Because of the unique magnetic properties, such as small remanence, coercivity, and low intrinsic damping, yttrium iron garnet (YIG) is a significant electronic material used in optical isolators, circulators, modulators, filters, oscillators, and phase shifters, and YIG films have been expected to be the promising material in spintronics studies.^[1–4]

The magnetic properties of YIG strongly depend on the chemical composition and crystal structure, therefore it is important to control the stoichiometric quantities, crystallization temperature, and pH to fabricate the crystals.^[4–11] Several methods are developed to fabricate YIG and its films, such as liquid phase epitaxy,^[5,6] co-precipitation,^[12–14] pulsed laser deposition,^[1,15,16] sol–gel,^[5–8] and metal-organic decomposition (MOD).^[12,17,18] The MOD technique, compared to the others, has a lower synthesis temperature as well as some other advantages: uncomplicated, chemical stable, homogeneous, and inexpensive. It is one promising method to conveniently guarantee the synthesis of fine YIG and its films.

However, the synthesis temperatures ( ) adopted by different research groups using the MOD method to synthesize YIG were quite different,^[14,17] varying from 800 °C to 1200 °C. The magnetic properties of the samples, such as saturation magnetization ( and the coercive field ( ), also changed with . The reason could be that there are YFeO and Fe O as well as YIG (Y Fe O ) in the products synthesized at the lower temperature, and Y Fe O itself has a sophisticated structure, where the ferric ions Fe may be on two kinds of sites with different magnetic properties, the tetrahedral and octahedral sites, noted as Fe (T) and Fe (O), respectively. The only method to determine the ratio of the two different Fe located in YIG is the Mössbauer spectroscopy analysis. However, few articles have been applied Mössbauer spectroscopy elaborately in the study of YIG synthesized by MOD.^[14,17]

In this paper, we investigate the synthesis process of the YIG by the MOD method crystallized at different temperatures from 900 °C to 1300 °C, verify the component and crystal structure of the YIG powders by scanning electron microscope (SEM), XRD and Mössbauer spectroscopy, and measure the samples’ magnetic properties by the vibrating sample magnetometer (VSM). We analyze the ratio of Fe (T) to Fe (O) in YIG as well as the ratio of YFeO and YIG in the samples, and find that the pure garnet phase with outstanding magnetic features, such as the minimum coercivity and maximum saturation magnetization, can be obtained at not lower than 1300 °C by the MOD method. Based on the analysis and discussion, the crystal structure dependence of magnetic properties of the YIG samples is studied thoroughly.

The MOD process began with the preparation of a carboxylates solution which contained stoichiometric composition of Y Fe O . The initial materials were yttrium nitrate (Y(NO 6H O), ferric nitrate (Fe(NO 9H O), ammonium hydroxide, and propanoic acid.

Yttrium and iron were synthesized to a metal-organic compound with a 3:5 molar ratio by two decomposition actions:^[19]

The YIG powders were prepared firstly by drying the MOD solution at 150 °C for 30 hours. Then the samples were pre-crystallized at 750 °C for 3 hours after grinding in the mortar for several minutes. Finally the ten portions of powders were annealed at 750 °C, 850 °C, 900 °C, 1000 °C, 1050 °C,1100 °C, 1150 °C, 1200 °C, 1250 °C, and 1300 °C for 6 hours, respectively.

An SEM Hitachi S4800 was used to study the morphology and microstructure of the ceramic samples. The components and crystal structures of the YIG powders were characterized by XRD (X’TRA). The ratios of the ferric ions on each kind of site were characterized by Fe Mössbauer spectroscopy obtained with a Wissel Mössbauer spectrometer ( Co/Pb) at room temperature. The magnetic properties of the powders were studied using the VSM (lakeshore 7304).

Scanning electron micrographs of the powders by MOD methods calcined at 900 °C, 1100 °C, 1200 °C, and 1300 °C show the change trend of the morphology in Fig. 1. Obviously, the obtained solids below 1300 °C are composed of no regular shaped grains coexisting with prolate spherically shaped particles. The higher leads to the more prolate spherical particles in the samples. It is interesting to note that an almost identical microstructure was observed for the sample synthesized at 1300 °C. Individual particles seem to be a prolate spheroid with a 1 μm diameter and 3 μm length, and partly join each other in Y connection style, which are not like the plate-like crystals synthesized by the sol–gel method.^[14,17]

Fig. 1.

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	Fig. 1. Scanning electron micrographs of Y Fe O synthesized by MOD method at (a) 900 °C, (b) 1100 °C, (c) 1200 °C, and (d) 1300 °C.

The components and crystal structures of the powders sintered at 750 °C–1300 °C are shown in Fig. 2, characterized in the XRD spectra. According to the XRD patterns, PDF #43-0507, #39-1489, and #39-0238, the sample pre-crystallized at 750 °C comprises just the YFeO and Fe O phases, without any phases of Y Fe O . The YIG phase appears in the 900 °C crystallized sample, but its diffraction peaks are weak. Increasing with , the diffraction peaks of the garnet become stronger. Especially, the characteristic peak near 32.3° finally becomes the highest peak as shown in the Fig. 2. While the diffraction peaks of YFeO and Fe O , such as the characteristic peak near 33°, gradually become weaker and finally hard to be recognized when the temperature heightens to 1200 °C. During this process, it is interesting to observe that the colors of garnet samples change from bronzing to khaki and eventually deep green.

Fig. 2.

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	Fig. 2. XRD pattern of the YIG powders sintered at 750 °C, 850 °C, 900 °C, 1000 °C, 1100 °C, 1200 °C, 1300 °C. F, P, and G stood for the Fe O , YFeO , and Y Fe O , respectively. It would be noticed that each peak due to YIG splits into two peaks due to Cu-K and Cu-K .

The crystallization process in MOD has been deduced as the following two steps:

Here, the process at different temperatures is verified from the Fe Mössbauer spectroscopy shown in Fig. 3. The sextet with an internal field ( of 51.59 T belongs to well crystallized α-Fe O . Another sextet with of 49.71 T belongs to perovskite structured yttrium orthoferrite YFeO . The doublet corresponds to small α-Fe O particles, which was born out of the collapse of α-Fe O sextet due to superparamagnetic relaxation.^[22] The total atomic ratio of Fe in both types of α-Fe O is 55% calculated from the fitting parameters (Table 1). As the annealing temperature is elevated to 850 °C, the total atomic ratio of Fe declines to 42%.

Fig. 3.

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	Fig. 3. Mössbauer spectroscopy of sample annealing at (a) 750 °C, (b) 850 °C, (c) 900 °C, (d) 1000 °C, (e) 1100 °C, (f) 1200 °C, (g) 1250 °C, and (h) 1300 °C.

Table 1.

Table 1.

Table 1. The fitting parameters of Mössbauer spectra. . Temperature/°C Sample Area I. S./mm s /T Q. S./mm s /mm s Temperature/°C 750 YIP 0.45 0.19 0.37 49.71 –0.02 0.32 Fe O 0.41 0.20 0.37 51.59 –0.20 0.26 Fe O 0.14 0.14 0.31 0.93 0.65 850 YIP 0.58 0.19 0.37 49.83 –0.01 0.36 Fe O 0.42 0.20 0.37 51.72 –0.21 0.27 900 YIP 0.39 0.19 0.37 49.75 –0.02 0.29 Fe O 0.38 0.19 0.37 51.72 –0.20 0.27 YIG-O 0.08 0.22 0.40 48.92 0.06 0.53 YIG-T 0.15 0.00 0.18 39.70 0.09 0.60 1000 YIP 0.31 0.19 0.37 49.82 –0.02 0.34 Fe O 0.29 0.25 0.43 51.82 –0.22 0.26 YIG-O 0.15 0.34 0.52 48.92 0.06 0.37 YIG-T 0.25 0.03 0.21 39.75 0.10 0.55 1100 YIP 0.22 0.19 0.37 49.71 –0.02 0.26 Fe O 0.19 0.19 0.37 51.66 –0.21 0.24 YIG-O 0.23 0.20 0.37 48.90 0.06 0.40 YIG-T 0.37 –0.03 0.15 39.57 0.07 0.54 1200 YIP 0.05 0.19 0.37 49.71 –0.02 0.17 Fe O 0.10 0.19 0.36 51.69 –0.21 0.21 YIG-O 0.33 0.20 0.38 48.90 0.05 0.38 YIG-T 0.52 –0.03 0.15 39.63 0.04 0.49 1250 YIP 0.01 0.19 0.37 49.71 –0.02 0.07 Fe O 0.09 0.19 0.36 51.61 –0.21 0.21 YIG-O 0.35 0.20 0.38 48.90 0.06 0.38 YIG-T 0.55 –0.03 0.15 39.58 0.03 0.47 1300 YIP 0.01 0.19 0.37 49.71 –0.02 0.08 Fe O 0.06 0.19 0.36 51.70 –0.22 0.18 YIG-O 0.36 0.20 0.38 48.90 0.07 0.38 YIG-T 0.57 –0.03 0.14 39.58 0.02 0.44

Table 1. The fitting parameters of Mössbauer spectra. .

As is elevated to 900 °C, a new sextet with a field of 39.70 T appears in the spectra. Its isomer shift and quadrupole splitting value are identical with the Fe on the tetrahedral sites of the YIG.^[23] In the binary system of Y O –Fe O with a little excess of Y, the reaction product tends to be a mixture of YFeO and YIG instead of non-stoichiometry YIG.^[24] Therefore, we deem that YIG is formed as the sample sintered at 900 °C, as judged also from the XRD pattern where the diffraction peaks of YIG appear in Fig. 2.

The sextet of corresponding Fe ions occupying octahedral sites in YIG has a superposition with the sextet of Fe ions in YFeO . As the octahedrite of FeO in YFeO and YIG have the same structure and much the same bond length,^[25] Mössbauer spectrums of the two types of Fe ions exhibit approaching resonance absorption peaks. Hence, we divided the superposed sextet into two sub-spectrums with of 49.7 T and 48.9 T for YFeO and octahedral sites occupied Fe in YIG as shown in Fig. 3(b).

With the increasing of annealing temperature, the changes of isomer shift, quadrupole splitting value, and hyperfine field indicate that YFeO transforms to YIG. It is clear to see that the YIG sample reaches 95% purity when is 1300 °C. More importantly, the ratio of Fe (O) to Fe (T) within YIG may not strictly meet to 2:3 as a result of a different recoil-free factor at a different lattice, which means the pure YIG powders have the fine crystal structure.

Figure 4 shows the room temperature magnetic hysteresis loops of the powders sintered at different temperatures. The samples sintered at the temperatures lower than 1000 °C are too rough to form the hysteresis loops, so the figure does not include the curves of them. It is clear that the samples are soft magnetism and saturated when the magnetic field is applied over 1000 Oe, therefore the M–H loops have been taken just up to 0.2 T. As seen in Fig. 5, following the , increases from 10.4 emu/g to 29.7 emu/g which is just near to the theoretical value of 30 emu/g, and decreases from 47 Oe to 5.5 Oe. It is easy to understand that these magnetic properties are dependent on the annealing temperature as the compositions and crystal structures are different at various temperatures, which can be comparably seen from the XRD pattern (Fig. 3) and Mössbauer spectroscopy (Fig. 4).

Fig. 4.

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	Fig. 4. Room temperature hysteresis loops of powders sintered at 1000 °C, 1050 °C, 1100 °C, 1150 °C, 1200 °C, 1250 °C, and 1300 °C. The low magnetic field parts of the curves are indicated as an inset figure to make clear the change of coercivity.

Fig. 5.

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	Fig. 5. Saturation magnetization and coercivity dependence of the annealing temperature at 1000 °C,1050 °C, 1100 °C, 1150 °C, 1200 °C, 1250 °C, and 1300 °C.

In Refs. [14] and [17], it had been mentioned that the main reason of the and changing with the annealing temperature was that the particle size became smaller as the temperature was raised. However, based on the measurements and analyses above, we deem that the exact reason is correlated to the morphology, composition, and the crystal structure, especially the iron ions ratio. of YIG powders closely depended on the quantity of Fe ions distributed in the octahedron and tetrahedron, which formed two coupled anti-ferromagnetic structures. The theoretical ratio of the ions on the two sites is 2:3 in the perfect YIG crystal structure, leading to the maximum . The ratio is gradually achieved with the increased, actually revealed from the Mössbauer spectroscopy in Fig. 3. As for the gradually diminishing , it can be explained qualitatively: when the particles become homogeneous prolate spheres and the crystal structure becomes more and more perfect as the goes up as shown in Figs. 1 and 2, the shape-induced anisotropic demagnetizing fields of the grains, as well as crystal defect numbers, porosity, and internal stress in particles, reduce step by step until the pure garnet phase, with the minimum by MOD methods, has been obtained at 1300 °C.

YIG powders are prepared by the MOD method at different crystallized temperatures from 900 °C to 1300 °C. The chemical composition and crystal structure of the samples are studied by SEM, XRD, and Mössbauer spectroscopy. The YIG phase is formed as the sample sintered at 900 °C, and the pure YIG is obtained at the temperature over 1300 °C. It is shown that the ratio of the number of ferric ions on the octahedral sites to those on the tetrahedral sites gradually conforms to the theoretical value 2:3 as the sintering temperature increases. The right iron ions ratio in the YIG sample leads to the smallest magnetic coercivity and highest saturation magnetization, which is significant and useful for the devices research based on pure YIG materials.