Recently, ultrahigh speed and large capacity information transmission system achieved much more attention with the development of the 5th generation communication techniques^[1]. In practical application process, the optical data signal is inevitably distorted due to optical interference and filtering effects^[2]. Therefore, the optical isolators^[3] are needed to achieve unidirectional transport by preventing the reflected light into the laser sources^[4]. Magneto-optic materials is the key material of this kind of nonreciprocal devices^[5⇓⇓-8]. Among many magneto- optical materials, yttrium iron ferrimagnetic garnet crystals (Y₃Fe₅O_12, abbreviated as YIG) are important representative with suitable magnetic and electrical properties^[9-10], that have been widely used as the key components in microwave communications and magneto-optical field due to their huge Faraday rotation angle^[11] and excellent light transmission function in near and mid-infrared wavelength region^[12⇓-14].

So far, the commercial magneto-optic materials are mainly Bi and rare-earth doped YIG single crystal thin films, which is deposited on Gd₃Ga₅O₁₂ (GGG) substrate from the lead-based flux using a liquid phase epitaxy (LPE) technique^[15-16] or pulsed laser deposition^[17-18]. The films may contain some divalent and tetravalent Pb impurities that further affect the magnetic properties^[19]. Moreover, the LPE method requires expensive GGG crystal substrates, and the maximum thickness of the film is only about 1 mm. YIG single crystal can also be grown by Floating Zone method^[20] with the size up to 8 mm in diameter and 50 mm in length^[14]. The fiber crystal has the advantages of good crystallinity and fast growth rate^[21]. However, this growth technique is inappropriate for large-scale production due to high cost and low uniformity^[22]. In this sense, it is of great significance to find a suitable method that can grow large size YIG crystal and realize large-scale productivity^[23].

Top seeded solution growth (TSSG) technique has strong applicability for refractory, volatile and inconsistent melting system^[24]. It has numerous advantages: (1) the growth process can be monitored in real time; (2) the seed growth reduces the nucleation and benefit to obtain large size crystal; (3) the crystal has high uniformity^[25]. Since 1958, Nielsen has begun to grow YIG single crystals from lead flux by molten salt method^[26]. In the following decades, lead-based flux was the main flux due to their low melting point and high fluidity, such as PbO-PbF₂^[27], PbF₂-B₂O₃^[28], PbO-PbF₂-B₂O₃^[29]. However, the crystal grown from the Pb-based flux contains some undesired impurities and intrinsic defects, which severely affect the microwave and magneto-optic properties^[19]. Based on this, the lead-free flux began to attract the attention of researchers, such as BaO-B₂O₃^[30], Na₂O-B₂O₃^[31], BaO-BaF₂-B₂O₃^[32]. In recent years, both the size and quality of YIG crystals have been improved gradually with the advent of the accelerated crucible rotation technique^[33] and modified seeding procedure^[34]. However, the preparation of large size and high quality YIG single crystal remains a huge and longstanding challenge.

In this work, the authors devoted to prepare large size YIG single crystals that could provide garnet crystals of satisfactory quality for commercial applications. The growth, crystal structure and basic properties of YIG single crystals prepared from the B₂O₃-BaF₂ flux were reported detailedly.

The YIG crystals were grown by TSSG technique with B₂O₃-BaF₂ fluxes. Compared with lead-based flux, this lead-free flux has numerous advantages of nontoxicity, high solubility, low solvent incorporation and non-corrosion for platinum crucible^[19]. The high purity starting chemicals (99.99%) of Y₂O₃, Fe₂O₃ and flux were weighed according to the stoichiometric composition of Y₃Fe₅O₁₂, and the optimal molar ratio of solute/flux. The weighed raw materials were thoroughly mixed in tumbling mixer and loaded into platinum crucible, then heated above the saturation temperature to 1200 ℃ for 48 h to ensure homogenization. A columnar [110]-orientated YIG crystal was used to determine the saturation temperature by repeated seeding trials and then grown at a rate of 0.2-0.5 ℃/h. After the growth process, the grown crystal was cooled down to room temperature at a rate of 30 ℃/h. Fig. 1 shows that the as-grown YIG crystals covered with the natural growth faces of (110) and (211) planes, which is mainly because the above-mentioned planes contain two and one easy directions of magnetization, respectively. After exploration and optimization of the growth process conditions, the size and weight of the single crystals were gradually increased, and the maximun size and weight of the as-grown YIG crystal can up to 43 mm×45 mm×11 mm and 60 g, respectively.

The X-ray diffraction (XRD) pattern was obtained by a diffractometer with Cu K_α radiation (Rigaku, Ultima IV, Japan). The structure data was collected on a single crystal X-ray diffractometer (Bruker D8, Mo K_α, λ= 0.071073 nm) at room temperature and the crystal structure was determined using the SHELXL software. The magnetic properties were measured by physical property measurement system (Quantum Design, model 6000, America). The dielectric properties were measured using an Alpha-broadband dielectric spectrometer (Novocontrol GmbH, Germany). The ferromagnetic resonance (FMR) data were obtained on the selected spheres at 5 GHz by a FMR tester. The crystal wafer was doubly- polished to optical grade with the thickness of 486 µm for transmission spectra scheme from 1000 nm to 2300 nm, which was measured by a UV-Vis-nearinfrared spectrophotometer (Perkin Elmer, Lamda 950, America). The X-ray photoelectron spectroscopy (XPS) characterization was performed by an XPS Spectrometer (Thermo Fisher, ESCALAB 250Xi, America).

In the crystal growth process, YFeO₃ phase is easily incorporated with YIG crystal at the condition of high temperature^[35]. Therefore, the high quality YIG seed crystal should be seeded to avoid the occurrence of YFeO₃ phase. Fig. 2(a) shows the XRD pattern of YIG crystal, exhibiting a pure garnet structure without any second phase in the apparatus resolution, which is accordance with the standard card (PDF#43-0507).

The garnet structure of YIG crystal was measured at room temperature, which is shown in Fig. 2(b). The crystal structure shows that YIG belongs to the cubic crystallographic system with a space group of Ia3d. The lattice parameters are a=b=c=1.23790 nm, α=β=γ=90° and Z=8, which is completely consistent with the previous data^[36]. Fig. 2(c, d) show the sublattice structure and oxygen coordination around the Y³⁺ and Fe³⁺ ions in the lattice. Obviously, YIG crystal possesses three sublattices, i.e. Fe³⁺ ions occupied the octahedral A-sites and tetrahedral D-sites, and Y³⁺ ions occupied the dodecahedral C-sites^[37]. The dodecahedral coordination Y³⁺ ions are diamagnetic and do not contribute to the optical or magnetic properties^[38]. Nevertheless, Fe³⁺ ions at A-sites and D-sites sublattices surrounded by oxygen ions are separated, i.e. the spins of two Fe³⁺ ions at A-sites are antiparallel to that of three Fe³⁺ ions at D-sites, forming a net magnetization and Faraday rotation through the ferromagnetically coupled interaction^[39-40]. The magnetic and magneto-optic properties of the as-grown YIG crystal are elaborated below.

For magnetic materials, the saturation magnetization (M_S), Curie temperature (T_C), dielectric constant (ε') and ferromagnetic resonance (FMR) linewidth are considered important indicators in the applications of linear and nonlinear microwave devices. Fig. 3(b) exhibits the magnetization-field (M-H) curve of YIG crystal at room temperature with the saturation magnetization of 25.43 emu/g (1 emu=10^-3 A/m²), showing a typical ferromagnetic characteristic. Fig. 3(a) shows the magnetic hysteresis loop in the magnetic field intensity of -1000- 1000 Oe to facilitate the observation of hysteresis. The coercive field (H_c) of YIG crystal is 8.61 Oe, and the remnant magnetization (M_r) is 0.84 emu/g. It is concluded that the coercive field of YIG crystal is small enough, indicating that the saturation magnetization can be achieved only with a small external magnetic field, which significantly reducing the requirements for external magnetic field in magneto-optic applications. Fig. 3(c) depicts the temperature-dependent magnetization for an applied magnetic field of 1000 Oe for YIG crystal. The ferrimagnetic to paramagnetic transition temperature (T_C) was about 560 K, which is comparable to the previous reported data^[38,41].

Fig. 4(a, b) depicts the temperature-dependent dielectric constant (ε') and dielectric loss (tanδ) at various frequencies of 10 Hz-1 MHz. Obviously, the temperature- dependent ε' exhibits very high frequency stability below 100 ℃, and tanδ at 1 MHz is only about 8.9×10^-4. This is completely different from the Lu₃Fe₅O₁₂^[42] and Ho₃Fe₅O₁₂^[43] system, which exhibiting the typical relaxor behavior.

It is well known that the impurities and intrinsic defects severely affect the microwave losses^[44]. Therefore, the quality of the YIG crystal with regard to microwave applications was measured using the FMR tester, which were obtained on the selected spheres at 5 GHz. The small YIG resonator spheres with the diamater of 0.5 mm are shown in Fig. 4(c). The FMR linewidth (ΔH) of the YIG spheres (signed by sample S1-S6) and the average ΔH are shown in Fig. 4(c, d). The values of ΔH reflect the presence of the fluctuations for the six sphere samples, which is mainly attribute to the sphericity or the crystal defects. As a result, the average ΔH is 0.679 Oe, which is comparable to the previous reported YIG crystal^[45].

The YIG wafers were polished to optical grade with no anti-reflective coating for transmission spectrum measurement in the near infrared region of 980 to 2300 nm, which is shown in Fig. 5. Obviously, it possesses an intense absorption edge from 980 nm which influence extends out to about 1180 nm, and then the optical absorption becomes extremely lower in the wavelength of ~1180 nm. It was more significant that the transmittance at 1310 and 1550 nm are both higher than 75%, exhibiting a high level of transparency. According to the previous reports, the ideal transparency for pure YIG in the near infrared region is about 75%^[46]. The results showed that the transmittance of the as-grown YIG crystal is very close to the theoretical value, which is consistent with the previously data.

In addition, the Faraday rotation angle of YIG crystal at the wavelength of 1310 and 1550 nm were 200 and 160 (°)·cm^-1, respectively, which is lower than the previously reported data (~220 (°)·cm^-1@1310 nm)^[47-48]. It is well known that the Faraday rotation effect is mainly attribute to the contribution of octahedral ferric ions^[38]. However, the Fe³⁺ ions are easily reduced to Fe²⁺ ions in a low oxygen atmosphere^[49] and the presence of Fe²⁺ ions will increase the optical absorption coefficient and weaken the Faraday rotation effect^[50]. Therefore, we can speculate that the YIG crystal under the growth condition of a low oxygen partial pressure should contain large amounts of Fe²⁺ ions.

In order to identify the above speculation, the XPS technique was employed to check the valence of Fe and O element. Fig. 6 exhibits the X-ray photoelectron spectrum of Fe2p and O1s lines and their Lorentzian- Gaussian dividing results of YIG crystal powders. As shown in Fig. 6(a), the Fe2p core level splits into 2p_1/2 and 2p_3/2 components binding energy of around 724.6 and 710.9 eV, respectively. However, the peaks for Fe²⁺ and Fe³⁺ ions exhibit a mixed peak and can be fitted by the Lorentzian-Gaussian fitting method^[51]. The fitting result show that the binding energy of Fe2p_1/2 is expected to be 725.23 eV for Fe³⁺ and 723.93 eV for Fe²⁺, and the binding energy of Fe2p_3/2 is expected to be 711.73 eV for Fe³⁺ and 710.38 eV for Fe^2+[39]. Therefore, it is confirmed that the oxidation state of Fe ions in the as-grown YIG crystal are in a mixed Fe³⁺ and Fe²⁺ states^[52]. The percentage of Fe²⁺ can be estimated by peak area according to the fitting curves, and the result shows that Fe²⁺ accounts for about 54.59% in octahedral A-sites and tetrahedral D-sites.

In addition, the valence fluctuation of Fe ions is always accompanied by the generation of oxygen vacancies. Fig. 6(b) shows the O1s spectrum of YIG crystal powders, which can be also splitted into two peaks. According to the fitting curves, the binding energy around 529.98 eV (O_a) represents the oxygen in the lattice, while the binding energy around 532.18 eV (O_b) is related to the oxygen vacancies. The percentage of oxygen vacancies can also be estimated by the integral area of the O_b peak, and the oxygen vacancies accounts for about 63.73%. The presence of Fe²⁺ valence state and oxygen vacancies must attribute to the low oxygen atmosphere at high- temperature growth process, which seriously increases the optical absorption coefficient and weaken the Faraday rotation effect. Therefore, the YIG crystal should be grown in oxygen atmosphere or nitrogen atmosphere in order to avoid generating the defects of Fe²⁺ and oxygen vacancies, and this will be a major focus in our future work.

Until now, there are many reports on the growth of YIG single crystals, but they are usually small in size and thickness. In this paper, we reported the growth optimization conditions for large-size YIG crystal by TSSG technique from lead-free B₂O₃-BaF₂ flux. The size and weight of the YIG crystal achieved 43 mm×46 mm× 11 mm and 60 g with the natural growth faces of (110) and (211) planes. The crystals exhibited excellent purity, satisfactory quality and excellent comprehensive performances with the narrow ferromagnetic resonance linewidth (0.679 Oe), high transparency (75%) and Faraday rotation angle (200 (°)·cm^-1@1310 nm and 160 (°)·cm^-1@1550 nm), which makes it a promising candidate for microwave and magneto-optic device applications. XPS result shows that the as-grown YIG crystals possess some Fe²⁺ and oxygen vacancies, which must be originated from the low oxygen atmosphere in the high-temperature growth process, and this problem may be solved by introducing the oxygen atmosphere or nitrogen atmosphere.