1 Introduction

Magnetic materials are important electronic materials that have a wide range of industrial and commercial applications. Magnetic materials can be regarded as being indispensable in modern technology. They are components of many electromechanical and electronic devices. Magnetic materials are also used as components in a wide range of industrial and medical equipment. Permanent magnet materials are essential in devices for storing energy in a static magnetic field. Major applications involve the conversion of mechanical to electrical energy and vice versa, or the exertion of a force on soft ferromagnetic objects. The applications of magnetic materials in information technology are continuously growing. There are five categories of permanent magnets: Samarium cobalt (SmCo) alloys [1,2,3], Steels alloys (Fe alloys containing Co, Cr and W) [3], neodymium iron boron alloys (NdFeB) [4], Alnico alloys (AlNiCo) [5], and hexaferrites [3]. Due to the lower raw material cost and compositional variability than other types of materials, hexaferrites were widely used in making permanent magnets.

Growing demand for hexaferrites (M, W, X, U, Y, Z types) for daily life uses has been recorded throughout the last decades. Among them, M-type strontium-based hexaferrite (SrFe12O19) has an expanded request for different industrial, military, and home equipment applications [6, 7]. In this context, SrFe12O19 emerges exceptional features involving relatively large saturation magnetization, high coercive force, outstanding Curie temperature, pronounced magneto-crystalline anisotropy, excellent chemical and thermal stability, high dielectric constant, low conductive losses, corrosion resistance, non-toxicity, and simple accessibility [8,9,10,11].

The improvement of magnetic properties of SrFe12O19 can be monitored by controlling the distribution of strontium and iron sites as well as the synthesis strategies. Several soft synthetic routes were employed to synthesize homogeneous nanomaterials [12,13,14,15]. The tartrate precursor technique was an appropriate synthesis method to attain single-phase nanocrystalline ferrite powder with narrow size distribution, uniform shape, and homogenous microstructure at relatively low calcination temperature [16].

Due to its superior properties, SrFe12O19 is a multidisciplinary technological component indispensable for refrigerator magnets, permanent magnets, recording media, automotive industries, microwave devices, sensors, biomedicine, photocatalyst, magneto-optics devices, magnetic fluids, sound devices, electric motors, generators, etc [9, 17,18,19,20]. Consequently, enhancing the magnetic feature of strontium hexaferrite has been a theme of comprehensive examination work. Nevertheless, the substitution of Fe3+ or Sr2+ sites is an effective route to monitor the magnetic properties of strontium hexaferrite. Many previous published works are focused on the manipulation of rare earth elements, Bi3+ [19], Mn3+ [20], Co3+ [21], Cr3+ [22], or combined of divalent or trivalent ions with rare earth elements [23,24,25,26]. However, the manipulation of Li+ substitution (as alkaline earth metal) at Sr2+ sites on the structural and magnetic properties of M-type strontium ferrite is not mentioned in the literature. Therefore, in this report, the impact of Li+ ion substitution at the Sr2+ sites on the structural, microstructure, and magnetic properties was investigated in details. Li+-substituted strontium hexaferrite (Sr1−xLi2xFe12O19) was fabricated trough tartaric acid precursor approach at different ratios of Li+ (0.2–0.6) and varied temperatures (1000–1200 °C).

2 Experimental

2.1 Materials and procedure

Anhydrous iron (III) chloride FeCl3 (Sigma Aldrich), lithium chloride anhydrous LiCl (Fluka), strontium chloride hexahydrate SrCl2·6H2O (RIEDEL-DE HAEN), and tartaric acid C4H6O6 (RIEDEL-DE HAEN) exploited in this work were employed without any further purification. Flow sheet diagram for synthesis Li ion-substituted SrFe12O19 powders (Sr1−xLi2xFe12O19 where x = 0.1, 0.2 and 0.3) using tartrate precursor approach is shown in Fig. 1. The next steps were similar to the previously published works elsewhere [9]. Table 1 represents the estimated weights of starting materials applied for synthesis of hexagonal ferrite powders at different Sr:Fe:Li molar ratios.

Fig. 1
figure 1

Flow sheet diagram for synthesis of Sr1−xLi2xFe12O19using tartrate precursor approach

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Table 1 Synthesis conditions of Li-substituted strontium hexaferrite powders
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2.2 Physical properties

Crystal structure of the synthesized powders was identified by an X-ray diffractometer, model Bruker AXS (D8-ADVANCE) with Cu Kα (λ = 1.54056 Å) radiation, operating at 40 kV and 10 mA. Crystallite size, lattice parameters, aspect ratio (a/c), and unit cell volume were estimated by equations in work published by Hessien et al. [9]. Scanning electron microscopy (SEM) was employed to demonstrate the change in microstructure through surface morphology and average particle size of the synthesized ferrite. Magnetic properties of the elaborated powders were defined using a vibrating sample magnetometer (VSM, 7410-LakeShore, USA) at room temperature in a maximum applied field of 20 kOe.

3 Results and discussion

3.1 Phase development

Li+-substituted strontium hexaferrite was synthesized with the Fe:Sr:Li molar ratios of 12:0.9:0.2 and annealed at different temperatures ranging from 1000 to 1200 °C for 2 h. The XRD patterns of the produced powders indicated that all peaks were perfectly corresponding to a single well-crystallined Sr1−xLi2xFe12O19 at annealing temperatures from 1100 to 1200 °C without any other impurity species (Fig. 2). The change of the Fe:Sr:Li molar ratios to be 12:0.8:0.4 indicated the presence of diffraction peaks linked to secondary impurity α-Fe2O3 species as presented in Fig. 3. Eventually, Fig. 4 illustrates the XRD diffraction analysis profiles with Li+ ratio of 0.6 at Fe:Sr ratio of 12:0.7. The evince α-Fe2O3 phase is predestined together with the M-type hexagonal phase under the various studied temperatures.

Fig. 2
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XRD patterns of Sr0.9Li0.2Fe12O19 synthesized from strontium–lithium–iron tartrate precursor treated at different temperatures (1000–1200 °C) for 2 h

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Fig. 3
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XRD patterns of Sr0.8Li0.4Fe12O19 synthesized from strontium–lithium–iron tartrate precursor treated at different temperatures (1000–1200 °C) for 2 h

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Fig. 4
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XRD patterns of Sr0.7Li0.6Fe12O19 from strontium–lithium–iron tartarate precursor treated at different temperatures (1000–1200 °C) for 2 h

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Applying Scherrer equation based on the main peak (107) of the XRD chart showed that there is variation in the crystallite size. With raising the temperature from 1000 to 1200 °C for all examined samples with Li+ ion contents, the crystallite size of Sr1−xLi2xFe12O19 was found to increase (Fig. 5; Table 2). When the annealing temperature was increased from1000 to 1200 °C, the nanocrystalline size of the Fe:Sr:Li molar ratio 12:0.9:0.2 was increased from 146.3 to 159.8 nm. The causes behind increasing the crystallite size with an increase of annealing temperature were the grain growing and internal stress decrease. The change of the Fe:Sr:Li molar ratio to 12:0.8:0.4 increased the nanocrystalline size. Further increasing Li+ ion with a ratio of 0.6 on the hexagonal phase decreased the nanocrystalline size, but still larger than that for hexagonal phase Li+ ion with a ratio of 0.2. Noticeably, these results indicate clearly that Li+ ion molar ratio affected on the crystallite size.

Fig. 5
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Variation of crystallite size and unit cell volume as function of Li+ ion ratio and annealing temperature

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Table 2 Crystallite size, lattice parameters, and unit cell volume on Sr1−xLi2xFe12O19 nanopowder synthesized at various annealing temperatures from 1000 to 1200 °C for 2 h
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Lattice parameters are important feature that identify and reveal the structural properties of crystalline materials. Recently, determination of lattice parameters through computational approaches has gained a significant importance due to the ability to reduce experimental temporal cost and expenses. The variation in the lattice parameters and unit cell volume of Sr1−xLi2xFe12O19 with Li+ ion ratio and annealing temperature is shown in Fig. 5 and Table 2. It can be shown that the lattice parameters and unit cell volume of Sr1−xLi2xFe12O19 are influenced by the annealing temperature as well as the Li+ ion content. The unit cell volume is effectively increased by the annealing temperatures, while Li+ ion content had small effects on unit cell volume. The variation in the lattice parameters and unit cell volume may be related to the difference between the ionic radii of Li+ (0.76 Å) Sr2+ (1.18 Å) and Fe3+ (0.64 Å). To determine the structure type, an examination of aspect ratio c/a can be used [8, 9, 16]. For a ratio in between 3.917 and 3.963, the M-type (magnetoplumbite) structure can be predicted. Table 2 showed that the c/a ratios of as-prepared samples (~ 3.9140–3.9175) are optimum M-type structure range.

3.2 Microstructure

Figure 6 displays SEM images of clear hexagonal ferrites generated through tartaric acid approaches at different Li+ molar ratios. Hexagonal platelet-like structure was distinct with well-defined hexagonal grains and prominent grain boundaries at 0.2 Li+ molar ratio, (Fig. 6a, b). This defined microstructure further confirms the well-crystalline SrFe12O19 phase. In contrast, further boosting Li+ ratio to 0.4 resulted in fine spherical particles pointing the formation of secondary iron oxide composed with the hexagonal platelet shape, with wide size distributions, as illustrated in Fig. 6c and d. Similar trends were also illustrated in Fig. 6e–h with the increase of Li+ ratios. Meanwhile, the α-Fe2O3 spherical shape was also increased.

Fig. 6
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SEM micrographs of Sr1−xLi2xFe12O19nanopowders at various temperatures; a, b x = 1 and 1100 °C, c, d x = 0.2 and 1100 °C, e, f x = 0.3 and 1100 °C, and g, h x = 0.3 and 1200 °C

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Qualitative elemental analyses, through energy-dispersive X-ray spectroscopy (EDX), were detected on Sr0.7Li0.6Fe12O19 sample after heat treatment at 1100 °C to perceive allocation of each metal ion at the formation process. The EDX shown in Fig. 7 clearly indicated the characteristic charts of elemental Fe, Sr, O, and Li, and the spot analysis of Sr0.8Li0.6Fe12O19 annealed at 1100 °C is presented in Table 3. It is clear that Fe, Sr, O, and Li elements were allocated between the plate-like structure (hexaferrite) and the small crystals, with larger concentration of Fe and Sr in the plate-like structure phase. Strontium was mostly presented in the plate-like structure, while lithium was mostly absent in the small crystals and in the plate-like structure. This behavior may be attributed to the loss of Li+ content at high temperature.

Fig. 7
figure 7

SEM mappings of Sr0.7Li0.6Fe12O19 obtained at 1100 °C for the distribution of a real image, b strontium, c iron, d oxygen, e lithium, and f the associated EDX spectra

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Table 3 Spot analysis range of constituent elements in the Sr0.7Li0.6Fe12O19phase, wt%
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3.3 Magnetic properties

Impacts of different annealing temperatures (1000, 1100, and 1200 °C) on the magnetic properties of Li+ ion-substituted SrFe12O19 powders are illustrated in Figs. 8, 9, and 10. Values of magnetic parameters such as saturation magnetization (MS), coercive force (HC), remanence magnetization, and squareness ratio (MR/MS) are shown in Table 4. The saturation magnetization improved with increasing the annealing temperature from 1000 to 1100 °C, then diminished with further increasing the temperature up to 1200 °C. Highest saturation magnetization (Ms = 60.88 emu/g) was acquired at 1100 °C for Sr0.9Li0.2Fe12O19 sample. Li et al. emphasized that the magnetic moment of hexagonal strontium ferrite platelet was improved as the thickness of nanoplatelet increased, based on magnetic force microscopy (MFM) with nanoscale spatial resolution [27] as profited in Fig. 6a, b. However, the decrease in the saturation magnetization with increasing the Li+ concentration at 1100 °C could correspond to the mass ratio of SrFe12O19 and α-Fe2O3.

Fig. 8
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Effect of calcination temperature on the hysteresis loop of Sr0.9Li0.2Fe12O19

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Fig. 9
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Effect of calcination temperature on the hysteresis loop of Sr0.8Li0.4Fe12O19

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Fig. 10
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Effect of calcination temperature on the hysteresis loop of Sr0.7Li0.6Fe12O19

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Table 4 Effect of Fe3+/Sr2+ mole ratio, La3+ ion substitution, and annealing temperature on the magnetic properties of the synthesized strontium hexaferrite
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On the other hand, the coercive force of the formed Li+-substituted SrM ferrites was extremely reduced to lower than 1.5 KOe. Typically, Hc is rebated by both the intrinsic and extrinsic properties, such as the crystal structure, magnetic anisotropy, chemical content, composition, crystalline structure, average crystallite size, morphology, stress, lattice defects and so on [28, 29]. Based on XRD patterns and SEM specimens, it can be concluded that Hc was much dependent on the crystallinity, the average particle size, and α-Fe2O3 content. In this regard, the high Hc value was related to the high average particle size and α-Fe2O3 content as well as magneto-crystalline anisotropy. The variation on the Ms and Hc values at different temperatures is shown in Fig. 11.The squareness ratios Mr/Ms of the hexagonal samples were low as the results of low crystallinity, formation of α-Fe2O3 phase, and the hexagonal ferrite were in multi-magnetic domains.

Fig. 11
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Variation of Saturation magnetization (Ms) and Coercivity (Hc) as function of Li+ ion ratio and annealing temperature

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4 Conclusions

A series of Li+ ion-substituted strontium hexaferrite at different Li+ concentrations, and annealing temperatures were tailored through on the tartaric acid precursor route. Based on structural properties, a single phase of hexagonal strontium ferrite was confirmed without impurity phase of Sr0.9Li0.2Fe12O19 powders at Fe:Sr:Li ratio of 12:1:0.2 and at different annealing temperatures from 1000 to 1200 °C. Increasing the Li+ concentration in substituted SrM ferrite was found to develop the formation of α-Fe2O3 as impurity phase. The lattice parameters, aspect ratio (a/c), and unit cell volumes of the formed hexaferrite powders were varied with Li+ content and annealing temperature. The SEM investigation showed that hexagonal powders at Fe:Sr:Li molar ratio of 12:1:0.2 presented platelet-like structure at annealing temperature of 1100 °C. However, wide particle distributions of platelet and spherical shapes were distinguished with rising Li+ molar ratio. EDX analysis revealed that Fe, Sr, O, and Li elements were spread between the platelet structure (hexaferrite) and the small spherical crystals, with a larger concentration of Fe and Sr in the plate-like structure phase. Highest saturation magnetization (Ms = 60.88 emu/g) was attained for Sr0.9Li0.2Fe12O19 sample at 1100 °C. Various coercivities (443–1474 Oe) can be accomplished at different synthesis conditions.