Boost the Crystal Installation and Magnetic Features of Cobalt Ferrite/M-Type Strontium Ferrite Nanocomposites Double Substituted by La³⁺ and Sm³⁺ Ions (2CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉)

Abstract

Spinel cobalt ferrite/hexagonal strontium hexaferrite (2CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉; x = 0.2, 0.5, 1.0, 1.5) nanocomposites were fabricated using the tartaric acid precursor pathway, and the effects of La³⁺–Sm³⁺ double substitution on the formation, structure, and magnetic properties of CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉ nanocomposite at different annealing temperatures were assayed through X-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry. A pure 2CoFe₂O₄/SrFe₁₂O₁₉ nanocomposite was obtained from the tartrate precursor complex annealed at 1100 °C for 2 h. The substitution of Fe³⁺ ion by Sm³–⁺La³⁺ions promoted the formation of pure 2CoFe₂O₄/SrFe₁₂O₁₉ nanocomposite at 1100 °C. The positions and intensities of the strongest peaks of hexagonal ferrite changed after Sm³⁺–La³⁺ substitution at ≤1100 °C. In addition, samples with an Sm³⁺–La³⁺ ratio of ≥1.0 annealed at 1200 °C for 2 h showed diffraction peaks for lanthanum cobalt oxide (La₃Co₃O₈; dominant phase) and samarium ferrite (SmFeO₃). The crystallite size range at all constituent phases was in the nanocrystalline range, from 39.4 nm to 122.4 nm. The average crystallite size of SrFe₁₂O₁₉ phase increased with the number of Sm³⁺–La³⁺ substitutions, whereas that of CoFe₂O₄ phase decreased with an x of up to 0.5. La–Sm co-doped ion substitution increased the saturation magnetization (Ms) value and the subrogated ratio to 0.2, and the Ms value decreased with the increasing number of double substitutions. A high saturation magnetization value (Ms = 69.6 emu/g) was obtained using a La³⁺–Sm³⁺ co-doped ratio of 0.2 at 1200 for 2 h, and a high coercive force value (Hc = 1192.0 Oe) was acquired using the same ratio at 1000 °C.

1. Introduction

The consolidation of spinel soft and hard ferrites in nanocomposites can considerably change the magnetic characteristics related to interfacial exchange coupling. Composite magnets comprising soft phases with high saturation magnetization, and hard phases with high coercivity, can be merged as permanent magnets. Nanocomposite magnets offer wide-band absorption, with a megahertz (spinel ferrites MFe₂O₄, where M = divalent ion, such as Cu, Zn, Cd, Co, Ni, Mn, or mixture of two divalent ions) to gigahertz (hard ferrites) range. This feature can be useful for minimizing the radar cross-section of the target and problems due to electromagnetic interference over wireless communications [1,2,3]. Hard hexagonal ferrites are classified as M-type hexaferrites, including MFe₁₂O₁₉, Y-type ferrites (M₂Me₂Fe₁₂O₂₂), W-type ferrites (MMe₂Fe₁₆O₂₇), X-type ferrites (M₂Me₂Fe₂₈O₄₆), U-type ferrites (M₄Me₂Fe₃₆O₆₀), and Z-type ferrite (M₃Me₂Fe₂₄O₄₁), where M is Ba, Sr, or Me is a small ionic radius such as Zn, Co, Ni [4]. Strontium hexaferrite (SrFe₁₂O₁₉) M-type ferrite is the most well-known hard ferrite, owing to its merits, such as low cost, high saturation magnetization, high magnetization, high coercive force, excellent corrosion resistance, eminent chemical stability, and high Curie temperature (470 °C). It has a wide application range, in the fields of automotives, home appliances, electronics, lighting, biomedical and diagnostic applications, and oil and energy [5]. In addition, the commercial applications of M-type hexaferrite for high-density recording media depend on the ferrite type. The production of major recording media and permanent magnets require inexpensive ferrites with good chemical stability and high resistance. For recording media such as video type, CVD, DVD, and USB, high coercivities complicate re-recording. However, high coercivity prevents the corruption of information in credit and identification cards exposed to stray magnetic fields [6]. The exchange coupling of CoFe₂O₄ as a spinel ferrite and SrFe₁₂O₁₉ as a hard ferrite composite has been scrutinized [7]. Dhabekar and Kant [8] synthesized a cobalt ferrite/strontium hexaferrite nanocomposite in a ratio of 1:1, with a facile co-precipitation strategy. The change in the dielectric properties of the elaborated composite was investigated at different temperatures, ranging from 100 °C to 400 °C. Using a similar technique, Dhabekar and Kant [9] fabricated a CoFe₂O₄/SrFe₁₂O₁₉ nanocomposite (1:2) with a saturation magnetization of 47.02 emu/g. However, Pan et al. [10] processed CoFe₂O₄/SrFe₁₂O₁₉ nanocomposites based on an electrospinning pathway at different Co²⁺/Sr²⁺ ratios. The maximum saturation magnetization of the nanocomposite was 62.8 emu/g. Panchal and Jotania [11] prepared a CoFe₂O₄-SrFe₁₂O₁₉ composite using the Self-propagating high-temperature synthesis SHS approach. The magnetic features and the particle size were monitored on the basis of the variations in the content of spinel ferrite in the composite with hexagonal strontium ferrite. Petrecca et al. [12] attempted to synthesize SrFe₁₂O₁₉/Zn_1-xFe₃O₄ nanocomposite by milling the materials in a weight ratio of 90:10 in mortar and then annealed at temperatures of 500–1100 °C for 2 h. Their aim was to expand the application range of the nanocomposite for moderate energy product applications involving the automotive and energy industries. Meanwhile, Algarou et al. [13] developed SrFe₁₂O₁₉ and Mg_0.5Cd_0.5Dy_0.03Fe_1.97O₄ nanocomposites for antifungal applications. The electrochemical performance of SrFe₁₂O₁₉/CoFe₂O₄ nanocomposite synthesized using the hydrothermal pathway was investigated. The specific capacitance was increased from 133 F g⁻¹ in pure Sr-ferrite to 634 F g⁻¹ at a Co/Sr ratio of 0.5 [14]. The magnetic properties of M-type hexaferrite was enhanced by the partial replacement of Sr or Fe or both. For example, the possibility of substituting Sr by La creates an increment of the magneto-crystalline anisotropy. Therefore, the impact of other rare earth elements, such as Nd and Sm, on double substitution affects the magnetic interactions and subsequently enhances magnetic features [15]. Herein, to the best of our knowledge, the present work is the first report to describe the synthesis of cobalt ferrite/M-type strontium hexaferrite with the tartaric acid precursor strategy or sol gel auto-combustion pathway, with tartaric acid used as a fuel. The effect of the co-doping of M-type hexaferrite with La³⁺ and Sm³⁺ ions was investigated at different concentrations. The structural and magnetic properties were reorganized through X-ray powder profiling, scanning electron microscopy, transmission electron microscopy, and vibration sample magnetometry.

2. Experimental

Spinel cobalt ferrite/strontium hexaferrite (CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉; x = 0.2, 0.5, 1.0, 1.5) nanocomposites with a cobalt ferrite/strontium ratio of 2:1 were synthesized using the tartrate precursor technique. Tartaric acid was used as the chelating agent and fuel. Pure chemical grades of cobalt chloride hexahydrate (CoCl₂.6H₂O), strontium chloride hexahydrate (SrCl₂.6H₂O), ferric chloride anhydrous (FeCl₃), lanthanum nitrate hexahydrate (La(NO₃)₃.6H₂O), samarium nitrate hexahydrate (Sm(NO₃)₃.6H₂O), and tartaric acid (C₄H₆O₆) were added to the aqueous solution, according to the precalculated ratios of Sr²⁺, Co²⁺, Sm³⁺, La³⁺, and Fe³⁺ ions (1.0:2.0:x, x, and 16-x). The prepared CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉ nanocomposites (x = 0.2, 0.5, 1.0, 1.5) had a content of 2:1 (Figure 1). The solutions were gently agitated on a hot plate magnetic stirrer at 90 °C until sticky gel precursors with good homogeneity were obtained. Thereafter, the formed sticky gel was dried in an oven at 110 °C overnight. Eventually, the formed precursors were annealed in a static atmosphere furnace at annealing temperatures of 1000–1200 °C for 2 h. X-ray diffraction (XRD) profiles were collected with a model Bruker AXS diffractometer, from Karlsruhe, Germany (D8-ADVANCE), with Cu Kα ((λ = 1.54056 A°) radiation, operating at 40 kV and 10 mA and used in evaluating the phase evolution in a 2θ range of 10°–70°. The average crystallite size was estimated using the well-known Debye–Scherrer equation, and according to the strongest peaks for CoFe₂O₄ at 2θ of 35.49°, SrFe₁₂O₁₉ at 2θ of 34.14°, and La₃Co₃O₈ at 2θ of 32.48° [16]. The surface morphology and the average grain sizes of the created nanocomposites were examined through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A vibrating sample magnetometer (Lake Shore Cryotronics, Westerville, OH, USA) was used in studying changes in magnetic properties. Magnetic features were characterized at room temperature in a maximum applied field of 20 kOe.

3. Results and Discussion

3.1. X-ray Diffraction Characterization

Figure 2 shows the XRD spectra of mixed soft or hard ferrites nanocomposites without substitution annealed at various temperatures (1000–1200 °C) for 2 h. Peak profiles linked to spinel cobalt ferrite (JCPDS # 79-1744) and hexagonal SrFe₁₂O₁₉ (JCPDS # 84-0757) as common phases and cubic α–Fe₂O₃ (JCPDS # 89-0599) as an impurity secondary phase were detected at 1000 °C. Spectrum peaks corresponding to the essential diffraction planes (220), (311), (400), (422), (511), and (440) of cubic cobalt ferrite were disclosed at 2θ values of 30.32°, 35.49°_,43.13°, 53.48°, 57.04°, and 63.05°; whereas peaks associated with the strongest diffraction planes (114), (107), (0 0 8) (110), (203), (2200), and (2011) of hexagonal SrFe₁₂O₁₉ were differentiated at 2θ values of 34.17°, 32.31°, 31.085°, 30.34°, 37.15°, 63.16°, and 56.8°. Otherwise, diffraction peaks at 2θ values of 33.17°, 35.66°, 54.06°, 49.46°, and 24.14° were attributed to the diffraction planes (104), (110), (116), (024), and (012), indicating the presence of an α–Fe₂O₃ phase. However, when the annealing temperature was increased from 1000 °C and 1200 °C within 2 h, well-defined cubic cobalt ferrite CoFe₂O₄ and M-type strontium hexaferrite SrFe₁₂O₁₉ nanocomposite was formed.

Figure 3 and Figure 4 show the SrFe_12−2xSm_xLa_xO₁₉ (x = 0.2, 0.5, 1.0, 1.5) co-doped with Sm³⁺ and La³⁺ ions at annealing temperatures of 1000–1100 °C and an annealing time of 2 h. Spinel ferrite and M-type ferrite formed in all the samples with various replacements at 1000–1100 °C. The intensities of the main peaks of strontium ferrite decreased, and the positions of the peaks slightly shifted to a low 2θ value after the concentrations of Sm³⁺ and La³⁺ ions increased.

Figure 5 illustrates the effect of co-doping SrFe_12−2xSm_xLa_xO₁₉ with Sm³⁺ and La³⁺ ions on the phase evolution of the nanocomposite synthesized from the tartaric acid precursors annealed at 1200 °C for 2 h. The XRD patterns confirmed that all the samples were well-defined nanocomposites of cubic CoFe₂O₄ and hexagonal strontium hexaferrite with Sm³⁺-La³⁺ double substitution of ≤0.5. However, in samples with Sm³⁺–La³⁺co-doped substitution of ≥1.0, the peaks of spinel cobalt and hexagonal ferrites dramatically decreased, and new diffraction patterns were observed. Diffraction peaks for the nonmagnetic phases of La₃Co₃O₈ (JCPDS # 89-1319) and SmFeO₃ (JCPDS # 74-1474) were observed as well.

The average crystallite size of 2CoFe₂O₄/SrFe₁₂O₁₉ nanocomposite was calculated using the XRD data and the Scherrer equation. The most intense peak of each constituent phase was used in the calculation (peak at 2θ of 35.49° for the CoFe₂O₄ phase, peak at 2θ of 34.14° for the SrFe₁₂O₁₉ phase, and peak at 2θ of 35.49° for the La₃Co₃O₈ phase). The calculated crystallite sizes are listed in

Table 1. Crystallite size variation for the formed phases with Sm³⁺-La³⁺ double substituted CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉ annealed at 1000, 1100 and 1200 °C elaborated using tartaric acid precursor approach (CoFe₂O₄ at 2θ = 35.49, SrFe₁₂O₁₉ at 2θ = 34.14, La₃Co₃O₈ at 2θ = 32.48).

Composition	Temperature, °C	Formed Phase	C. Size, nm
2CoFe2O4/SrFe12O19	1000	CoFe2O4	79.6
		SrFe12O19	42.2
	1100	CoFe2O4	94.6
		SrFe12O19	51.8
	1200	CoFe2O4	109.7
		SrFe12O19	59.5
2CoFe2O4/SrFe11.6Sm0.2La0.2O19	1000	CoFe2O4	75.1
		SrFe12O19	60.9
	1100	CoFe2O4	83.2
		SrFe12O19	89.4
	1200	CoFe2O4	91.4
		SrFe12O19	101.1
2CoFe2O4/SrFe11Sm0.5La0.25O19	1000	CoFe2O4	72.6
		SrFe12O19	76.2
	1100	CoFe2O4	79.5
		SrFe12O19	86.6
	1200	CoFe2O4	86.1
		SrFe12O19	105.4
2CoFe2O4/SrFe10Sm1.0La1.0O19	1000	CoFe2O4	70.1
		SrFe12O19	80.1
	1100	CoFe2O4	74.9
		SrFe12O19	96.8
	1200	CoFe2O4	88.2
		SrFe12O19	113.7
		La3Co3O8	39.4
2CoFe2O4/SrFe9Sm1.5La1.5O19	1000	CoFe2O4	76.0
		SrFe12O19	87.9
	1100	CoFe2O4	84.1
		SrFe12O19	107.3
	1200	CoFe2O4	91.7
		SrFe12O19	122.4
		La3Co3O8	51.1

. Notably, crystallite size increased with annealing temperature in all dominant phases (2CoFe₂O₄, SrFe₁₂O₁₉, and La₃Co₃O₈). However, the crystallite size range in all the studied samples, at all constituent phases, was in a nanocrystalline range of 39.4 to 122.4 nm. In the spinel cubic CoFe₂O₄ phase, crystallite size decreased at Sm³⁺–La³⁺ dual displacement of up to 0.5 (at 1200 °C, from 109.7 nm at x of 0 to 86.1 nm at x of 0.5) and then increased again at a higher Sm³⁺–La³⁺ binary substitution (to 91.7 nm at x of 1.5; 1200 °C). Indeed, the crystallite size of hexagonal SrFe₁₂O₁₉ increased with Sm³⁺–La³⁺ double substitution at all x values. It increased from 59.5 nm at x of 0.0 to 122.4 nm at x of 1.5 (annealed at 1200 °C). The La₃Co₃O₈ phase formed only at x of ≥1.0, and its crystallite size increase from 39.4 nm at x of 1.0 to 51.1 nm at x of 1.5. The reduction in the crystallite size with La ion content can be imputed to phase transition and the difference between ionic radii of La³⁺ (1.36 Å), Sm³⁺ (1.24 Å), and Fe³⁺ (65 Å).

3.2. Microstructure

Figure 6 shows SEM images of the produced spinel ferrite or M-type hexaferrite at different annealing temperatures. A fine spherical structure with random grain orientation and hexagonal platelet-like structures related to strontium M-type ferrite was observed in the samples annealed at 1000 °C (Figure 6a,b). This temperature was insufficient for the formation of crystallite phases of soft or hard nanocomposites. As the temperature was increased to 1100 °C (Figure 6c,d), the grains started to grow again, and the SEM micrographs showed a clear aggregation of crystallite hexagonal platelet particles (SrFe₁₂O₁₉) with sharp planes of crystals coated by small spherical or cubic grains (CoFe₂O₄). The same structures were observed in the samples annealed at 1200 °C. In addition, multiple layers of hexagonal platelet-like structures coated by cubic grains (CoFe₂O₄) and crystals with uniform coarse structures were observed. The accumulation of spherical (or cubic) grains on the surface of the hexagonal platelet-like structure in the SrFe₁₂O₁₉ hard ferrite indicated magneto-dipole interactions among the particles of the soft and hard ferrites [17]. However, the grain size of the hard ferrite phase was larger than that of the soft ferrite phase. Figure 7 shows the TEM micrographs of the synthesized nanocomposites at different annealing temperatures without metal substitution. Large hexagonal particles with small particles were observed in the nanocomposites. The TEM micrographs showed that the sizes of the hard and soft ferrite nanocomposites were several tens of nanometers at an annealing temperature of 1000 °C (Figure 7a,b). The size range of the synthesized nanocomposite increased to several hundreds of nanometers at an annealing temperature of 1200 °C (Figure 7e,f). The effect of Sm³⁺ and La³⁺ ion substitution on the produced 2CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉ (x = 0.2, 0.5, 1.0, 1.5) annealed at 1100 °C is evident in the SEM micrograph presented in Figure 8. The composition of SrFe₁₂O₁₉ indicated a hexagonal platelet-like structure with accumulated spherical grains and the pseudo-cubic shapes of different components on the surface of the hexagonal plate. At an x of 0.2, the morphology showed a platelet-like structure of hexagonal strontium hexaferrite, accumulated spherical grains, few rod-like structures, and the pseudo-cubic structure of cobalt ferrite specimen (Figure 8a,b). The number and size of the platelet-like structures of hexagonal ferrite and rod-like structures of soft ferrite increased at an La³⁺–Sm³⁺ ion ratio of 0.5 (Figure 8c,d). Furthermore, at an x of ≥1.5 (Figure 8e–h), some plate-like hexagonal strontium hexaferrite (the distribution of spherical or pseudo-cubic grains structures corresponding to soft ferrite), La₃Co₃O₈, and SmFeO₃ were observed. To clarify the distribution of each element (Sr, Co, Fe, and O) in the structure, EDX spot analysis of the 2CoFe₂O₄/SrFe₁₂O₁₉ nanocomposite powders (without substitution) annealed at 1200 °C was conducted. The range of analysis for all reported phases is provided in

Table 2. Spot Analysis Range of Constituent Elements in the 2CoFe₂O₄/SrFe₁₂O₁₉, wt% nanocomposite powders (without substitution) annealed at 1200 °C fabricated using a tartraic acid precursor approach.

Element	Co		Sr		Fe		O
	Mass%	Atom%	Mass%	Atom%	Mass%	Atom%	Mass%	Atom%
Plate-Like structure	4.81–5.54	2.82–3.16	10.15–10.49	3.55–3.85	57.73–72.94	35.69–65.85	7.52–27.31	24.14–58.94
Small cubic crystals	17.08–21.87	9.13–14.61	1.41–3.43	0.40–0.88	49.16–59.13	27.72–41.70	17.59–31.86	43.29–62.72

. The concentration range of cobalt in the small cubic structure phase (9.13–14.61 atom %) was higher than that in the plate structure (2.82–3.16 atom %). Moreover, the concentration range of strontium in the small pseudo-cubic structure phase (0.40–0.88 atom %) was lower than that in the plate structure (3.55–3.85 atom %). These results indicated that the spherical structure phase was spinel CoFe₂O₄, whereas the plate structure phase was hexagonal ferrite (SrFe₁₂O₁₉). However, the concentration range of iron in the plate structure (35.69–65.85 atom %) was higher than the spherical structure phase (27.72–41.70 atom %), whereas the concentration range of oxygen in the plate structure (24.14–58.94 atom %) was lower than that in the spherical structure phase (43.29–62.72 atom %). The concentration ranges of iron and oxygen indicated that the spherical structure phase was spinel CoFe₂O₄ and the plate structure phase was hexagonal ferrite (SrFe₁₂O₁₉).

3.3. Magnetic Properties

Figure 9 includes the field dependence of the magnetization of 2CoFe₂O₄/SrFe₁₂O₁₉ samples synthesized at different temperatures (1000–1200 °C) for 2 h. The measurements were performed at room temperature, and the maximum applied field was 20 kOe. The magnetic properties were intermediate between the two phases. The formed nanocomposites showed single-phase magnetic characteristics, indicating that the magnetic hard and soft phases were exchange-coupled [18]. The saturation magnetization Ms value increased with annealing temperature. The M_s value gradually increased from 53.5 emu/g at 1000 °C to 67.4 emu/g at 1200 °C. Variations in saturation magnetization were observed as the crystallite size increased in the two formed magnetic phases. Furthermore, the saturation magnetization increased with the decreasing ratio of nonmagnetic species of α–Fe₂O₃. However, the coercive force H_c of the composite decreased with increasing annealing temperature. As a result, the α-Fe₂O₃ presented showed a high intrinsic coercive force at an annealing temperature of 1000 °C. Furthermore, changes in this value may be attributed to increase in the soft cobalt ferrite phase at increased annealing temperature [19]. Coercivity decreased with crystallite size and with lattice imperfections, voids, and porosity, attributed to the multidomain formation and the facile movement of domain walls [4]. Figure 10, Figure 11 and Figure 12 show the magnetic hysteresis loops (M–H curves) of the 2CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉ nanocomposite powder synthesized at different temperatures (1000–1200 °C).

Table 3. Effects of Sm³⁺-La³⁺ substitution and annealing temperature on the magnetic properties of 2CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉ elaborated at different La-Sm concentrations and annealing temperatures tailored using a tartaric acid precursor approach.

Composition	Temperature, (°C)	Magnetic Properties
		Saturation Magnetization Ms, (emu/g)	Retentivity Mr, (emu/g)	Coercivity Hc, (Oe)	Mr/Ms
2CoFe2O4/SrFe12O27	1000	53.5	21.3	1097.6	0.3981
	1100	62.5	17.4	648.0	0.2784
	1200	67.4	14.1	431.13	0.2092
2CoFe2O4/ SrFe11.6Sm0.2La0.2O19	1000	55.0	23.8	1192.0	0.4327
	1100	63.9	22.4	959.0	0.3505
	1200	69.6	21.5	795.9	0.3089
2CoFe2O4/ SrFe11Sm0.5La0.5O19	1000	50.6	23.4	1169.2	0.4625
	1100	53.2	16.5	828.5	0.3102
	1200	56.0	16.9	716.7	0.3018
2CoFe2O4/ SrFe10Sm1.0La1.0O19	1000	33.4	10.1	618.4	0.3024
	1100	44.5	10.3	468.9	0.2315
	1200	48.5	7.5	329.5	0.1546
2CoFe2O4/ SrFe9Sm1.5La1.5O19	1000	31.0	9.7	651.7	0.3129
	1100	33.5	8.3	503.7	0.2478
	1200	36.1	6.6	384.4	0.1828

lists the estimated values of magnetic parameters, namely, saturation magnetization (M_s), residual magnetization (M_r), coercivity field (H_c), and squareness ratio (Mr/Ms). The impacts of La³⁺ and Sm³⁺ content on the saturation magnetization and coercive force of the formed nanocomposites are illustrated in Figure 13 and Figure 14. Remarkably, the saturation magnetization and coercive force of 2CoFe₂O₄/SrFe_11.6Sm_0.2La_0.2O₁₉ increased with annealing temperature. The lanthanide element is usually established in the octahedral site (12k, 2a, 4f₂), because of volume impact. Moreover, La³⁺ and Sm³⁺ ions preferentially occupy the 4f₂ site. Fe³⁺ ions occupy five sites in the strontium hexaferrite structures (three octahedral sites with spin up direction (12k, 2a) and spin down direction (4f₂), one tetragonal site with spin down direction (4f₁), and one trigonal bipyramidal site with spin up direction (2b). At the 4f₂ site, Fe with spin down moment is replaced by rare earth elements, indicating that it has a lower magnetic moment (La⁺³ and Sm⁺³ ions have 1.5 µ_B, unpaired electron [4f⁵] and 0 µ_B [4f⁰], respectively) than Fe⁺³ ion (5 µ_B). The overall magnetic moment increased, and, thus, the total magnetization of the nanocomposite increased. However, as the substitution ratios increased from 0.5 to 1.5, La³⁺ and Sm³⁺ ions occupied the spin-up sites (2a, 2b, and 12 k), or La³⁺ ions were substituted by the Fe³⁺ ions in the spin up sites, the net magnetic moment and magnetization decreased. This result may be attributed to the formation of the impure secondary phases of the nonmagnetic phases of La₃Co₃O₈ and SmFeO₃, as indicated by the XRD profile data_. Furthermore, high concentrations of La⁺³ and Sm⁺³ ions steadily altered the magnetization of the Fe⁺³ ion, from a collinear spin to a noncollinear spin. Additionally, substitution by La⁺³ and Sm⁺³ decreased the strength of the super-exchange interaction between Fe⁺³-O-Fe⁺³, leading to the non-collinear or spin-canting arrangement of magnetic moments [20,21]. The squareness ratio Mr/Ms ranged from 1.5 to 0.46, confirming that the formed nanocomposites were in multimagnetic domains [16].

4. Conclusions

The results are summarized as the follows:

• Soft or hard ferrite nanocomposite, based on cobalt ferrite CoFe₂O₄/strontium hexaferrite SrFe₁₂O₁₉ in a ratio of 2:1, was successfully synthesized for the first time with tartaric acid precursor, at annealing temperatures of 1100–1200 °C and an annealing time of 2 h.
• The simultaneous insertion La³⁺ and Sm³⁺ ions into CoFe₂O₄/SrFe_12−2xSm_xLa_xO₁₉ nanocomposites (x = 0.2, 0.5, 1.0, 1.5) indicated that the two phases did not deteriorate until x > 1.5 because of the impurities in the La₃Co₃O₈ and SmFeO₃ phases produced.
• The microstructures of the two formed magnetic phases were affected by the ratio between the CoFe₂O₄ and SrFe₁₂O₁₉ phases and the La3+ and Sm3+ ion substitution ratio. The morphology of the pure nanocomposites displayed hexagonal platelet-like structures with pseudo-cubic shapes.
• EDX spot analysis revealed that Fe, Sr, O, and Co elements were distributed in the cubic and hexagonal plate-like structures, and large concentrations of Fe and Sr were found in the plate- like structures.
• The occupation of La-Sm co-doped ions in the lattice sites of the spin down magnetic moments 4f₂ led to an increase in net magnetic moments in the magnetic structure of M-type hexaferrite, which in turn increased the Ms value. However, when the substitution ratios increased from 0.5 to 1.5, La³⁺ and Sm³⁺ occupied the spin up sites (2a, 2b, and 12 k), or when the La³⁺ and Sm³⁺ ions were substituted by the Fe³⁺ ions in the spin up sites, the net magnetic moment and magnetization decreased.
• A high saturation magnetization value (Ms = 69.6 emu/g) and high coercive force value (Hc = 1192.0Oe) were obtained at annealing temperatures 1200 °C and 1000 °C, respectively, at a La³⁺–Sm³⁺ co-doped ratio of 0.2.
• These nanocomposites are potential materials for microwave absorber devices and hyper-frequency applications, such as multilayer chip inductors and LC filters.