JP5568073B2 - Bismuth-substituted rare earth iron garnet crystal, method for producing the same, and Faraday rotator - Google Patents

Description

  The present invention relates to a bismuth-substituted rare earth iron garnet crystal used in magneto-optical elements such as optical isolators, optical circulators, and optical attenuators, and a method for producing the same.

  For magneto-optical elements such as optical isolators, bismuth-substituted rare earth iron garnet crystals grown on substrate crystals by liquid phase epitaxy have been used in the past, but in garnet crystals, flux components used during production As a result, lead was mixed from lead oxide.

  In recent years, environmental regulations have become stricter. For example, the regulation of lead content is virtually non-compliant with the RoHS Directive “European Parliament and Council Directive on Restriction of Use of Specific Hazardous Substances in Electrical and Electronic Equipment”. It is required to be 1000 ppm or less, which is considered to be contained.

  Therefore, as a method for solving this problem, a method of reducing the molar concentration of lead oxide constituting the melt (Patent Document 1) has been proposed. However, the melt is constituted by reducing the molar concentration of lead oxide. The density | concentration of bismuth oxide increases and the fault that the melt | dissolution of platinum generally used as a crucible material progresses.

The fact that platinum is dissolved in the melt means that not only the crucible material is thinned, but platinum becomes Pt ⁴⁺ and is mixed into the garnet single crystal, and the valence of iron ions that should originally be Fe ^{3+ It} is changed to an ion species having light absorption in the wavelength region used by the Faraday element such as ²⁺ (Problem 1). Furthermore, since Pt ⁴⁺ enters the [a] site of the garnet single crystal, increasing the saturation magnetization, and adding a large amount of nonmagnetic elements to suppress this causes the disadvantage that the Faraday rotation ability is reduced (Problem 2).

  As a method for preventing the problem caused by melting of the platinum crucible, for example, Patent Document 2 proposes to use gold as a crucible material. However, the melting point of gold is 1063 ° C., which is close to about 1000 ° C., which is adopted as a temperature at which the crystal components are completely dissolved, so that there is a disadvantage that the crucible is deformed or sometimes broken. If the temperature at which this crystal component is completely dissolved is 1000 ° C. or less in order to avoid damaging the crucible, the crystal component cannot be completely formed into a garnet structure, so that “foreign matter” is generated and grown. There has been a problem that the crystallinity of the bismuth-substituted rare earth iron garnet crystal is deteriorated, the crystal is easily broken, and the quenching performance of the grown crystal is deteriorated.

JP 2007-165668 A JP 2008-237776 A JP 2011-011944 A

In order to suppress light absorption in the wavelength range used in the Faraday element of Problem 1, a small amount of PbO may be added, or a Group I element such as Na or a Group II element such as Mg may be added in the form of an oxide. (Patent Documents 2 and 3). However, since Fe ²⁺ generated in the melt is small, and Fe ²⁺ is generated one after another due to dissolution of the platinum crucible and the platinum jig even while the garnet epitaxial film is grown, PbO, NaOH, MgO to be added It is almost impossible to determine the amount, and as a result, it has been difficult to stably suppress the light absorption of the Faraday element for optical communication that requires a thick film of 300 μm or more. In addition, handling PbO and NaOH, which are regarded as harmful and deleterious substances, involves danger, and there is a demand to avoid them if possible.

Next, Problem 2 will be described in detail.
The bismuth-substituted rare earth iron garnet crystal has a garnet crystal structure, and this crystal structure is composed of three types of cation sites represented by {c}, [a], and (d), and the {c} site is a formula of the composition formula The amount is 3.0, the [a] site is 2.0 in the formula of the composition formula, the (d) site is 3.0 in the formula of the composition formula, and the bismuth element and the rare earth element are located at the {c} site. Fe elements are located at the [a] and (d) sites and are antiferromagnetically coupled. When [a], (d) sites are all occupied by Fe element, [a] site contains 2.0 Fe element, and (d) site contains 3.0 elemental Fe element. Because of the antiferromagnetic coupling, a magnetic moment of 3.0 to 2.0, that is, 1.0 as a subtraction amount, is related to the magnitude of the saturation magnetization. Note that the Faraday rotation ability, which is the ability to rotate the polarization plane of light, involves an Fe element of 5.0 in the formula amount regardless of the site.

As described above, increasing the amount of bismuth oxide increases the amount of Pt ⁴⁺ in the melt, and this Pt ⁴⁺ enters the [a] site in the bismuth-substituted rare earth iron garnet crystal. ] The magnetic moment applied at the site increases, that is, the saturation magnetization increases. Bismuth-substituted rare earth iron garnet crystals are used as a Faraday rotator in a magnetically saturated state with a magnet arranged around it, so that the magnetic field for saturation increases due to the shape of the magnet arranged around it. Contrary to the downsizing that is a market requirement, this is not preferable.
For this reason, in general, by adding a Ga element that is similar to Fe ions but has a slightly smaller ion radius, the saturation magnetization is lowered mainly by substituting the Fe element at the (d) site. Done. However, since Ga ions replace the Fe element at the [a] site, the amount of Ga element added increases, and an increase in the amount of addition leads to dilution of the amount of Fe element, resulting in degradation of the Faraday rotation capability. Degradation of Faraday rotatory power increases the thickness required to rotate the polarization plane generally required for a Faraday rotator by 45 degrees, that is, increases the thickness of the epitaxial film of the bismuth-substituted rare earth iron garnet crystal. Therefore, crystal growth becomes difficult.

  As a method for suppressing the degradation of the Faraday rotation ability, in Patent Document 3, a divalent Ca element is added, and an element having a relatively small ionic radius, such as an Al element, is selected preferentially (d) site. In other words, a technique is disclosed in which the saturation magnetization is lowered by increasing the substitution amount of (d) sites while suppressing the decrease in the total amount of Fe element of [a] and (d) sites. However, in Patent Document 3, as described in the Examples, a large amount of PbO is added together with the addition of CaO, and this is not a technique that does not substantially contain lead. This seems to be because PbO was added because light absorption loss cannot be suppressed stably only by adding CaO.

  The present invention has been made in view of the above-mentioned problems, does not substantially contain lead that is considered to be highly harmful, has excellent crystal characteristics, and has a light transmission property when used as a Faraday rotator. Another object of the present invention is to provide a bismuth-substituted rare earth iron garnet crystal in which deterioration of the Faraday rotation ability is not observed.

In order to achieve the above object, the present invention provides a bismuth-substituted rare earth iron garnet crystal, the bismuth-substituted rare earth iron garnet crystal having a composition formula (BiR) _3-a M1 _a Fe _5- bcd Al _b Pt _c M2 _d O ₁₂
(R: one or more elements selected from lanthanoid metals and Y,
M1: one or two elements selected from Ca and Sr,
M2: one or two elements selected from Ge and Si,
0.01 <a <0.1, 0.15 ≦ b + d ≦ 0.6, 0.01 ≦ c ≦ 0.04, a = c + d)
A bismuth-substituted rare earth iron garnet crystal is provided.

  Such a bismuth-substituted rare earth iron garnet crystal does not substantially contain lead, and there is almost no deterioration in light transmittance and Faraday rotation capability when the Faraday rotator is used, and no increase in saturation magnetization. For this reason, the thickness of the crystal can be suppressed, and the crystal characteristics are excellent. Therefore, it becomes a high-quality bismuth-substituted rare earth iron garnet crystal that is not harmful.

At this time, it is preferable that R in the composition formula is Tb and Yb, Tb and Ho, or Tb and Gd.
With such an element, the saturation magnetization is lower, and a bismuth-substituted rare earth iron garnet crystal having a square hysteresis is obtained when the magnetization process is performed.

Further, the present invention provides a Faraday rotator having a thickness of 700 μm or less, which is composed of the bismuth-substituted rare earth iron garnet crystal of the present invention.
Thus, if it is comprised from the bismuth substitution rare earth iron garnet crystal | crystallization of this invention, it will become a Faraday rotator which has sufficient Faraday rotation ability in thickness of 700 micrometers or less, and can be produced easily.

The present invention also provides a Faraday rotator composed of a bismuth-substituted rare earth iron garnet crystal according to the present invention, having a saturation magnetization of 200 gauss or less and having a square hysteresis. .
Thus, if the composition formula is composed of the bismuth-substituted rare earth iron garnet crystal of the present invention in which R is Tb and Yb, Tb and Ho, or Tb and Gd, the saturation magnetization is 200 gauss or less. A high-quality Faraday rotator with square hysteresis.

Also, a method for producing a bismuth-substituted rare earth iron garnet crystal in which a bismuth-substituted rare earth iron garnet crystal is grown in a flux composition melted in a platinum crucible by a liquid phase epitaxial method, the composition formula (BiR) _3-a M1 _a Fe _5-b-c-d Al _b Pt _c M2 _d O ₁₂
(R: one or more elements selected from lanthanoid metals and Y,
M1: one or two elements selected from Ca and Sr,
M2: one or two elements selected from Ge and Si,
0.01 <a <0.1, 0.15 ≦ b + d ≦ 0.6, 0.01 ≦ c ≦ 0.04, a = c + d)
A method for producing a bismuth-substituted rare earth iron garnet crystal represented by the formula:

  By producing a bismuth-substituted rare earth iron garnet crystal in this way, there is almost no deterioration of optical transparency and Faraday rotability when the Faraday rotator is made substantially free of lead and an increase in saturation magnetization. No crystals can be produced. For this reason, the thickness of the crystal can be suppressed, and an excellent crystal characteristic can be easily manufactured. Further, since a platinum crucible can be used, problems such as deformation of the crucible at the growth temperature do not occur. Therefore, it is possible to stably produce a high-quality bismuth-substituted rare earth iron garnet crystal that is not harmful.

  As described above, according to the present invention, lead that is considered to be highly harmful is not substantially contained, crystal characteristics are excellent, and light transmittance and Faraday rotability when a Faraday rotator is used are deteriorated. It is possible to provide a bismuth-substituted rare earth iron garnet crystal in which no crack is observed.

  When manufacturing a conventional bismuth-substituted rare earth iron garnet crystal, there are problems such as contamination of lead into the crystal which affects the environment, light absorption of the manufactured garnet crystal, deterioration of Faraday rotation ability, and the like.

In order to solve the above problems, the present inventor has made various studies on bismuth-substituted rare earth iron garnet crystals obtained by growing from a melt containing no lead oxide by a liquid phase epitaxial method using a platinum crucible.
As a result, by adding divalent calcium carbonate, strontium carbonate, calcium oxide, or strontium oxide and tetravalent germanium oxide or silicon oxide in the melt, the characteristics of the bismuth-substituted rare earth iron garnet crystals obtained. Was confirmed to be excellent, and the present invention was completed.

Hereinafter, although this invention is demonstrated in detail as an example of an embodiment, this invention is not limited to this.
Bismuth-substituted rare earth iron garnet crystal of the present invention, formula _{(BiR) 3-a M1 a} Fe 5-b-c-d Al b Pt c M2 d O 12
(R: one or more elements selected from lanthanoid metals and Y,
M1: one or two elements selected from Ca and Sr,
M2: one or two elements selected from Ge and Si,
0.01 <a <0.1, 0.15 ≦ b + d ≦ 0.6, 0.01 ≦ c ≦ 0.04, a = c + d)
It is represented by
Note that (BiR) _3-a represents that the sum of the Bi amount and the R amount is 3-a.

Such a bismuth-substituted rare earth iron garnet crystal has excellent crystal characteristics because it contains substantially no lead and can suppress the thickness of the epitaxial film. Further, Fe ²⁺ generated by Pt ⁴⁺ generated along with the dissolution of platinum which is a crucible material when growing a crystal becomes a divalent cation and M1 which is an element located at the {c} site of the garnet crystal structure. As a result, the formation of Fe ²⁺ can be suppressed without leading to dilution of the Fe element, and the light absorption can be reduced. Furthermore, saturation magnetization can be adjusted by including M2, which is a tetravalent cation and is an element mainly located at the (d) site of the garnet crystal structure. By combining with M1, light absorption can be stably performed. It becomes the crystal | crystallization which can suppress and can suppress the dilution of Fe element.

  In the present invention, in the above composition formula, R is one selected from a lanthanide metal or Y (yttrium) in consideration of optical characteristics and magnetic characteristics, and in consideration of lattice constant compatibility with the growth substrate. Alternatively, two or more elements may be used. Among these, Y, Gd, Tb, Ho, Yb, and Lu that do not absorb light in the optical communication wavelength band are preferably used. In particular, if Tb and Yb, Tb and Ho, or Tb and Gd are selected as R, the amount of addition of Al element can be increased to reduce the saturation magnetization to 200 gauss or less, and the magnetization process can be performed under an external magnetic field of about 5K Oersted. By performing the above, a Faraday rotator having a square hysteresis can be obtained.

M1 has a relatively large ionic radius among the elements that are divalent cations, and is selected from Ca and Sr that enter the {c} site of the garnet crystal structure.
M2 has a relatively small ionic radius among elements that become tetravalent cations, and is mainly selected from Ge or Si entering the (d) site of the garnet crystal structure.

Further, M1, which is a divalent element contained in the crystal in the present invention, enters the {c} site of the garnet crystal structure, and M2 mainly enters the (d) site, thereby minimizing the dilution of the Fe element. The saturation magnetization can be adjusted to 800 gauss or less as desired while minimizing the deterioration of the Faraday rotation capability.
On the other hand, conventionally used Pb can be both Pb ²⁺ and Pb ⁴⁺ with one kind of element, and was an extremely favorable element for improving optical properties. I can not use it.

In the present invention, the substitution amount with the Fe element is 0.15 ≦ b + d ≦ 0.6, 0.01 ≦ c ≦ 0.04.
When the thickness of the Faraday rotator exceeds 700 μm, the crystal for manufacturing the rotator requires a lapping allowance and a polishing allowance, and thus a thickness of 750 μm or more is required. It will be difficult. Therefore, if the composition of the crystal of the present invention is within the above range, a Faraday rotator having a thickness of 300 μm to 700 μm can be provided.

Here, in the present invention, since a charge in the crystal becomes neutral, a = c + d is required, and when the crystal is grown from a melt constituting the flux with bismuth oxide substantially not containing lead oxide, C of 0.01 ≦ c ≦ 0.04 is a large value that cannot be said to be an impurity. The amount of Ca or Sr that neutralizes the tetravalent Pt ions is slightly greater than the amount of Pt ions, so that Ge or Si indicated by M2 must be included in the crystal, and other than platinum, the divalent and tetravalent ions can be included. A valent cation is allowed to coexist in the melt. As a result, even when the epitaxial film to be grown is thick, that is, when crystal growth continues for a long time, the light absorption loss when the Faraday rotator is used without changing the valence of Fe ³⁺ is suppressed. Can do.

The bismuth-substituted rare earth iron garnet crystal of the present invention as described above can be produced, for example, by the following production method of the present invention.
In the production method of the present invention, a garnet unit held in a platinum jig in a melt (flux composition for producing a garnet crystal) in which a metal oxide composing a garnet crystal is melted in a platinum crucible. A crystal substrate is inserted, and the composition formula (BiR) _3-a M1 _a Fe _5-b-c-d Al _b Pt _c M2 _d O _{12 is} obtained by liquid phase epitaxial method.
(R: one or more elements selected from lanthanoid metals and Y,
M1: one or two elements selected from Ca and Sr,
M2: one or two elements selected from Ge and Si,
0.01 <a <0.1, 0.15 ≦ b + d ≦ 0.6, 0.01 ≦ c ≦ 0.04, a = c + d)
A bismuth-substituted rare earth iron garnet crystal represented by

The garnet single crystal substrate used for growing the garnet crystal film of the present invention includes, for example, samarium gallium garnet (hereinafter abbreviated as SGG), neodymium gallium garnet (hereinafter abbreviated as NGG), GGG-based SOG, NOG [added to Shin-Etsu Chemical Co., Ltd., a product, which is substituted by adding at least one of Ca, Mg, Zr, Y to gadolinium gallium garnet (hereinafter abbreviated as GGG) Name]. In these substrates, Sm ₂ O ₃ , Nd ₂ O ₃ , Gd ₂ O ₃ or, if necessary, a substitution agent such as CaO, MgO, ZrO ₂ is charged in a crucible together with a predetermined amount of Ga ₂ O ₃ , After being melted by heating to a melting point or higher in a high-frequency induction furnace, the single crystal can be pulled from this solution by the Czochralski method.

Also, the melt (flux composition) may be a rare earth element oxide, Bi ₂ O ₃ , Fe ₂ O ₃ , Al ₂ O ₃ that is an element that can be replaced with an iron element, or an element that becomes a divalent cation. Carbonates such as CaCO ₃ and SrCO ₃ that form Ca ²⁺ and Sr ²⁺ with relatively large ionic radii, and SiO ₂ and GeO that form Si ⁴⁺ and Ge ⁴⁺ that have smaller ionic radii than Fe ³⁺ even with tetravalent cation elements. _An oxide such as ₂ is charged in a platinum crucible and heated to 1000 to 1200 ° C. to dissolve it. A single crystal can be grown on the surface of the inserted garnet single crystal substrate from the supercooled melt at a growth temperature of 700 to 950 ° C. by a liquid phase epitaxial method.

  In this way, carbonates and oxides of the above-mentioned nonmagnetic metal elements having the above-mentioned characteristics that can be substituted with general bismuth oxide, iron oxide, and iron elements are mixed with bismuth-substituted rare earth iron garnet crystals. By growing a bismuth-substituted rare earth iron garnet single crystal from a melt composition in which the rotation ability, that is, the thickness necessary to rotate the polarization plane by 45 degrees is adjusted to a desired value, the thickness is 300 μm. A bismuth-substituted rare earth iron garnet crystal of ˜700 μm can be obtained. That is, the bismuth-substituted rare earth iron garnet crystal of the present invention can be produced by making the composition in the flux satisfy the above formula and growing the bismuth-substituted rare earth iron garnet crystal therefrom. Using this, for example, a Faraday rotator incorporated in an optical isolator having a wavelength of 1.3 to 1.6 μm can be obtained.

In the present invention, since PbO or PbF ₂ containing lead as a flux component is used in a trace amount or not at all, lead is contained in a garnet single crystal obtained by growing from that amount exceeding a regulated value of 1000 ppm. There is nothing.

EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not limited to these.
Example 1
The bismuth-substituted rare earth iron garnet crystal of the present invention was grown by liquid phase epitaxy.
As the garnet single crystal substrate, a NOG wafer (trade name, Shin-Etsu Chemical Co., Ltd.) having a lattice constant of 12.496 mm and a thickness of 1.2 mm and a diameter of 76.2 mm was used. As metal oxides for growing bismuth-substituted rare earth iron garnet crystals, 9000 g of bismuth oxide (Bi ₂ O ₃ ), 128 g of boron oxide (B ₂ O ₃ ), 465 g of ferric oxide (Fe ₂ O ₃ ), A platinum crucible was charged with 80 g of terbium oxide (Tb ₄ O ₇ ), 10 g of europium oxide (Eu ₂ O ₃ ), 23 g of aluminum oxide (Al ₂ O ₃ ), ₂ g of germanium oxide (GeO ₂ ) and 5 g of calcium oxide (CaO). This was heated to 1100 ° C. to melt it. Then, a bismuth-substituted rare earth iron garnet single crystal film was grown from the melt on the above-described NOG wafer at a growth temperature of 760 to 770 ° C. by a liquid phase epitaxial method. In this growth, no microcrystal was precipitated in the melt, and a bismuth-substituted rare earth iron garnet single crystal having a film thickness of 680 μm and good crystallinity could be obtained.

This bismuth-substituted rare earth iron garnet single crystal was analyzed by ICP emission spectrometry. As a result, it was found that the formula Bi _1.00 Tb _1.76 Eu _0.20 Ca _0.04 Fe _4.56 Al _0.40 Pt _0.03 Ge were those represented by _0.01 O _12.
This single crystal was cut and polished, and finished to have a Faraday rotation angle of 45 degrees at a wavelength of 1.55 μm. As a result, the thickness was 550 μm. Further, after applying an anti-reflective coating made of SiO ₂ and Ti ₃ O ₅ , the size is set to 1.0 × 1.0 mm, a magnetic field of 6000 Oersted is applied thereto, and a single magnetization is applied. When the optical absorption loss at .55 μm was examined, it was a low loss of 0.08 dB. The saturation magnetization value was measured and found to be 650 gauss.

(Example 2)
The bismuth-substituted rare earth iron garnet crystal of the present invention was grown by liquid phase epitaxy.
As a garnet single crystal substrate, a NOG wafer having a diameter of 76.2 mm and a lattice constant of 12.496 mm and a thickness of 1.2 mm was used. Moreover, as a metal oxide for growing a bismuth-substituted rare earth iron garnet crystal, bismuth oxide (Bi ₂ O ₃ ) 9500 g, boron oxide (B ₂ O ₃ ) 83 g, ferric oxide (Fe ₂ O ₃ ) 450 g, A platinum crucible containing 68 g of terbium oxide (Tb ₄ O ₇ ), 15 g of ytterbium oxide (Yb ₂ O ₃ ), 60 g of aluminum oxide (Al ₂ O ₃ ), ₂ g of germanium oxide (GeO ₂ ) and 7.4 g of strontium carbonate (SrCO ₃ ) And heated to 1100 ° C. to melt it. From this melt, a bismuth-substituted rare earth iron garnet single crystal film was grown on the above-mentioned NOG wafer at a growth temperature of 760 to 770 ° C. by a liquid phase epitaxial method. In this growth, no microcrystal was precipitated in the melt, and a bismuth-substituted rare earth iron garnet single crystal having a film thickness of 690 μm and good crystallinity could be obtained.

This bismuth-substituted rare earth iron garnet single crystal was analyzed by ICP emission spectrometry, which showed that the formula Bi _1.15 Tb _1.64 Yb _0.15 Sr _0.06 Fe _4.40 Al _0.55 Pt _0.04 Ge were those represented by _0.02 O _12.
This single crystal was cut and polished, and finished to have a Faraday rotation angle of 45 degrees at a wavelength of 1.55 μm. As a result, the thickness was 600 μm. Further, after applying a non-reflective coating made of SiO ₂ and Ti ₃ O ₅ , the size is set to 1.0 × 1.0 mm, and a magnetic field of 6000 Oersted is applied thereto to form a single magnet. When the optical absorption loss at 0.55 μm was examined, it was 0.07 dB, which was extremely low loss. Further, the result of measuring the saturation magnetization was 45 gauss. This 1.0 × 1.0 mm chip was magnetized under an external magnetic field of 5000 Oersted, and then a magnetic field was applied in the direction opposite to the magnetization direction, and the holding force Hc was measured. As a result, it was 1400 Oersted. It was confirmed that the material had square hysteresis.

(Examples 3-4, Comparative Examples 1-3)
As in Examples 1 and 2, except that the crystal components of the bismuth-substituted rare earth iron garnet crystals were variously changed to grow the garnet crystals of Examples 3 to 4 and Comparative Examples 1 to 3 with the composition shown in Table 1, and the wavelength 1 The thickness of 45 ° rotation angle at 0.55 μm, light absorption loss, and saturation magnetization were examined in the same manner as in Examples 1 and 2.
The measurement results are shown together in Table 1.

In Comparative Examples 1 and 2, “3-a” and “5-bcd” in the composition formula of the present invention were not satisfied, and many cracks were observed in the epitaxial film after epitaxial growth. This is because the thickness of the epitaxial film increases and crystal growth becomes difficult.
In Comparative Example 3, “M2” in the composition formula of the present invention was not included, and the light absorption loss increased. When polishing was continued from the surface side of the epitaxial film, the light absorption loss gradually decreased, and it was confirmed that the light absorption loss increased with the crystal growth.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

Claims (5)

A bismuth-substituted rare earth iron garnet crystal, the bismuth-substituted rare earth iron garnet crystal having a composition formula (BiR) _3-a M1 _a Fe _5- bc _-d Al _b Pt _c M2 _d O ₁₂
(R: one or more elements selected from lanthanoid metals and Y,
M1: one or two elements selected from Ca and Sr,
M2: one or two elements selected from Ge and Si,
0.01 <a <0.1, 0.15 ≦ b + d ≦ 0.6, 0.01 ≦ c ≦ 0.04, a = c + d)
A bismuth-substituted rare earth iron garnet crystal characterized by the following:   2. The bismuth-substituted rare earth iron garnet crystal according to claim 1, wherein the R in the composition formula is Tb and Yb, Tb and Ho, or Tb and Gd.   A Faraday rotator having a thickness of 700 μm or less, comprising the bismuth-substituted rare earth iron garnet crystal according to claim 1.   3. A Faraday rotator comprising the bismuth-substituted rare earth iron garnet crystal according to claim 2, wherein the saturation magnetization is 200 gauss or less and a square hysteresis is provided. A method for producing a bismuth-substituted rare earth iron garnet crystal by growing a bismuth-substituted rare earth iron garnet crystal in a flux composition melted in a platinum crucible by a liquid phase epitaxial method, the composition formula (BiR) _3-a M1 _a Fe _5-b-c-d Al _b Pt _c M2 _d O ₁₂
(R: one or more elements selected from lanthanoid metals and Y,
M1: one or two elements selected from Ca and Sr,
M2: one or two elements selected from Ge and Si,
0.01 <a <0.1, 0.15 ≦ b + d ≦ 0.6, 0.01 ≦ c ≦ 0.04, a = c + d)
A process for producing a bismuth-substituted rare earth iron garnet crystal represented by the formula: