 10 g of chopped tamarind fruits. This mixture was boiled during 1 h. The red–brown mixture was cooled at room temperature and filtered. Tamarind fruit aqueous extract (pH = 2) was obtained (TE sample).

2.2 Synthesis of CoFe2O4 nanoparticles through the precursor method

An amount of iron(III) nitrate (4.04 g) and cobalt(II) nitrate (1.455 g) in molar ratio 2Fe3+:1Co2+ was dissolved in 20 ml distilled water, under stirring. After 20 min, this solution was added slowly into 100 ml aqueous tamarind fruit extract. The pH was raised to 10 by adding an ammonia solution 25%. A black precipitate was formed. The suspension was maintained under stirring at 80 °C/8 h. After 8 h, the black precipitate (multimetallic compound precursor) was separated by centrifugation and dried on P4O10 (MCP sample). Thermal treatment at 500 °C/1 h was necessary to obtain cobalt ferrite nanoparticles (CF sample).

2.3 Synthesis of tamarind/CoFe2O4 nanoparticles through the two-step wet chemical process

The first step consisted in the dissolution of the metal nitrates (4.04 g iron(III) nitrate, 1.455 g cobalt(II) nitrate representing the molar ratio 2Fe3+:1Co2+) in 20 ml distilled water. After that, an ammonia solution 25% was added until the pH rose to 10. The resulting black magnetic compound was maintained at 80 °C/8 h. The magnetic compound (CoFe2O4) was filtered and washed. In the second step, this synthesized CoFe2O4 was added into 100 ml aqueous tamarind fruit extract. The obtained suspension was maintained under stirring at 40 °C/2 h and then at room temperature for 70 h. It was filtered and dried on P4O10 (tamarind/CoFe2O4—TCF sample).

2.4 Characterization techniques

Powder X-ray diffraction patterns were recorded using Rigaku's Ultima IV multipurpose diffraction system, a Cu target tube (λ = 1.54060 Å) and a graphite (002) monochromator, with working conditions of 30 mA and 40 kV. The data were collected at room temperature between 10 and 85° in 2θ, with a 0.02° step size and a scanning rate of 2°/min. Phase identification was performed using Rigaku’s PDXL software, connected to the ICDD PDF-2 database. The lattice constants were refined using diffraction line position and the crystallite size was calculated from the diffraction line width (full width at half maximum, FWHM) using the Scherrer equation.

The microstructure of the obtained powders was studied by scanning electron microscopy (SEM) in a FEI Quanta 3D FEG apparatus operating at accelerating voltages between 2 and 20 kV, using secondary electron images. Elemental analysis measurements were taken with the same microscope with an energy-dispersive X-ray spectrometer (EDS) using an excitation electron beam at an accelerating voltage of 20 kV.

The thermal analysis TG-DSC was performed with a Netzsch STA 449C Jupiter apparatus, from room temperature up to 900 °C, with a heating rate of 10 °C/min, under a flow of 20 mL min−1 dried air.

The FTIR spectra were recorded on KBr pellets using a JASCO FTIR 4100 spectrophotometer.

The local structure was determined by a conventional Mössbauer spectrometer, using a 57Co (Rh matrix) source, mounted on a drive unit with constant acceleration. The spectra were acquired at room temperature, in transmission geometry. The NORMOS computer program [42] was used for the least-squares fitting of the Mössbauer spectra. The isomer shifts were reported relative to α-Fe at room temperature. The magnetic measurements were taken with a Superconducting Quantum Interference Device (SQUID) from Quantum Design. The hysteresis loops were acquired at 5 and 300 K under an applied magnetic field of 5 T.

3 Results and discussion

3.1 Characterization of the cobalt ferrite precursor (MCP)

The FTIR spectra of the tamarind fruit extract and precursor (MCP sample) were recorded within the range of 4000–400 cm−1. The positions of the infrared bands and their assignments are listed in Table 1. They are in good agreement with the literature data [45, 46].

Table 1 Characteristic bands of the tamarind fruit extract (TE sample) and multimetallic compound (MCP sample)
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The band located at ~ 1741 cm−1 in the spectrum of the tamarind fruit extract assigned to ν(C=O) of carboxyl groups is replaced by two bands νasym(COO)− (1616 cm−1) and νsym(COO)− (1353 cm−1) in the spectrum of the precursor (MCP sample). This suggests that the carboxyl groups from the tamarind fruit extract have been deprotonated in the alkaline reaction medium and coordinated to the metal ions, leading to the formation of a multimetallic complex [46]. The intense bands between 1900 and 900 cm−1 (1741, 1627, 1407, 1265, 1130 and 1074 cm−1) represent the “fingerprint” of the tamarind fruit.

The thermal behavior of the MCP sample by comparison with that of tamarind fruit extract was investigated (Fig. 1).

Fig. 1
figure 1

TG-DSC curves of the tamarind fruit extract (TE sample) and the precursor (MCP sample)

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In the case of the tamarind fruit extract, the first step up to 190 °C, involving a mass loss of 23.32%, was assigned to the release of the volatile fractions from tamarind and is associated with a very weak endothermic effect on DSC curve. Between 190 and 600 °C, three partial superposed processes are identified and assigned to the oxidative degradation of the organic compounds found in the extract (weight loss 63.79%). After 600 °C, there is a weight loss of 7.38% due to the oxidation of carbonaceous mass.

The thermal decomposition of the precursor (MCP sample) is different from that of the tamarind extract. In the case of the precursor, the first step, up to 120 °C, with a mass loss of 4.52%, associated with a weak endothermic peak on the DSC curve, corresponds to the loss of water molecules. Between 120 and 420 °C, a complex combustion process of the organic components associated with three exothermic effects on DSC curve takes place (weight loss 79.02%).

It can be noted that the final decomposition temperature of the precursor is lower than that of the tamarind fruit extract. This behavior supports the formation of a complex coordination compound and, also, suggests the catalytic effect of Fe3+/Co2+ cations on the thermal decomposition of the carboxylate anions [11].

The XRD powder diffraction patterns recorded on the MCP sample shows that the compound is polycrystalline, probably, due to the presence of the carboxylate anions (tartarate/malate) which coordinate to the metal ions (Fig. 2). It is well known that the tartarate/malate compounds of metal ions crystallize as two- and/or three-dimensional structures [47].

Fig. 2
figure 2

X-ray diffraction of multimetallic compound (MCP sample)

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The magnetic characterization of MCP sample is displayed in Fig. 3. Both hysteresis loops (5 and 300 K) highlight the paramagnetic behavior of this precursor that contains a large amount of organic ligand.

Fig. 3
figure 3

Magnetization vs magnetic field curves of the MCP sample measured at two different temperatures: a 5 K and b 300 K

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3.2 Characterization of the cobalt ferrite samples

Figure 4 shows the XRD patterns recorded for the CF and TCF samples. The measured XRD pattern of the sample obtained through the precursor method (CF) can be identified as a spinel-type structure with a cubic crystal system, with symmetry described by the space group Fd-3 m. The diffraction lines matched well against the diffraction data of CoFe2O4 (ICDD file no. 022-1086). Two minor unidentified diffraction lines are also present (Fig. 4a). However, similar results were obtained for the sample from the two-step wet chemical process (TCF), except that no additional diffraction lines were observed and broader and less intense diffraction lines were recorded.

Fig. 4
figure 4

X-ray diffractograms of the cobalt ferrite obtained by: a the precursor method (CF) and b the two-step wet chemical process (TCF)

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By using the Bragg’s law of cubic system (a = b = c, α = β = γ = 90°), the lattice parameter, a, was calculated by using the following equation:

$$a = d\sqrt {h^{2} + k^{2} + l^{2} }$$
(1)

where h, k and l are the Miller indices of lattice planes, a is the lattice constant and d is the lattice spacing of CoFe2O4, with multiple diffraction lines with known Miller indices. The lattice parameter a was 8.3778(15) Å for CF sample and 8.389(3) Å for TCF sample; both values are smaller than the indexed value of 8.3920 Å, recorded in the ICDD file 022–1086. These values are comparable to those reported for cobalt ferrite obtained through a green approach using garlic and tulsi extracts [33] and lower that those reported for cobalt ferrite synthesized through combustion method [24].

The average crystallite size was estimated by Scherrer formula:

$$D = \frac{0.9 \times \lambda }{{\beta_{d} \times \cos \theta }},$$
(2)

where D is the average crystallite size, λ is the incident radiation, βd is the corrected broadening, θ is the Bragg angle and it has been calculated as 13 nm for CF sample and 5 nm, respectively, for TCF sample. Comparing with the literature data, it is observed that these sizes are smaller than those of cobalt ferrite obtained through chemical or other green synthesis methods [3, 23, 24, 33].

Crystallinity is determined by the following equation, using the crystal and amorphous phase scattering intensity obtained:

$${\text{Crystallinity}} = \frac{{\text{Scattered intensity from crystalline phase}}}{{{\text{Scattered intensity from crystalline phase}} + {\text{Scattered intensity from amorphous phase}}}}$$
(3)

For the CF sample, no amorphous line can be detected; thus, the degree of crystallinity was considered 100%. For the TCF sample, the degree of crystallinity was calculated as 86.103%.

The microstructure of the CF powder (Fig. 5) studied by SEM revealed micron-sized agglomerates of fine size nanoparticles (Fig. 5a), with primary particle sizes of 10–15 nm (Fig. 5b), corresponding to single crystalline domains. This is in accordance with the XRD results. An initial stage of sintering was induced by the mild thermal treatment at 500 °C, leading to the formation of a mixture of primary nanocrystallites, secondary particles, sized ~ 50 to 100 nm that correspond to clusters of a few nanocrystallites and micron-sized polycrystalline agglomerates (Fig. 5a-b). Figure 5c is a histogram showing the particle size distribution measured from SEM images. EDS elemental chemical analysis (Fig. 5d) detects Co, Fe, O, components of the cobalt ferrite, as main elements, as well as several other minor elements, mainly K, but also Ca, P, Mg from tamarind fruits. Quantitative analysis gave an elemental composition (at.%) of 13:26:52:4:3 for Co:Fe:O:C:K, respectively, with all other minor element component contents each below 1 at.%.

Fig. 5
figure 5

a–b SEM micrographs, c grain size distribution histogram and d EDS spectrum of the CF sample

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The microstructure of the TCF powder (Fig. 6) studied by SEM revealed micron-sized hard solid blocks (Fig. 6a), which appear clearly granulated in the nanoscale, these “grains” having typical sizes of 50–150 nm (Fig. 6b), forming dense solid microblocks. Figure 6c is a histogram showing the grain size distribution measured from SEM images. EDS elemental chemical analysis (Fig. 6d) detects the elemental components of the cobalt ferrite and a significant amount of carbon. Quantitative analysis gave an elemental composition (%at.) of 11:22:40:24:2 for Co:Fe:O:C:K, respectively, with other minor elements content below 1 at.% each. According to XRD results, TCF contains very fine cobalt ferrite nanocrystallites of ~ 5 nm; therefore, the observed coarser granulation corresponds to the presence of carbon in the agglomerated granules. We can conclude from the SEM, EDS and XRD results that the presence of carbon residues in the agglomerated granules, from the organic components in dried tamarind extract, together with minor elements: K, Ca and P, present in tamarind, lead to carbon-rich particles embedding ferrite nanocrystallites.

Fig. 6
figure 6

a–b SEM micrographs, c grain size distribution histogram and d EDS spectrum of the TCF sample

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The FTIR spectra of CoFe2O4 (CF) and tamarind/CoFe2O4 (TCF) were recorded in the 4000–400 cm−1 range. In both spectra (Fig. 7), the spinel ferrite characteristic bands can be observed between 400 and 800 cm−1, at 570 cm−1 (ν1) and 418 cm−1 (ν2) and are assigned to the stretching vibrations of Fe–O bond in the tetrahedral and octahedral sites, respectively.

Fig. 7
figure 7

FTIR spectra of the cobalt ferrite samples: a CF and b TCF

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Besides the very well-defined bands characteristic of the cobalt ferrite, the FTIR spectrum of CF sample shows some weak bands characteristic to the vibration of carboxylate anions and hydroxyl groups which remain even after calcination at 500 °C/1 h.

The FTIR spectrum of TCF sample evidenced the bands characteristic of cobalt ferrite and, also, some bands assigned to the vibration of νOH of water molecules (3318 cm−1) and carboxylate anions from the fingerprint of tamarind fruit extract, in the range 1700–900 cm−1. This suggests the formation of bonds between the components of tamarind fruit extract and the metal ions present on the surface of the cobalt ferrite.

Thermal analysis of the tamarind/cobalt ferrite (TCF sample) was performed in order to confirm the binding of the carboxyl groups from tamarind extract to the cobalt ferrite (Fig. 8).

Fig. 8
figure 8

TG-DSC curves of the tamarind/cobalt ferrite (TCF)

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The first step in the process, occurring up to 120 °C, corresponds to the loss of the water molecules (weight loss 2.74%). In the range of 120–420 °C, a highly exothermic process takes place, corresponding to the oxidative degradation of the organic components of tamarind fruit extract (weight loss 47.57%). The decomposition is complete below 500 °C. The process ends with the formation of the cobalt ferrite nanoparticles (residual mass 49.69%).

In this case, the final decomposition temperature is also lower than that of the tamarind extract. The FTIR spectrum of this compound, correlated with its thermal decomposition profile, confirms the successful coating of the cobalt ferrite nanoparticles with components from tamarind extract.

The local structures of CoFe2O4 (CF) obtained by the thermal decomposition of MCP at 500 °C/1 h and of the tamarind/CoFe2O4 nanoparticles obtained by the two-step wet chemical process (TCF) are evidenced in the Mössbauer spectra (Fig. 9).

Fig. 9
figure 9

Mössbauer spectra of the cobalt ferrite samples: a CF and b TCF

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The Mössbauer spectra illustrate a main magnetic contribution consisting of a relatively broad sextet pattern which was fitted via a probability distribution of hyperfine magnetic fields, shown on the right side of each corresponding spectrum. Discrete sublattices of sextets and doublets were also found in the spectrum of the TCF sample. The spectrum of the CF sample evidences relaxation processes due to the thermal spin fluctuations, in agreement with the reduced size of the nanoparticles revealed by XRD. A wide magnetic pattern, having the isomer shift value, IS, of 0.41(± 0.01) mm/s (specific to Fe3+ ions), negligible quadrupole splitting, QS = -0.07(± 0.01) mm/s, and several large peaks in the hyperfine magnetic field (Bhf) distribution, accounts for the relaxed cobalt ferrite. The magnetic part in the spectrum of TCF consists of two magnetic sextets assigned to Fe3+ ions located in the tetrahedral (Bhf = 46.6(± 0.1) T, IS = 0.32(± 0.01) mm/s, QS≈0 mm/s) and octahedral (Bhf = 43.8(± 0.1) T, IS = 0.3(± 0.01) mm/s, QS = 0.02(± 0.02) mm/s) sites of the cobalt ferrite structure (equal contributions of about 20% of the full spectrum, in accordance with the inverse spinel structure of CoFe2O4) and a hyperfine magnetic field distribution having IS = 0.56(± 0.04) mm/s, median value of Bhf = 28.1 T (32% of the spectrum) related to relaxation mechanisms. A part of the magnetic contribution related to CoFe2O4 collapsed into a central superparamagnetic doublet with hyperfine parameters typical to Fe3+ ions (IS = 0.34(± 0.01) mm/s, QS = 0.73(± 0.01) mm/s, Γ = 0.64(± 0.01) mm/s), similar to other reports [48] and a contribution to the spectrum of 25%. A secondary phase having a contribution of only 3% related to Fe2+ ions (IS = 0.86(± 0.02) mm/s, Γ = 0.38(± 0.06) mm/s) in a very distorted configuration is also present. Magnetic relaxation phenomena are more pronounced in the spectrum of the TCF sample, relative to the CF sample, due to smaller crystallite sizes revealed by XRD results. Both spectra are similar to recent results of Palade et al. [19].

Figures 10 and 11 display the hysteresis loops for the CF and TCF samples acquired at low temperature (5 K) and room temperature (300 K), respectively, in a magnetic field of 5 T.

Fig. 10
figure 10

a, b Hysteresis loops at selected temperatures for the CF sample and a’, b’ extrapolation of magnetization (M) versus inverse of magnetic field (1/B−1) according to the “law of approach of the magnetization to saturation”

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Fig. 11
figure 11

a, b Hysteresis loops at selected temperatures for the TCF sample and a’, b’ extrapolation of magnetization (M) versus inverse of magnetic field (1/B−1) according to the “law of approach of the magnetization to saturation”

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Both samples are ferrimagnetic at 5 K and became superparamagnetic at room temperature (300 K). The saturation magnetization (Ms) was estimated using "law of approach of the magnetization to saturation” [49]:

$$M(B) = M_{s} \left( {1 - \frac{a}{B}} \right)$$
(4)

where M and Ms define the magnetization and saturation magnetization, B is the magnetic field corrected for the demagnetizing effect of the sample surface and a is a constant. According to Eq. (4), the extrapolation of magnetization (M) versus B−1 is linear and at B−1 → 0, the extrapolation is the Ms (Figs. 10a’, b’, 11a’, b’).

The Ms values obtained at 5 K were 57.20 emu/g for CF sample and 38.76 emu/g for TCF sample. At 300 K, the obtained Ms values were 52.12 emu/g for CF and 31.93 emu/g for TCF sample. The magnetic coercive field values at 5 K are 0.61 T for CF and 0.34 T for TCF (nanoparticles are in the blocked state at this temperature). The CF sample has a very small coercive magnetic field at 300 K (0.03 T) due to the presence of a small amount of impurities highlighted by X-ray diffraction (Fig. 4a) which are not superparamagnetically relaxed even at this temperature. The remanent magnetization values (Mr) at 5 K were 41.12 emu/g for CF sample and 18.51 emu/g for TCF sample. At 300 K, the CF sample has a Mr value of 26.65 emu/g explainable in the context of the presence of impurities. Increased magnetic parameters for the sample prepared via the precursor method having larger crystallite sizes relative to the sample prepared by the two-step wet chemical method are in agreement with other reports on spinel ferrite nanoparticles of tens of nanometers [50].

Table 2 comparatively summarizes the values of magnetic parameters obtained by our synthesis methods and other methods reported so far. The saturation magnetizations obtained through the precursor method and the two-step wet chemical process have significantly lower values both at low temperatures (5 K) and at 300 K than those reported in the literature (Table 2).

Table 2 A comparative study of the magnetic parameters obtained in our work and other reported synthesis methods
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The highest value of saturation magnetization for this type of material was obtained on bulk (80.8 emu/g at room temperature and 93.9 emu/g at 5 K) [52]. The decrease of 13.9 emu/g of the saturation magnetization at room temperature compared to the one recorded at 5 K in bulk materials was attributed to the variation of the electronic populations on CoFe2O4 orbitals with temperature [19]. For our samples, CF and TCF, the difference between Ms recorded at 300 K and that at 5 K is smaller than in bulk, 5.08 and 6.83 emu/g, respectively. This smaller decrease of Ms for nanoparticles compared with the bulk could be explained by the thermal fluctuations of the magnetic moment in a single energy minimum when temperature increase [19]. At low temperature, when spin orientation thermal fluctuations are negligible, the difference between the Ms of our samples and that of bulk materials can be explained by the presence of an outer layer belonging to the nanoparticles surface which has an insignificant contribution to Ms [19]. A similar trend was also found in the values of the remanent magnetizations and coercive magnetic fields, respectively. According to the literature, for spherical nanoparticles with cubic magnetocrystalline anisotropy, the squareness ratio (Mr/Ms) should be 0.83 [53]. In our study, the maximum squareness ratio of 0.71, close to the theoretical one (0.83), was obtained for CF sample at 5 K. At 300 K, the Mr/Ms ratio for the same sample was 0.51, similar to the value reported for a system of non-interacting single magnetic domain nanoparticles with uniaxial symmetry (R = 0.5) [54], suggesting a uniaxial effective anisotropy for this sample, at room temperature. For TCF, the squareness ratio at 5 K was 0.47, much smaller than for CF sample. This decrease can be attributed to surface effects that become more important as particle sizes decrease, leading to a large anisotropy energy in nanoparticles.

4 Conclusions

An aqueous extract of Tamarindus indica fruit can be used as chelating agent to obtain CoFe2O4 nanoparticles through the precursor method and the two-step wet chemical process. XRD showed that both approaches have resulted in the formation of CoFe2O4 cubic spinel structure with an average crystallite size of 13 nm (CF) and 5 nm (TCF). The values of the magnetic parameters were determined and correlated with the synthesis method and structural and morphological properties. Local structure and relaxation mechanisms were described via Mössbauer spectroscopy. Magnetic relaxation is more evident in the TCF sample due to smaller crystallite sizes. The values of saturation magnetization at 300 K are lower compared to those obtained at 5 K (e.g., 52.12 emu/g at 300 K and 57.20 emu/g at 5 K for CF sample) due to both changing of the population of electron orbitals and thermal fluctuations of the magnetic moment. Surface effects are predominant in the TCF sample due to the decrease in particle sizes. In summary, we showed that these green, plant-based methods are simple alternatives to the chemical methods. Moreover, the results obtained on the cobalt ferrite samples make them suitable for potential application in the field of magnetic materials.