1 Introduction

Strontium hexaferrite belongs to a hexagonal M-type magnetoplumbite structure MFe12O19, where M = Sr, Ba, Pb, and space group P6 3/mmc ferrimagnetic material with easy magnetization in the direction of c-axis [1,2,3]. M-type hexaferrites have attracted considerable attention because of chemically stability, low production price, high Curie temperature (720 K), magnetocrystalline anisotropy along c-axis, high-saturation magnetization and high coercivity, due to these beneficial properties, the M-type hexaferrites have been widely used in the permanent magnetic material, high-density magnetic recording media, sensors, telecommunication, microwave devices, ceramic magnets in loud speakers, and the rotors in small DC motors [1,2,3,4,5]. Strontium hexaferrite nanoparticles can be prepared by several preparation techniques including the solid-state method, ball milling, hydrothermal method, co-precipitation techniques, the traditional sol-gel process, and sol-gel auto-combustion method [2]. Among these different methods, the sol–gel auto-combustion method has attracted considerable attention because of its relatively simple synthesis program [6].

In order to improve the inherent magnetic properties of M-type hexaferrite, and to make these materials suitable for different applications, many researchers have focused on modifying the magnetic properties of strontium ferrite by the substitution Fe3+ ions with other cations or cation combinations such as Co2+, Ni 2+, Al3+, Zr–Mn, Mg–Zr, and Pr–Ni [7,8,9,10,11]. In the present study, the substitution of chromium trivalent ion (Cr3+) with (x = 0, 0.25, 0.5, 0.75, 1) contents on the Fe site of SrFe12O19 compound, which were prepared by sol–gel auto-combustion method have been investigated. The main aim of this study is to synthesize chromium-substituted hexaferrite nanosize materials by a suitable methodology and to evaluate the possible changes in its structure and magnetic properties.

2 Experimental Details

2.1 Materials

Strontium nitrate anhydrous (Sr(NO3)2), iron (III) nitrate nonahydrate (Fe(NO3) 3.9H 2O) and chromium (III) nitrate nonahydrate (Cr(NO3)3.9H 2O) as inorganic reactants; citric acid (C6 H 8O7) as complexing agent, ammonium hydroxide as a neutralizing agent, and double-distilled water as solvent were used to prepare Cr-substituted strontium hexaferrite nanoparticles by sol-gel auto-combustion method.

2.2 Sample Synthesis

In this study, SrFe12−xCrxO19 (x = 0 − 1) M-type hexagonal ferrites were prepared by sol-gel auto-combustion method. The aqueous sol was prepared by dissolving Fe(NO3)3.9H 2O (Merck 99%), Sr(NO3)2 (Merck 99%), and Cr(NO3)3.9H 2O (Merck 99%), according to appropriate stoichiometric proportion, in double-distilled water and then mixed together by constant stirring on a hotplate at 50 ∘C. Then, citric acid was added to the solution (citric acid to total metal ions molar ratio of 1:1) for the enhancement of nitrates dissolution and subsequently stirred for 45 min at 70 ∘C. Afterward, the pH of the final solution was adjusted to about 7 by adding ammonium. The dark brown solution were stirred and heated for several hours at 80 ∘C until the sol turned into a dried gel. The obtained gel was heated at 250 ∘C for 1 h to eliminate the residual organic compound. The combustion reaction equation of the gels can be described simply as the following:

$$\begin{array}{@{}rcl@{}} &&{\kern-.6pc}\text{Sr}(\text{NO}_{3})_{2} +\mathrm{x}\,\mathrm{C}\mathrm{r}(\text{NO}_{3})_{3} . 9\mathrm{H}_{2} \mathrm{O}+(12-\mathrm{x})\,\text{Fe}(\text{NO}_{3})_{3}. 9\mathrm{H}_{2}\text{O}\\ &&{\kern-.8pc}+\,\mathrm{y}\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{7} +\mathrm{z}\,\text{NH}_{4} \text{OH}+\mathrm{H}_{2}\mathrm{O}\xrightarrow{~~~80^{\circ}\mathrm{C}~~~} \text{Sol--gels}\\ &&{\kern-.6pc}\text{Sol--gels}{\kern-.5pt} \xrightarrow{\text{~~~Air ignition~~~}}{\kern-.5pt}\text{SrFe}_{12-\mathrm{x}}\mathrm{C}\mathrm{r}_{\mathrm{x}}\mathrm{O}_{19}{\kern-.5pt}+{\kern-.5pt}\mathrm{m}\,\text{Co}_{2}{\kern-.5pt}+{\kern-.5pt}\mathrm{n}\,\text{NO}{\kern-.5pt}+{\kern-.5pt}\mathrm{p}\,\mathrm{H}_{2}\mathrm{O} \end{array} $$

Finally, the as-burnt powder was ground and then calcined at 1000 ∘C in air for 5 h with a heating rate of 5 ∘C/min to remove any organic residue and to form the hexaferrite phase. A schematic diagram of the synthesis process is showed in Fig. 1.

Fig. 1
figure 1

Schematic diagram of synthesis process

Full size image

2.3 Powders Characterization

Fourier transform infrared (FT-IR) spectra of powders (as-burnt and calcined) were recorded by a Shimadzu FTIR 4300 spectrometer in a wavenumber range of 400–4000 cm−1. The crystallographic properties of the powders were characterized by the D8 Advance (Bruker) X-ray diffractometer with Cu–Kα radiation (the wavelength λ = 1.54056 Å) and the 2 𝜃 range from 25 ∘ to 75 ∘. Transmission electron microscopy (TEM) images were obtained by using an LEO 912 AB system operating at 120 kV. The magnetic properties were measured by a vibrating sample magnetometer (VSM, Lake Shore 7404) with a maximum field strength of 20 kOe. All the measurements were performed at room temperature.

3 Results and Discussion

3.1 FT-IR Analysis

The FT-IR spectra of the as-burnt and calcined samples at 1000 ∘C for 5 h in the range of 400–4000 cm−1, are shown in Fig. 2. As can be seen, for the as-burnt sample the characteristics of absorption of C–N = O are at wave number 562 cm−1, C–C at 874 cm−1, C–O at 1104 cm−1, C–H at 1444 cm−1, C = O at 1617 cm−1, and O–H at 3425 cm−1, respectively [12, 13]. Moreover, for calcined samples, three characteristics of vibration frequencies in the range of 440–450, 550–560, and 600–625 cm−1 appear. These frequency modes, which correspond to the formation of octahedral (440–450 cm−1) and tetrahedral (550–560, 600–625 cm−1) clusters, confirm the presence of metal-oxygen (M–O) stretching bands in strontium hexaferrite [14,15,16]. In addition, due to the shorter bond length of tetrahedral clusters, these vibrational modes are higher than that of octahedral clusters [16]. There is also a minor absorption peak around 1630 cm−1, which was due to symmetric and asymmetric vibrations of the coordinated carboxylate groups [17]. Furthermore, the absorption peak at the range of 3418–3430 cm−1 is associated with the hydroxyl of bonds (−OH) [17]. It can be also observed that with the increase of Cr3+ ion content (i.e., x), the value of FT-IR characteristic frequencies for SrFe12−xCrxO19 samples were shifted to a higher wavenumber due to Cr3+ ions with a lighter atomic weight than Fe3+ ions and the wavenumber being inversely proportional to the atomic weight [7]. According to the mentioned results, it can be concluded that with the calcination of as-burnt powders at 1000 ∘C, all of the organic compounds were completely eliminated; consequently, bonds between metals and oxygen formed, which resulted in formation of crystal structure. From the comparison of the un-doped and doped samples, it is observed that chromium substitution in the structure of strontium hexaferrite does not have a significant effect on the bonding position.

Fig. 2
figure 2

FT-IR patterns of SrFe12−xCrxO19 (x = 0–1) calcined powders together with as-burnt SrFe12O19 sample

Full size image

3.2 XRD Analysis

X-ray diffraction patterns of SrFe12−xCrxO19, (x = 0, 0.25, 0.5, 0.75, 1) nanoparticles calcined at 1000 ∘C for 5 h in air are shown in Fig. 3. The main diffraction peaks related to strontium M-type hexaferrite were appeared at 2 𝜃 of 30.36 ∘ , 32.32 ∘, 34.17 ∘, 37.13 ∘, 40.34 ∘ , 55.18 ∘, 56.73 ∘, and 63.13 ∘. These diffraction planes were respectively ascribed to (110), (107), (114), (203), (205), (217), (2011), and (220) [18, 19]. All of these diffraction planes are relevant to the diffraction peaks of M-type hexaferrite (JCPDS card (#33-1340)). Furthermore, an additional peaks corresponding to extra phase of α-Fe2O3 also appeared [18, 19]. The appearance of a second phase of α-Fe2O3 probably due to the fact that the calcination temperature was low or the calcination time was too short. Furthermore, It is reported that the single phase of pure M-type hexaferrites can be obtained at higher sintering temperatures >1000 ∘C [19, 20].

Fig. 3
figure 3

XRD patterns of SrFe12−xCrxO19 (x = 0–1) powders

Full size image

As can be seen in the X-ray diffraction patterns, the intensity of α-Fe2O3 peaks increased with increase in the concentration of chromium content. It means that the substitution of Fe3+ ions by Cr3+ ions in the M-type hexagonal lattice is imperfect [19]. The average crystallites size (D) of the samples were calculated from X-ray line broadening of the reflection of (114), (107), and (203) by using Debye–Scherrer’s equation and were found to be in the range of 35 to 48 nm. In addition, The lattice parameters (a and c), cell volume (v), and X-ray density (ρ X−ray) have been calculated from the XRD data using the following equations:

$$ D=\frac{0.89\lambda} {\beta \cos \theta} , $$
(1)
$$ \frac{1}{d^{2}}=\frac{4}{3}\left( {\frac{h^{2}+hk+k^{2}}{a^{2}}} \right)+\frac{l^{2}}{c^{2}}, $$
(2)
$$ V=\frac{\sqrt 3} {2}a^{2}c, $$
(3)
$$ \rho_{\text{X-ray}} =\frac{ZM}{N_{A} V}, $$
(4)

where λ is the wavelength of the X-ray radiation, 𝜃 is the diffraction angle, and β is the full width at half maximum (FWHM)), hkl are the corresponding indices of each line in the pattern, Z is the number of formula units in a unit cell, M is the molecular mass of the sample, and N A is the Avogadro’s number [7, 19, 20]. The obtained results are listed in Table 1.

Table 1 The structural parameters of SrFe12−xCrxO19 (x = 0–1) samples
Full size table

It can be seen (4) that the value of X-ray density (ρ X-ray) directly depends on the molecular weight of the sample and cell volume. The value of X-ray density slightly increases from 5.15 to 5.20 g cm −3 with the increase in concentration of the Cr3+ ion (Table 1), which is due to insignificant decrease in the cell volume of the respective samples [21]. As can be seen, the lattice parameters (a and c) decrease with increasing x value. As we know, the radius of Cr3+ ions (0.63 Å) is almost equal to that of radius of Fe3+ ions (0.64 Å) [22]. Therefore, the change in the lattice parameters is not due to the effect of size but might be due to the change of exchange energy after Cr3+ ion substitution [22]; however, the detailed mechanism needs to be further studied. From the results of Table 1, it is observed that the “c/a” values were calculated for the SrFe12−xCrxO19 (x = 0 − 1) samples, ranged from 3.914 to 3.924.

3.3 TEM Analysis

Figure 4 shows the TEM images of SrFe12−xCrxO19 (x = 0, 1) particles prepared by sol–gel auto-combustion method. As it is observed from Fig. 4, the average grain size of all the substituted hexaferrites is almost similar, and they adhere each other due to their magnetic attraction between the particles [23]. These nanoparticles are hexagonal platelet-like crystals which are also observed for BaFe12O19 and PbFe12O19 hexaferrites and the average size of the SrFe12−xCrxO19 (x = 0, 1) particles is in the range 50–65 nm.

Fig. 4
figure 4

TEM images of SrFe12−xCrxO19 nanoparticles a x = 0 and b x = 1

Full size image

3.4 Magnetic Analysis

The hysteresis loops for SrFe12−xCrxO19 (x = 0 − 1) which were measured at room temperature are shown in Fig. 5. It has been reported that the width and the shape of the hysteresis loop depend on different parameters such as chemical composition, cation distribution, grain size, and porosity [24]. From Fig. 5, it is observed that the magnetization did not saturate because the maximum magnetic field (20 kOe) of VSM used in the present study are unable to saturate the samples. Therefore, the saturation magnetization was determined from the extrapolation of the plot M versus 1/H [7]. The obtained results were presented in Table 2.

Fig. 5
figure 5

Hysteresis loops of SrFe12−xCrxO19 (x = 0–1) powders

Full size image
Table 2 The magnetic parameters of SrFe12−xCrxO19 (x = 0 − 1) samples
Full size table

The values of saturation magnetization (M s ), remanence (M r ), squareness ratio (M r / M s ), and coercivity (H c ) were calculated from the hysteresis loops and listed in Table 2. As can be seen (Fig. 6), the saturation magnetization at first slightly increased and reaches a maximum value at x = 0.25; which express that a small amount of the chromium doping leads to the increase of saturation magnetization. This increase probably due to the fact that the magnetic moment of Cr3+ (3 μB) ion is smaller than that of Fe3+ (5 μB) ion [22, 25]. It is well known that in a unit cell of strontium hexaferrite, the Fe3+ ions are distributed in five different sublattice sites: (2a, 12k, and 2b) sites having spin up direction, and (4f 1 and 4f 2) sites having spin down [7]. When substitution occurred (x ≤ 0.25) the number of Cr3+ ions entering 4f 2 site (spin-down) is more than that of in 12k and 2a sites (spin-up) [22, 25]; therefore, the saturation magnetization increased while for x > 0.25; the saturation magnetization decreased continuously. These changes may be due to the two reasons: the first one is due to the appearance of non-magnetic phase α-Fe2O3 and the second one may be also due to the fact that at high doping content (x > 0.25), Cr3+ ion substitutes Fe3+ ion in 12k and 2a sites having spin-up [22]. Therefore, as mentioned above, the saturation magnetization decreased because chromium saturation magnetization is less than iron magnetization [25]. On the other hand, the value of coercivity at first, decreases rapidly and reaches a minimum at x = 0.5; then, H c increases slightly (Fig. 6). It has been reported that the coercivity is affected by important parameters such as particle size, ion substitution, morphology, interface structure, crystal defects, magnetocrystalline anisotropy, and strain [26]. However, among these parameters, the ion substitution and particle size are undoubtedly two of the most important parameters [26]. In this research, the decrease of coercivity with increase in Cr3+ concentration up to x = 0.5 may be due to decrease in magnetocrystalline anisotropy. Furthermore, the increase of H c at the high doping content may be due to the appearance of non-magnetic phase α-Fe2O3 [22]. The squareness ratio (SQR) is calculated from the magnetic data and their values are listed in Table 2. It has been reported that large squareness ratio is favored in many applications such as the magnetic recording media and permanent magnets [7]. In the present study, the squareness ratios are found in the range of 0.422–0.451, indicating that materials are in single magnetic domain.

Fig. 6
figure 6

H c and M s of SrFe12−xCrxO19 samples as a function of x

Full size image

4 Conclusion

The sol–gel auto-combustion method has been successfully employed for the synthesis of chromium-substituted strontium hexaferrite nanomaterials. The X-ray diffraction patterns of samples confirmed the formation of hexagonal magnetoplumbite phase with the space group of P6 3/mmc. However, the small amounts of impurity phases (α-Fe2O3) also appeared. The FT-IR spectra of samples after calcination process showed three absorption peaks in the range of 440–450, 550–560, and 600–625 cm−1 originated from the metal oxygen (M–O) stretching band in strontium hexaferrite. The TEM images showed nearly hexagonal platelet shape for strontium ferrite nanoparticles. The magnetic properties including M s , M r , SQR, and H c were investigated. The saturation magnetization of chromium-substituted Sr hexaferrite at first increases slightly and then decreases continuously due to the appearance of non-magnetic phase α-Fe2O3. The coercivity of the samples at first, decreases rapidly and reaches a minimum at x = 0.5, then increases slightly.