Terbium gallium garnet crystal and growth method thereof
By using fluoride inhibitors and specific atmosphere control during the growth of terbium gallium garnet crystals, the problems of composition deviation and defects caused by Ga2O3 volatilization were solved, and the growth of high-quality terbium gallium garnet crystals was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-23
- Publication Date
- 2026-03-31
AI Technical Summary
In the prior art, the volatility of Ga2O3 during the growth of terbium gallium garnet crystals leads to compositional deviation, increased crystal defects, and abnormal growth morphology, affecting the integrity of the crystal structure and optical uniformity.
Terbium fluoride and/or gallium fluoride are used as inhibitors, combined with a mixed atmosphere of nitrogen and oxygen and appropriate gas pressure, to control the volatilization of Ga2O3 in high-temperature melt, generate a low-volatility mesophase or a stable fluorogallate complex, maintain stoichiometric stability and optimize the physical properties of the melt.
It significantly alleviates structural defects and growth anomalies caused by component deviation, improves the quality and optical uniformity of terbium gallium garnet crystals, and enhances crystal density and utilization.
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Figure CN121760047A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of crystal preparation technology, and more specifically, to a terbium gallium garnet crystal and its growth method. Background Technology
[0002] Terbium gallium garnet (TGG) has the chemical formula Tb3Ga5O. 12 Magneto-optical crystal (MAC) is an important magneto-optical crystal material widely used in key optical components such as Faraday isolators and circulators in high-power laser systems. Its excellent magneto-optical properties, high thermal conductivity, and good chemical stability make it one of the irreplaceable core materials in current medium- and high-power laser systems.
[0003] In existing technologies, TGG crystals are typically grown using the Czochralski method. The raw material synthesis generally involves combining high-purity terbium oxide (Tb₄O₇) and gallium oxide (Ga₂O₃) in a process called Tb₃Ga₅O₇. 12 After precise weighing and thorough mixing of the stoichiometric proportions, a bulk polycrystalline material was synthesized through a solid-state reaction at high temperature. Subsequently, the polycrystalline material was placed in an iridium crucible and then placed in a Czochralski furnace for melting and single-crystal pulling growth.
[0004] However, in actual crystal growth, Ga2O3 exhibits significant volatility in its high-temperature molten state. Even with the pre-addition of excess Ga2O3 to the raw material ratio to compensate for its loss, it is still difficult to effectively suppress its continuous volatilization in the high-temperature environment. This volatilization behavior leads to a series of serious problems: 1) Composition deviation: The Ga content in the melt continuously decreases, causing the stoichiometric ratio at the actual growth interface to deviate from Tb3Ga5O 12 The ideal ratio of Ga2O3 to Ga2O3 affects the integrity of the crystal structure; 2) an increase in crystal defects, including bubbles, inclusions, increased dislocation density, and even cracking; 3) abnormal growth morphology, manifested as obvious eccentric growth, spiral growth stripes, etc., which seriously affect the optical uniformity and processing utilization of the crystal. Although some studies have attempted to alleviate the volatilization problem by optimizing the furnace atmosphere (such as introducing a small amount of O2 to suppress Ga2O3 decomposition), improving the crucible lid design, or using a closed growth system, the effects have been limited. Summary of the Invention
[0005] The purpose of this application is to provide a terbium gallium garnet crystal and a growth method thereof, which can improve the crystal quality of the terbium gallium garnet crystal.
[0006] The embodiments of this application are implemented as follows: A first aspect of this application provides a method for growing terbium gallium garnet crystals, comprising: weighing terbium oxide and gallium oxide in a molar ratio of 1:2-2.4 and mixing them to form a powder mixture; heating the powder mixture to pre-synthesize it into a block mixture; using terbium fluoride and / or gallium fluoride as an inhibitor, placing the block mixture and the inhibitor into a Czochralski furnace for Czochralski growth to generate terbium gallium garnet crystals.
[0007] As one possible implementation, when the block mixture and inhibitor are placed in a Czochralski furnace for Czochralski growth, the growth atmosphere is nitrogen and oxygen.
[0008] As one possible implementation, when the blocky mixture and the inhibitor are placed in a Czochralski furnace for Czochralski growth, the gas pressure inside the Czochralski furnace is 1.1-2 atm.
[0009] As one possible implementation method, the flow ratio of nitrogen to oxygen is between 1:2 and 4.
[0010] As one possible implementation method, when pre-synthesizing a powder mixture by heating it to obtain a block mixture, the heating temperature is between 1500-1700℃ and the mixing time is between 4-10 hours.
[0011] As one possible implementation method, when the inhibitor is terbium fluoride, the mass ratio of terbium fluoride to the bulk mixture is 1-5:100.
[0012] As one possible implementation method, when the inhibitor is gallium fluoride, the mass ratio of gallium fluoride to the bulk mixture is 1-5:100.
[0013] As one possible implementation, when the inhibitor is terbium fluoride and gallium fluoride, the mass ratio of terbium fluoride, gallium fluoride and the bulk mixture is 1-5:1-5:100.
[0014] As one possible implementation method, terbium oxide and gallium oxide are weighed separately according to a molar ratio of 1:2-2.4 and mixed to form a powder mixture, including: weighing terbium oxide and gallium oxide separately; putting terbium oxide and gallium oxide into a mixer and mixing for 10-20 hours, with the mixer speed being 10-20 rpm.
[0015] A second aspect of this application provides a terbium gallium garnet crystal, which is prepared using the above-described terbium gallium garnet crystal growth method.
[0016] The beneficial effects of the embodiments of this application include: The method for growing terbium gallium garnet crystals provided in this application includes: weighing terbium oxide and gallium oxide in a molar ratio of 1:2-2.4 and mixing them to form a powder mixture; pre-synthesizing the powder mixture by heating to obtain a bulk mixture; using terbium fluoride and / or gallium fluoride as an inhibitor, placing the bulk mixture and the inhibitor into a Czochralski furnace for Czochralski growth to generate terbium gallium garnet crystals. During the Czochralski growth process, in the high-temperature melt formed, fluoride ions (F... - It can be used with Ga³ + or Tb³ + The fluorides combine to form intermediate phases such as GaOF and TbOF, or stable fluorogallium salt complexes. These substances have significantly lower volatility than pure Ga₂O₃, thereby reducing the overall vapor pressure of the Ga component and dynamically maintaining a melt composition close to the ideal stoichiometry during crystal growth. Furthermore, the fluorides not only suppress the continuous volatilization of Ga₂O₃, but also improve the physical properties of the melt, such as reducing viscosity and optimizing solid-liquid interface stability, resulting in a smoother and more uniform crystal growth front. Combined with the suppression of Ga₂O₃ volatilization, this significantly alleviates structural defects and growth anomalies caused by compositional deviations, thus improving crystal quality. Therefore, the terbium gallium garnet preparation method provided in this application can improve the crystal quality of terbium gallium garnet crystals. Attached Figure Description
[0017] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.
[0018] Figure 1 A flowchart illustrating the preparation method of terbium gallium garnet crystals provided in this application embodiment; Figure 2 Images of finished terbium gallium garnet crystals provided in embodiments of this application; Figure 3 Absorption curves of terbium gallium garnet crystals provided in embodiments of this application; Figure 4 The transmittance curve of the terbium gallium garnet crystal provided in the embodiments of this application. Detailed Implementation
[0019] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. The described embodiments are only some embodiments of this application, not all embodiments. Similar reference numerals and letters in the following drawings indicate similar items. Once an item is defined in one drawing, it does not need to be further defined in other drawings.
[0020] The terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product is in use. They are used only for the convenience of describing this application and should not be construed as limiting this application. The terms "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.
[0021] Unless otherwise expressly specified and limited, the terms "set up," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to connections within two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.
[0022] Please refer to the reference. Figure 1 This application provides a method for growing terbium gallium garnet crystals, including: S100: Weigh terbium oxide and gallium oxide separately according to a molar ratio of 1:2-2.4, and mix them to form a powder mixture; The chemical formulas for terbium oxide (Tb₄O₇), gallium oxide (Ga₂O₃), and terbium-gallium garnet (Tb₃Ga₅O₇) are: Tb₄O₇, ... and Tb₃Ga₅O₇. 12 When the molar ratio of terbium oxide to gallium oxide is 1:2-2.4, terbium gallium garnet can be synthesized in subsequent processes.
[0023] For example, 4-5 kg of terbium oxide and 2-3 kg of gallium oxide can be weighed.
[0024] S200: Pre-synthesize a powder mixture by heating to obtain a block mixture; Heating the above powder mixture to carry out a solid-phase reaction (pre-synthesis) generates a dense, blocky precursor, which helps to ensure uniformity of subsequent melting.
[0025] S300: Terbium fluoride, and / or gallium fluoride as an inhibitor, the bulk mixture and the inhibitor are placed in a Czochralski furnace for Czochralski growth to produce terbium gallium garnet crystals.
[0026] Specifically, the bulk mixture and inhibitor can be added to an iridium crucible, which is then placed into a Czochralski furnace. The control parameters are adjusted to grow the Czochralski crystal.
[0027] More specifically, the pre-synthesized bulk mixture is loaded into a crucible together with terbium fluoride (TbF3) and / or gallium fluoride (GaF3) as inhibitors, and terbium gallium garnet crystals are grown by the Czochralski method.
[0028] During the Czochralski process, fluoride ions (F) form in the high-temperature melt. - It can be used with Ga³ + or Tb³ + The combination generates intermediate phases such as GaOF and TbOF, or forms stable fluorogallate complexes. These substances have significantly lower volatility than pure Ga2O3, thereby reducing the overall vapor pressure of the Ga component and dynamically maintaining a melt composition close to the ideal stoichiometry during crystal growth.
[0029] Furthermore, fluorides not only suppress the continuous volatilization of Ga2O3, but also improve the physical properties of the melt, such as reducing viscosity and optimizing solid-liquid interface stability, making the crystal growth front smoother and more uniform. Combined with the suppression of Ga2O3 volatilization, this significantly alleviates structural defects and growth anomalies caused by compositional deviations, thereby improving crystal quality. Therefore, the terbium gallium garnet preparation method provided in this application can improve the crystal quality of terbium gallium garnet crystals.
[0030] Optionally, when the block mixture and inhibitor are placed in a Czochralski furnace for Czochralski growth, the growth atmosphere is nitrogen and oxygen.
[0031] In this embodiment, a mixed atmosphere of nitrogen and oxygen is used as the growth environment. On the one hand, the introduction of oxygen helps to suppress the reduction and decomposition of Ga2O3 at high temperatures and slows down its volatilization tendency; on the other hand, nitrogen, as an inert diluent gas, can regulate the total pressure in the furnace and stabilize the gas phase composition, avoiding excessive local oxidation or drastic atmospheric fluctuations that could disturb the crystal growth interface.
[0032] In the presence of fluorine inhibitors, a moderate oxygen partial pressure helps maintain the stability of the fluorine oxide mesophase (such as GaOF and TbOF), thereby more effectively inhibiting the volatilization of Ga by fluorides. Therefore, the synergistic effect of nitrogen-oxygen mixed atmosphere and fluoride inhibitors further ensures the long-term stability of the melt composition, thereby improving the quality of terbium gallium garnet crystals and reducing defects.
[0033] As an feasible approach, when placing the block mixture and inhibitor into a Czochralski furnace for Czochralski growth, the gas pressure inside the Czochralski furnace is 1.1-2 atm.
[0034] In this embodiment, the gas pressure inside the Czochralski furnace is set between 1.1 and 2 atm. Pressurization can further suppress the volatilization of gallium oxide in the high-temperature molten state. Specifically, increasing the ambient gas pressure can directly reduce the vapor escape rate of volatile components. In addition, applying a moderate positive pressure on the basis of a nitrogen and oxygen mixed atmosphere can effectively maintain the gas phase balance on the melt surface and reduce the component deviation caused by the continuous loss of Ga2O3.
[0035] In addition, an environment slightly above atmospheric pressure helps to compress the tiny bubbles that may form in the melt, reducing the probability of the formation of pores and inclusions inside the crystal, thereby improving the density and optical uniformity of the crystal and enhancing its quality.
[0036] Optionally, the flow ratio of nitrogen to oxygen is between 1:2 and 4.
[0037] During the Czochralski growth process, the nitrogen to oxygen flow ratio is controlled between 1:2 and 4, which ensures a high proportion of oxygen. This high proportion of oxygen helps maintain the oxidation state of Ga2O3 and effectively inhibits its volatilization due to reduction or thermal decomposition at high temperatures. Meanwhile, an appropriate amount of nitrogen, as an inert carrier gas, not only dilutes the reaction atmosphere and prevents local over-oxidation, but also stabilizes the gas flow in the furnace and reduces disturbance to the surface of the melt.
[0038] In the presence of fluoride inhibitors (such as TbF3 or GaF3), a moderately oxygen-rich environment is conducive to the formation of stable fluoride oxide mesophases (such as GaOF and TbOF). The mesophase has low volatility and high melting point, which allows gallium components to remain in the melt and further slows down Ga loss.
[0039] As an feasible method, when pre-synthesizing a powder mixture by heating it to obtain a block mixture, the heating temperature is between 1500-1700℃ and the mixing time is between 4-10 hours.
[0040] Specifically, the powder mixture is kept at a high temperature of 1500–1700℃ for 4–10 hours to cause a solid-phase reaction, generating a dense, uniformly composed block precursor, i.e., a block mixture.
[0041] Prolonged high temperatures can promote the full reaction of raw materials, initially forming a polycrystalline phase with a structure close to terbium gallium garnet, providing stoichiometrically stable and low-volatility molten raw materials for subsequent Czochralski growth.
[0042] Specifically, within a temperature range of 1500–1700℃, diffusion and solid-state reactions occur between oxide particles, gradually forming the crystalline phase of garnet or its precursor intermediate phase. This temperature range is higher than the sintering initiation temperature of terbium gallium garnet crystals but lower than its complete melting point (approximately 1800℃), ensuring sufficient reaction while avoiding premature melting that could lead to localized compositional inhomogeneity or excessive volatilization of Ga2O3. A holding time of 4–10 hours ensures reaction depth and uniformity, resulting in highly homogeneous internal composition, reduced porosity, and increased density in the bulk product.
[0043] Optionally, when the inhibitor is terbium fluoride, the mass ratio of terbium fluoride to the bulk mixture is 1-5:100.
[0044] As an feasible approach, when the inhibitor is gallium fluoride, the mass ratio of gallium fluoride to the bulk mixture is 1-5:100.
[0045] Optionally, when the inhibitor is terbium fluoride and gallium fluoride, the mass ratio of terbium fluoride, gallium fluoride and the bulk mixture is 1-5:1-5:100.
[0046] When the inhibitors are terbium fluoride and gallium fluoride, the volatilization of components during high-temperature melting is jointly suppressed from both ends of the terbium source and the gallium source through a dual-element synergistic regulation mechanism. In particular, it addresses the problem that Ga2O3 is extremely volatile at about 1800℃, and achieves high-precision maintenance of the stoichiometry of the melt.
[0047] As one feasible method, terbium oxide and gallium oxide are weighed out separately at a molar ratio of 1:2-2.4 and mixed to form a powder mixture, including: S110: Weigh out terbium oxide and gallium oxide respectively; S120: Place terbium oxide and gallium oxide into a mixer and mix for 10-20 hours. The speed of the mixer is 10-20 rpm.
[0048] Mixing with a feeder for 10-20 hours significantly improves the uniformity of raw material mixing and reduces localized Tb- or Ga-rich areas. Furthermore, the feeder's rotation speed of 10-20 rpm achieves highly uniform micro-mixing while avoiding the introduction of impurities, lattice damage, or localized agglomeration caused by high-speed stirring. Prolonged low-speed mixing allows oxide particles of different sizes and densities to be fully dispersed, forming a homogeneous powder mixture, laying the foundation for the uniformity and completeness of subsequent pre-synthesis reactions.
[0049] The mixer can be a drum mill or a planetary ball mill, without grinding media or using a low-wear method.
[0050] To further verify the beneficial effects of the embodiments of this application, this application provides comparative examples and four embodiments, the specific information of which is as follows: Comparative example: Weigh 4-5 kg of Tb4O7 and 2-3 kg of Ga2O3 powdered raw materials, put them into a mixer (10-20 rpm) and mix for 10-20 hours to form a powdered mixture; pre-synthesize the powdered mixture at 1500-1700℃ (4-10 hours) to obtain a blocky mixture; add 6-8 kg of the blocky mixture into an iridium crucible, put the iridium crucible into a Czochralski furnace, introduce N2 and O2 (ratio 1:2-4), maintain the total pressure at 1.1-2 atmospheres, adjust the growth parameters, the terbium gallium garnet crystals obtained by growth have poor crystal quality, obvious defects, spiral growth, and low utilization rate.
[0051] Example 1: Weigh 4-5 kg of Tb4O7 and 2-3 kg of Ga2O3 powdered raw materials, put them into a mixer (10-20 rpm) and mix for 10-20 hours to form a powdered mixture; pre-synthesize the powdered mixture at 1500-1700℃ (in air) for 4-10 hours to obtain a blocky mixture. Take 6-8 kg of the blocky mixture and add it to an iridium crucible, along with 1-5% of TbF3 and 1-5% of GaF3 granular crystals by mass of the blocky mixture. Place the iridium crucible into a Czochralski furnace, introduce N2 and O2 (ratio 1:2-4), maintain the total pressure at 1 atmosphere, adjust the growth parameters, and the resulting terbium gallium garnet crystal has average crystal quality and utilization rate.
[0052] Example 2: Weigh 4-5 kg of Tb4O7 and 2-3 kg of Ga2O3 powdered raw materials, put them into a mixer (10-20 rpm) and mix for 10-20 hours to form a powdered mixture; pre-synthesize the powdered mixture at 1500-1700℃ (in air) for 4-10 hours to obtain a blocky mixture. Take 6-8 kg of the blocky mixture and add it to an iridium crucible, along with 1-5% of the total mass of GaF3 granular crystal material. Place the iridium crucible into a Czochralski furnace, introduce N2 and O2 (ratio 1:2-4), maintain the total pressure at 1.1-2 atmospheres, and adjust the growth parameters. The resulting terbium gallium garnet crystal is of generally poor quality, with obvious defects and low utilization rate.
[0053] Example 3: Weigh 4-5 kg of Tb4O7 and 2-3 kg of Ga. -2O3 powdered raw material is placed in a mixer (10-20 rpm) and mixed for 10-20 hours to form a powder mixture. The powder mixture is then pre-synthesized at 1500-1700℃ (in air) for 4-10 hours to obtain a block mixture. 6-8 kg of the block mixture is added to an iridium crucible, along with 1-5% of TbF3 granular crystal material by mass of the block mixture. The iridium crucible is placed in a Czochralski furnace, and N2 and O2 (ratio 1:2-4) are introduced. The total pressure is maintained at 1.1-2 atmospheres. The growth parameters are adjusted. The resulting terbium gallium garnet crystal is of generally poor quality, with obvious defects and low utilization rate.
[0054] Example 4: Weigh 4-5 kg of Tb4O7 and 2-3 kg of Ga2O3 powdered raw materials, put them into a mixer (10-20 rpm) and mix for 10-20 hours to form a powdered mixture; pre-synthesize the powdered mixture at 1500-1700℃ (in air) for 4-10 hours to obtain a blocky mixture. Take 6-8 kg of the blocky mixture and add it to an iridium crucible, along with 1-5% of TbF3 and 1-5% of GaF3 granular crystals by mass of the blocky mixture. Place the iridium crucible into a Czochralski furnace, introduce N2 and O2 (ratio 1:2-4), maintain the total pressure at 1.1-2 atmospheres, adjust the growth parameters, and grow terbium gallium garnet crystals with no obvious defects, no spiral growth, high crystal quality, and high utilization rate.
[0055] To provide a clear understanding of the specifics of each embodiment, the details of each embodiment are summarized in Table 1: Table 1 Parameter Table of Embodiments
[0056] In Example 4, the performance of terbium gallium garnet was tested. Figure 2 The image shown is of terbium gallium garnet from Example 4. As can be seen from the image, there are no obvious defects, no spirals, and the crystal quality is high. Figure 3 The absorption curve for terbium gallium garnet is shown, with the highest absorption being only 2250 ppm / cm. Figure 4 The transmittance curve for terbium gallium garnet (TGG) is shown, with a transmittance exceeding 80%, indicating excellent crystal quality. The TGG crystal grown in Example 4 has a Field constant of 40.1 rad / T·m at 1064 nm. Furthermore, the actual crystal utilization rate is an important indicator of crystal growth. Table 1 compares the utilization rates of TGG crystal slices grown using different methods. The TGG crystal grown in Example 4 has a significantly higher utilization rate than other methods, exhibiting the best crystal quality.
[0057] This application also provides a terbium gallium garnet crystal, prepared using the above-described terbium gallium garnet crystal growth method. The terbium gallium garnet crystal provided in this application embodiment is described in detail and will not be repeated here.
[0058] The above description is merely an optional embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.
[0059] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this application will not describe the various possible combinations separately.
Claims
1. A method for growing terbium gallium garnet crystals, characterized in that, include: Terbium oxide and gallium oxide were weighed out in a molar ratio of 1:2-2.4 and mixed to form a powder mixture. The powder mixture is heated to pre-synthesize it into a block mixture; Terbium fluoride, and / or gallium fluoride, as an inhibitor, are used to place the bulk mixture and the inhibitor into a Czochralski furnace for Czochralski growth to produce terbium gallium garnet crystals.
2. The method for growing terbium gallium garnet crystals according to claim 1, characterized in that, When the blocky mixture and the inhibitor are placed in a Czochralski furnace for Czochralski growth, the growth atmosphere is nitrogen and oxygen.
3. The method for growing terbium gallium garnet crystals according to claim 2, characterized in that, When the block mixture and the inhibitor are placed in a Czochralski furnace for Czochralski growth, the gas pressure inside the Czochralski furnace is 1.1-2 atm.
4. The method for growing terbium gallium garnet crystals according to claim 2, characterized in that, The flow rate ratio of nitrogen to oxygen is between 1:2 and 4.
5. The method for growing terbium gallium garnet crystals according to claim 1, characterized in that, When the powder mixture is heated to pre-synthesize into a block mixture, the heating temperature is between 1500-1700℃ and the mixing time is between 4-10 hours.
6. The method for growing terbium gallium garnet crystals according to claim 1, characterized in that, When the inhibitor is terbium fluoride, the mass ratio of terbium fluoride to the block mixture is 1-5:
100.
7. The method for growing terbium gallium garnet crystals according to claim 1, characterized in that, When the inhibitor is gallium fluoride, the mass ratio of gallium fluoride to the bulk mixture is 1-5:
100.
8. The method for growing terbium gallium garnet crystals according to claim 1, characterized in that, When the inhibitor is terbium fluoride and gallium fluoride, the mass ratio of terbium fluoride, gallium fluoride and the bulk mixture is 1-5:1-5:
100.
9. The method for growing terbium gallium garnet crystals according to claim 1, characterized in that, The step of weighing terbium oxide and gallium oxide in a molar ratio of 1:2-2.4 and mixing them to form a powder mixture includes: Weigh out terbium oxide and gallium oxide respectively; Terbium oxide and gallium oxide are placed in a mixer and mixed for 10-20 hours at a speed of 10-20 rpm.
10. A terbium gallium garnet crystal, characterized in that, It is prepared using the growth method of terbium gallium garnet crystal according to any one of claims 1 to 9.