1 Introduction

The foundation of contemporary technology is strongly rooted in magnetic phenomena, which play a vital role in various devices such as sensors, microwave devices, random access memories, and computer hard drives [1, 2] In this context, ferrimagnetic materials have garnered significant interest from the scientific community due to their intrinsic magnetic properties [3,4,5]. Among these materials, the yttrium iron garnet (YIG—Y3Fe5O12) stands out as the ferrimagnet with the narrowest linewidth ever reported in the literature for a monocrystalline form (0.2 Oe) [6,7,8,9,10,11,12]. YIG is a ferrimagnetic compound belonging to a family of complex oxides, distinguished by its remarkable chemical stability, low electrical conductivity, and low magnetic loss [8, 13]. In addition to its intrinsic properties, it is possible to optimize its characteristics for specific applications through precise control of synthesis parameters, including the introduction of dopants into its structure [14, 15]. The influence of dopants on the YIG structure is a critical aspect, as magnetization is strongly dependent on the three-dimensional crystalline structure and the composition of the garnet. Factors such as the composition of magnetic ions, grain size, and even synthesis temperature play crucial roles [14, 16,17,18].

The controlled insertion of dopants and subsequent heat treatments have the capability to significantly alter the YIG structure, impacting its relevant properties, as evidenced in numerous studies in the literature [7, 16, 19, 20]. However, it is important to highlight that, despite the numerous investigations conducted in this field, the specific influence of lanthanum (La) as a dopant on the YIG structure is still relatively unexplored in the literature [11]. This knowledge gap sparks significant interest, as lanthanum can play a significant role in finely tuning the magnetic and structural properties of the material. In this context, this work presents a comparative study of the influence of different sintering temperatures (900°, 1000 °C, and 1100 °C) of Y3Fe4,97La0,03O12 nanoparticles regarding their structural and magnetic properties. The in-depth analysis provided herein aims to augment the existing knowledge in the literature, bridging the current gaps and introducing new perspectives for the application of this material. Specifically, we explore the potential applications of the prepared material in areas such as wireless communications, where efficiency and bandwidth are critical, and in energy storage devices, where materials with low magnetic losses are essential for enhancing efficiency and longevity.

2 Experimental Procedure

The compound Y3Fe4,97La0,03O12 was synthesized using the sol–gel method, following a previously reported methodology [9]. Yttrium nitrate Y(NO3)3, iron nitrate Fe(NO3)3, lanthanum nitrate La(NO3)3, and citric acid C6H8O7 with high purity (≥ 98.5%) purchased from Sigma Aldrich Brazil were used as starting materials. The desired amounts of nitrates were weighed and dissolved in 50 mL of distilled water. Citric acid (0.01 mol) was added to the solution, while the pH was changed to 2 by adding ammonium hydroxide. The solution, under constant stirring, was heated to 90 °C until gel formation. The dry gels were divided into three alumina crucibles and sintered at temperatures of 900 °C (YIGLa09), 1000 °C (YIGLa10), and 1100 °C (YIGLa11) for 4 h.

Structural analyses were carried out by X-ray diffraction (XRD) techniques in a Rigaku model SmartLab with Cu target of 1.5412 Å with 2θ in a range of 10 to 80° and using Bragg–Brentano geometry. Raman spectroscopic measurements were performed on the OmegaScope™ 1000 (AIST-NT Technology) with a 100 × objective lens, excitation laser with a wavelength of 671 nm, and iHR320 spectrometer (Horiba Scientific). A laser power of 15 mW was adopted. In the analyses, a diffraction grating of 1800 lines/mm, 8 accumulations, and acquisition time of 10 s were adopted. The Scanning Electron Microscopy (SEM) images were obtained in a TESCAN MIRA3 Microscope, while the magnetic measurements were made in a Physical Property Measurement System (PPMS – Quantum Design, Evercool II Model). Ferromagnetic resonance (FMR) measurements were conducted at room temperature using a homemade spectrometer with a 9.5-GHz resonant cavity; X-ray photoelectron spectroscopy (XPS) measurements were performed on the Ipê beamline at LNLS, Campinas-SP.

3 Results and Discussion

Figure 1 shows the XRD patterns and Rietveld refinement of the Y3Fe4,97La0,03O12 nanoparticles sintered at 900 °C, 1000 °C, and 1110 °C temperatures. All samples show crystallographic planes, characteristic of the YIG centrosymmetric cubic structure, which were confirmed by the crystallographic card ICSD 33931. The sample sintered at 900 °C showed the YIG single-phase formation (Bragg positions, red lines), while for the samples calcined at 1000 °C and 1100 °C, a segregated secondary phase (Bragg positions, blue lines), belonging to the YFeO3 compound, was confirmed by the crystallographic card ICSD 23822. This result indicates that it is possible to substitute Fe3+ cations by La3+ cations in the YIG crystalline structure, considering thermal treatments below of 900 °C.

Fig. 1
figure 1

XRD patterns of YIGLa samples treated at different temperatures: a 900 °C (YIGLa09), b 1000 °C (YIGLa10), and c 1100 °C (YIGLa11)

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The secondary phase formation may be a consequence of the distinct nature between Fe3+ and La3+ cations. At higher temperatures, there is a tendency for the structure to become more disorganized. As the La3+ cations can only occupy the octahedral sites (a-sites) of the YIG and considering that they have a larger ionic radius in relation to the Fe3+ cations, probably some Fe3+ cations, located in the a-sites, are expelled by the La3+ cations. Considering that there is a greater affinity for binding between Fe3+ cations and Y3+ cations, a secondary phase could be formed in the compound [14]. On the other hand, the structural parameters, lattice constant (a), volume (V), and crystallite size (D), were obtained from the refinement (Table 1). No significant changes in a and V were observed as a function of temperature; however, the values obtained are greater than those reported for undoped YIG synthesized by the sol–gel method at different temperatures [14]. This is related to the larger ionic ratio of La3+ (1.032 Å) in relation to Fe3+ (0.645 Å) [21]. Table 2 also shows the sizes of crystallites obtained from refinement. Note that D increases with the temperature, which could be related to nucleation and growth, producing crystallites whose sizes depend on temperature because there is a better diffusion process [14]. Finally, the values of Rwp, Rp, and \(\chi\)2 confirm the quality of the Rietveld refinement method that was carried out using the gsas/expgui software[22].

Table 1 Structural parameters obtained from refinement for the Y3Fe4,97La0,03O12 compound
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Table 2 Parameters extracted from the fitting process applied to the FMR measurements of the YIGLa09, YIGLa10, and YIGLa11 samples under a fixed excitation frequency of 9.5 GHz
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The SEM images of the La-doped YIG samples sintered at 900 °C, 1000 °C, and 1100 °C are displayed in Fig. 2 (a), (b) and (c). The samples present agglomerates of nanoparticles with elongated morphology, which is a characteristics of material garnet structure [23]. The particle cluster formation is associated with a long-range magnetic dipole–dipole interaction [6, 24]. It can be noticed that, with the increase in temperature, there is an increase in the size of the particles, which agrees with the results obtained by XRD for the crystallite sizes.

Fig. 2
figure 2

SEM images of YIGLa samples treated at different temperatures: a 900 °C (YIGLa09), b 1000 °C (YIGLa10), and c 1100 °C (YIGLa11)

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Based on the result of SEM, the distribution of the average grain diameter of samples with different synthesis temperatures was obtained, as shown in Fig. 3. As the grains have elongated shapes, for each grain the perpendicular diameters of the grain were measured and an average was taken between the two measurements. Additionally, an adjustment was made using the normal (Gaussian) distribution. The YIGLa09 sample presented the majority of grains with an average diameter between 150 and 400 nm. The YIGLa10 sample presented a narrower Gaussian fit, having the grain size concentrated between a smaller length range; approximately 85% of the grains have average diameters between 150 and 300 nm. The YIGLa11 sample presented a wider Gaussian fit, indicating that the grains have greater variation in their size. The sample sintered at 1100 °C was the only one that presented grains with average diameters greater than 550 nm. Therefore, the difference in synthesis temperature of the samples interferes not only with the addition of dopant to the structure, but also with the size and growth of the grain. Similar results were reported in the literature by several authors [25,26,27].

Fig. 3
figure 3

Histograms for particle size distribution of samples with different synthesis temperatures

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To comprehend the structural influence of temperature on the doping of YIG with La ions, Raman spectroscopy was performed (Fig. 4). This spectroscopic technique is capable of detecting the formation of distinct phases and the emergence of structural defects in doped materials [8, 24]. In the spectra of the obtained materials, YIGLa09, YIGLa10, and YIGLa11, the active Raman modes were identified according to group theory, with T2g, Eg, and A1g. Among the theoretically predicted 25 active modes (3 A1g + 8 Eg + 14 T2g), only 12 were identified through deconvolution of the obtained spectrum, owing to inherent instrumental limitations of the equipment [28]. The bands at 163.8 ± 0.7, 184.0 ± 0.5, and 228.9 ± 0.4 cm−1, corresponding to the T2g modes, are attributed to the translational motions of Fe3+ ions within the tetrahedral sites (d-site). The T2g mode, associated with the most intense band at 262.2 ± 0.2 cm−1, also signifies movements of the d-sites, as well as motions of Y3+ ions within the dodecahedral sites (c-site) of the face-centered cubic (bcc) crystal structure with the Ia3d (Oh10) space group characteristic of YIG. The band at 410.4 ± 0.4 cm−1 (Eg) can be associated with the translational motions of cubic distortions of Y3+ ions coordinated with oxygen in c-sites. In wavenumbers above 330 cm−1, the bands correspond to vibrational modes: Eg (332.9 ± 0.5 cm−1), for Y3+ ions in c-sites; T2g (365.6 ± 0.8 cm−1) for Fe3+ ions in d-sites; and T2g (436 ± 2 cm−1), A1g (497 ± 3 cm−1), T2g (572 ± 2 cm−1), and A1g (721 ± 1 cm−1) for Fe–O bonds in tetrahedral and octahedral (a-site) sites [23, 24].

Fig. 4
figure 4

Raman spectroscopy of YIGLa samples treated at different temperatures: 900 °C (YIGLa09), 1000 °C (YIGLa10), and 1100 °C (YIGLa11)

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The largest variations in the positions of the last four bands can be attributed to the substitution of La3+ dopant within the a-site structure, with an increasing shift of the A1g bands as temperature rises. In the peak analysis, the emergence of an asymmetry in the most intense band was also observed, indicated by asterisk (273.6 ± 0.8 cm−1), which can be linked to the presence of the secondary phase YFe2O3. The YFeO3 phase is likely formed due to the charge imbalance caused by the La3+ doping during the YIG synthesis, which affects the reactive stoichiometry [8]. This information aligns with the XRD analysis, which revealed the presence of characteristic peaks corresponding to yttrium iron oxide YFeO3 [29].

Figure 5 shows the X-ray photoelectron (XPS) spectra of Y3Fe4,97La0,03O12 heat treated at 900 °C (YIGLa09), 1000 °C (YIGLa10), and 1100 °C (YIGLa11); this type of analysis provides a comprehensive understanding of the composition surface and chemical bonds in lanthanum-doped yttrium iron garnets. The obtained spectra reveal intricate details about the oxidation states, crystal lattice defects, and lanthanum dopant interactions in the synthesized samples. The energy sweep spectra (survey) shown in Fig. 5 exhibit consistent profiles across all samples, highlighting typical features of the YIG structure. In addition, besides the peaks related to the elements in the sample, peaks related to carbon can also be observed. These survey results effectively capture the overall surface composition and confirm the presence of the desired elements.

Fig. 5
figure 5

X-ray photoelectron survey spectra for of YIGLa samples treated at different temperatures: a 900 °C (YIGLa09), b 1000 °C (YIGLa10), and c 1100 °C (YIGLa11)

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High-resolution spectra of energy levels of key elements provide deeper insights into surface interactions and electron configurations. The specific characteristics observed in each spectrum of elements reveal subtle information. In the yttrium (Y) spectra (Fig. 6(b)), the doublets of the Y 3d states confirm the presence of Y3+ ions. The positions of the peaks correspond to the characteristic division of the doublet, indicating the electronic configuration of yttrium in the YIG structure. Variations in peak intensities and positions between samples can be attributed to differences in ion synthesis and distribution conditions [30].

Fig. 6
figure 6

High-resolution X-ray photoelectron spectra for the elements with their respective related orbitals for YIGLa samples treated at different temperatures: a 900 °C (YIGLa09), b 1000 °C (YIGLa10), and c 1100 °C (YIGLa11)

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The oxygen (O) spectra (Fig. 6(d)) expose oxygen-related interactions and defects in the crystal lattice. The presence of multiple peaks at different binding energies can be attributed to different oxygen environments. The lowest binding energy peak is related to the crystal lattice oxygen, while the highest binding energy peaks are associated with oxygen defects. These findings corroborate the structural disorder and defects previously observed in the samples.

The lanthanum (La) spectra (Fig. 6(f)) exhibit consistent behavior across all samples, reflecting a uniform interaction between La3+ ions and the YIG matrix. This uniformity suggests that the lanthanum dopant maintains its oxidation state and binding characteristics regardless of the synthesis temperature.

The XPS results are in line with other analytical techniques reported in the literature, such as Mössbauer spectroscopy and electron paramagnetic resonance (EPR), which confirmed the presence of Fe3+, Fe2+ ions, and the lanthanum dopant. The Fe3+:Fe2+ atomic concentration ratios further reinforce the prevalence of Fe2+ ions, which is attributed to factors such as oxygen deficiency and the inclusion of ions in the garnet structure.

The characteristics related to defects due to lack of oxygen observed in the oxygen spectra support the argument of atomic disorder and defects in the crystal lattice. The shifts in the binding energies and variations in the intensity of these characteristics corroborate the interpretation established in the literature about materials similar in their composition as YFeO2 identified in the X-ray diffraction analysis, further strengthening their interpretation [30].

Summarizing, a comprehensive XPS analysis of the synthesized YIG samples offers valuable insights into their chemical states, interactions, defects, and any unintended phases present. These results contribute to a deeper comprehension of both the structural and compositional aspects. Furthermore, the entire XPS analysis is in alignment with data obtained from other studies in the literature. The consistency of XPS results with other characterization techniques reinforces the credibility of the conclusions derived from this study [8].

The curves in Fig. 7 show the magnetization measurements as a function of the applied field. The measurements were made at 300 K and returned very close coercivity values for the three samples (between 35 and 45 Oe). The remanence values are also significantly close, being between 4.5 and 4.9 emu/g. These values are representative of this type of compound as has been reported in recent works [24]. Furthermore, they are consistent with the results of the structural analysis, especially when we compare the crystallite size of all samples. Thus, the quantity most evidently affected by the effects of the preparation temperature is the saturation magnetization. Thus, the quantity most evidently affected by the effects of the preparation temperature is the saturation magnetization. Specifically, were obtained values of 27.7 emu/g for sample treated at 900 °C, 22.4 emu/g for 1000 °C, and 26.2 emu/g for 1100 °C. The values presented here are among the expected values in YIG doped with other ions [14]. The numerical data presented here were obtained by adjusting the Approach Law equation, \(M\left(H\right)={M}_{S}\left(1-{\left({H}_{A}/H\right)}^{2}\right)\), for fields H > 1000 Oe [31,32,33].

Fig. 7
figure 7

Magnetic moment as a function of the applied field (M–H) of YIGLa samples, treated at different temperatures: 900 °C (YIGLa09), 1000 °C (YIGLa10), and 1100 °C (YIGLa11)

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The adjustment is shown in Fig. 7(b). Specifically, were obtained values of 27.7 emu/g for sample treated at 900 °C, 22.4 emu/g for 1000 °C, and 26.2 emu/g for 1100 °C. The values presented here are among the expected values in YIG doped with other ions [14]. Figure 7(c) shows the behavior of saturation magnetization as a function of the synthesis temperature of the samples. The sample treated at 900 °C (YIG0La9) presents the highest Ms value. Sample YIGLa09 is a single-phase YIG with La inserted into the Fe sites as shown in XRD and Raman analysis. In samples YIGLa10 and YIGLa11 a secondary phase of YFeO3 is formed. The secondary phase weakens the coupling between the sublattices, being the responsible for the decrease in magnetization [34,35,36]. Comparing the saturation magnetization values of samples YIGLa10 and YIGLa11 it is possible to notice an increase from 22.66 to 26.39 emu/g. Yttrium iron garnet is a ferrimagnetic compound. Therefore, the total magnetic moment of the YIG depends on the individual contribution of each sublattice. The tetrahedral and octahedral sites are occupied with Fe3+ magnetic ions, which are aligned non-parallel. The dodecahedral sublattice does not contribute to the total magnetic moment, as it is filled mainly by non-magnetic Y3+ ions. Although samples YIGLa10 and YIGLa11 form a secondary phase of YFeO3, YIGLa11 presented a larger crystallite size than YIG10, being 79.72 nm and 93.76 nm, respectively. Therefore, the increase in crystallites is possibly responsible for the increase in Ms comparing the samples treated at 1000 °C and 1100 °C.

From the same adjustment, effective HA anisotropy fields of 213, 193, and 219 Oe were obtained for samples prepared at 900 °C, 1000 °C, and 1100 °C, respectively. These values have the contributions of the YIG magneto-crystalline anisotropy, the shape and size of crystals constituting the compounds. In addition, the secondary phase effects, reported in the X-ray diffraction analyses in Fig. 1, are relevant. The YFeO3 compound is an antiferromagnetic orthoferrite with a weak ferromagnetism and antiferromagnetism (Néel temperature of 644 K) [37].

The sample prepared at 900 °C is a YIG single phase with La inserted into the Fe sites, as shown in the XRD analysis. For the sample treated at 1000 °C, the secondary YFeO3 phase is formed and for the sample prepared at 1100 °C, the crystals of this secondary phase are bigger. The crystal size of this secondary phase is determinant in its magnetic response; therefore, the variations in Ms and HA for the compound (YIG + YFeO3) are modified by the interactions between the referred phases.

FMR measurements allow for the calculation of damping parameter values, exchange constants, and anisotropy constants. This enables the investigation of the influence of structural aspects on magnetic properties. Figure 8 presents the FMR spectra measured at room temperature with a fixed frequency of 9.5 GHz for the YIGLa09, YIGLa10, and YIGLa11 samples.

Fig. 8
figure 8

FMR spectra measured at room temperature for the YIGLa samples treated at different temperatures: 900 °C (YIGLa09), 1000 °C (YIGLa10), and 1100 °C (YIGLa11), excited at a fixed frequency of 9.5 GHz

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An adjustment was performed to determine crucial parameters such as resonance field (HR) and linewidth (ΔH). The Lorentzian function [38, 39] was employed for fitting purposes. The spectra depicted in Fig. 7 exhibit dual-resonance curves for each sample; thus, the fitting function is expressed as follows:

$$F\left(H\right)=a+bH+c_1\cdot\frac{{\Delta H}_1}{\left(H-H_{R1}\right)^2+{\Delta H}_1^2}+c_2\cdot\frac{{\Delta H}_2}{\left(H-H_{R2}\right)^2+{\Delta H}_2^2}$$

Here, the terms indexed as i = 1 and 2 correspond to the parameters of the first and second resonance curves, respectively. The “a” and “b” are the fitting parameters, “C” is associated to the peak amplitudes, “HRi” stands for the resonance field, and ΔHi signifies the linewidth. The parameter values resulting from the fitting procedure are documented in Table 2.

In the literature, values of HR approximately around 2.6 kOe are reported for single-phase YIG samples excited at frequencies close to 9.5 GHz [40, 41]. This suggests that the first absorption curve (with a lower resonance field) corresponds to the YIG FMR signal. The second absorption curve (HR2 ≈ 3.7 kOe) is attributed to the FMR signal of the secondary phase, as confirmed by XRD.

Structural analysis using X-ray diffraction (XRD) revealed a single phase for the sample treated at 900 °C and a secondary phase for the samples treated at 1000 °C and 1100 °C. However, FMR analysis demonstrated the presence of the secondary phase in all samples. This discrepancy arises due to the higher sensitivity of magnetic measurements. Furthermore, it can be observed that as the temperature increases, the YIG resonance curve becomes progressively less distinct compared to the secondary phase curve. For the sample treated at 1100 °C, the YIG phase curve becomes almost imperceptible. This indicates an increased formation of the secondary phase as the temperature rises, corroborating the results obtained from XRD.

4 Conclusions

In this study, we investigate the impact of calcination temperature on both structural and magnetic attributes of the Y3Fe4,97La0,03O12 compound. Through X-ray diffraction analysis, we confirm the formation of a single-phase YIG structure at 900 °C, whereas at 1000 °C and 1100 °C, phase separation becomes evident, attributed to the YFeO3 compound. Notably, higher temperatures lead to an increase in average crystalline size, likely influenced by temperature-favored nucleation and growth mechanisms. The samples exhibit agglomerated morphology with irregular elongated shapes. Magnetic characterization unveils temperature-related alterations in the compound’s magnetic moment. At 1000 °C and 1100 °C, the secondary phase introduces additional effects to the compound’s magnetism, leading to variations in saturation magnetization and effective anisotropy field.