JP2843433B2 - Bi-substituted magnetic garnet and magneto-optical element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in bismuth (Bi) -substituted magnetic garnets that are suitable for magneto-optical devices such as optical isolators, optical circulators, and optical switches using the Faraday effect. Things.

[Related Art] In an optical transmission circuit using a semiconductor laser as a light source, when reflected light returned from optical components such as connectors and switches, a light-receiving element, etc. enters the semiconductor laser, laser oscillation becomes unstable and transmission quality deteriorates. It is known. As a countermeasure against this, an optical isolator that blocks reflected return light to a semiconductor laser has been proposed, and its practical use is currently progressing.

The optical isolator makes good use of the nonreciprocity of the Faraday effect, which is a phenomenon of rotation of the plane of polarization of transmitted light among magneto-optical effects.

Shows the Faraday effect, as one of the Faraday rotator materials used in the near infrared region of wavelength 1.3 ~ 1.55μm,
Magneto-optical garnet crystals are made. Among them, the yttrium-iron-garnet (YIG) crystal was first used as a Faraday rotator in an optical isolator and is still widely used today.

In recent years, the miniaturization and cost reduction of optical isolators have rapidly progressed, and along with this, there is a strong demand for miniaturization of elements and reduction of manufacturing costs. It is an essential condition to reduce the size of the Faraday rotator, which is a main component. By the way, when YIG or the like is used as a Faraday rotator, it is necessary to have a length sufficient to rotate the incident polarization plane by a predetermined amount (for example, 45 degrees), and this length is the Faraday rotator of the material constituting the Faraday rotator. It is proportional to the magnitude of the rotation coefficient. Therefore,
In order to further reduce the size of the Faraday rotator, it is necessary to select a material having a large Faraday rotation coefficient.

As a material having a large Faraday rotation coefficient, a so-called Bi-substituted magnetic garnet in which Bi is dissolved is generally known (for example, see Japanese Patent Application Laid-Open No. 64-27212). It is also known that this material can be manufactured by a CVD method, a sputtering method, a flux method, a liquid phase epitaxial (LPE) method, or the like.

However, among these production methods, the CVD method and the sputtering method mainly have problems in crystallinity, and have not yet reached the stage of practical use.The flux method, together with the garnet components yttrium oxide and iron oxide, has a flux component. Melting lead oxide or boron oxide, keeping the melt at or above the liquidus temperature to make it uniform, and then gradually cooling the melt to grow garnet crystals by natural nucleation.
In the conventionally known methods, there are disadvantages such as that the concentration is likely to vary due to segregation of the garnet component, and the crystal is liable to crack with the solidification of the flux. For this reason, recently, an improved flux method in which these disadvantages are improved has been proposed. However, in this improved method, there is a problem that it takes a long time to grow a crystal to grow a natural nucleus, precision processing is required to make a device, and the mass productivity is poor.

Under such circumstances, recently, the production by the LPE method, which was originally developed as a method for growing a garnet thin film for a magnetic bubble element, has attracted particular attention, and various studies have been conducted on the development of a Bi-substituted magnetic garnet thick film using this method. Is in a situation.

By the way, as is well known, in Bi-substituted magnetic garnet, the Faraday rotation coefficient increases in proportion to the Bi substitution amount ([Thin Solid Films] 114 (1984),
P69-107 etc.).

However, since the segregation coefficient of Bi with respect to the garnet crystal is extremely small, it is very difficult to increase the substitution amount.

In order to substitute a large amount of Bi, it is necessary to increase the segregation coefficient of Bi, but as a method for achieving this,
Searches for the optimal component system, optimization of LPE conditions, and the like have been proposed.

As an example of searching for the optimal component system, set the rare earth site of YIG to B
Gd _2.1 Bi _0.9 Fe _4.6 Al _0.1 Ga _0.3 O substituted with i and Gd elements
_Twelve materials have been reported as materials having a large Faraday rotation coefficient (see “Materials Science” Vol. 23, No. 3 (1987), p. 151). However, according to the study of the present inventors, it has been found that this material has a problem to be solved such that the temperature coefficient of the Faraday rotation coefficient is large. For example, the temperature coefficient of the Faraday rotation coefficient of YIG is about 0.04 deg / ° C., but the temperature coefficient of the above material is 0.10 deg / ° C., which is more than twice that of YIG. Therefore, when this material is used for a Faraday rotator of an optical isolator, the isolation (Es) of the optical isolator deteriorates. Es is represented by Es = −10 · log · sin ² (Δθ). Here, Δθ (deg) is a deviation of θ from 45 degrees.

If the change in the oscillation wavelength of the semiconductor laser is not considered, 0
In the temperature range of 5050 ° C., YIG can obtain 36 dB of Es, but the above material is reduced to 29 dB. Generally, it is desired that the optical isolator has Es of 30 dB or more. To secure this Es, a temperature coefficient of 0.07 deg / ° C. or less is required.

[Problems to be Solved by the Invention] As described above, miniaturization of the Faraday rotator is indispensable for miniaturization of the optical isolator. To achieve this, for example, Bi
Is also known to dramatically increase the Faraday rotation coefficient by replacing a large amount, but in this case, there is a problem to be solved that the temperature coefficient of the Faraday rotation coefficient also increases as described above. I found that there was.

When the lattice constant of the LPE film and the substrate are mismatched,
Although film cracking or substrate cracking may occur, simply replacing Bi ions having a large ion radius will increase the lattice constant of the film. Therefore, it is necessary to select a substrate having a larger lattice constant that matches the lattice constant of the Bi-substituted magnetic garnet film. However, the types of garnet substrates that can be used industrially are limited. Therefore, it is common practice to replace the rare earth site or the Fe site with various elements, and to strictly adjust the amount of the replacement to match the lattice constant of the film to the substrate. Sometimes.

Also, as described above, optimization of LPE conditions has been proposed as another method for replacing a large amount of Bi. This method mainly involves increasing the degree of supercooling (ΔT = saturation temperature−film growth temperature) and lowering the film growth temperature (T _G ).
However, natural nuclei are more likely to be generated as ΔT increases.
This natural nucleus becomes a seed crystal in the flux method, but is an unfavorable product in the LPE method because it causes a film defect. Further, with the increase of ΔT and the lowering of _TG , the flux components Pb, crucible, and Pt of the substrate fixing jig material in the melt tend to be mixed into the YIG film.

It is known that light absorption increases when Pb or Pt is mixed in the film ("Journal of the Japan Society of Applied Magnetics", VOL.10, N
o.2, (1986) 161). Also, Pt contributes to film defects ("J, Magn, Soc, Jpn", vol. 11, SI (1987) P. 347),
It is known that the optical properties of the defective portion are deteriorated (see “Japan Society of Applied Magnetics”, vol. 10, No. 2, (1986) 147). Therefore, it is considered that the degree of supercooling ΔT is preferably about 30 ° C. or less, and the film growth temperature _TG is about 730 ° C. or more.

An object of the present invention is to realize a magnetic garnet in which a large amount of Bi is replaced and the Faraday rotation coefficient is improved, and the temperature characteristics of the Faraday rotation coefficient are also good, and natural nuclear defects are reduced. It is an object of the present invention to provide an overall excellent magneto-optical garnet crystal.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a compound of the general formula Lu
_{3- (X + Y + Z + Q)} Y _X Tb _Y Ho _Z Bi _Q Fe ₅ O ₁₂ (where X, Y and Z are
Garnet having a composition of 0.1 to 1.0, and Q is a number of 0.5 to 2.0).

Hereinafter, the basic concept of the composition limitation in the present invention will be described.

There are few types of garnet substrates for LPE that can be used industrially, usually Gd ₃ Ga ₅ O ₁₂ (GGG) and Ca-Mg-Zr substitution
GGG substrates dominate the market, with Sm ₃ Ga ₅ O ₁₂ and Nd ₃ Ga ₅ O ₁₂
Substrates are only partially used. Therefore,
Generally, the composition of the film is controlled to match the lattice constant of the substrate. As described above, the lattice constant of the film increases with an increase in the Bi substitution amount.
It is considered that reducing the lattice constant of the base material for replacing Bi becomes a basis for replacing a large amount of Bi. First, Lu ₃ Fe ₅ O ₁₂ (“T
hin Solid Films ”114 (1984), p. 33). Then, in the present invention, Lu is positioned as a role of balance of rare earth sites, and the amount of Lu added is determined according to the amount of Bi substitution. Therefore, in the present invention
The amount of Lu is, as shown in the above composition formula, Lu _{3- (X + Y + Z + Q)}
Range.

Next, as described above, Gd is known as a host rare earth element that enables a large amount of Bi substitution, but has a drawback of deteriorating the temperature characteristics of the Faraday rotation coefficient. The present inventors have made various studies and found that the addition of Y is effective as an element capable of replacing Bi in a large amount without adversely affecting the magneto-optical characteristics. FIG. 1 shows the result of examining the relationship between the Bi amount and the Y amount in the film. As can be seen from this figure, no effect is exhibited when the value x of the Y amount is less than 0.1 in the above composition formula. On the other hand, if it is larger than 1.0, the amount of other elements to be replaced with rare earth sites is reduced, which is not preferable. Desirably, it should be 0.2 to 0.9.

It is said that deterioration of the temperature characteristic of the Faraday rotation coefficient due to Bi substitution is mainly caused by a change in the sign of the Faraday rotation coefficient. That is, although the Faraday rotation coefficient dramatically increases due to the Bi substitution, the sign becomes negative at the same time, and the value decreases as the temperature rises. Therefore, in order to improve the temperature characteristics of the Faraday rotation coefficient, attempts have been made to form a crystal having positive and negative signs of the Faraday rotation coefficient into a solid solution. The present inventors have also paid attention to the solid solution method, and as a result of studying various elements, in the present invention, the Faraday rotation coefficient Tb having a positive sign
And found Ho to be promising. Fig. 2 shows the results of examining the effect of the ratio of the amount of Ho and Tb in the film on natural nucleation, and Fig. 3 shows the relationship between the amount of Ho + Tb in the film and the temperature coefficient of the Faraday rotation angle. Show. As can be seen from these figures, when the content Y or Z of Ho or Tb in the above composition formula is less than 0.1, no improvement effect is observed. Note that 2.0
If it is larger than this, the amount of Bi substitution decreases, which is not preferable. Desirably, it is in the range of 0.2 to 1.6. The ratio between Tb and Ho may be in the range of 4/1 to 1/4.

If Bi is less than 0.5 in the above composition formula, a sufficient Faraday rotation coefficient cannot be obtained, and the required film thickness is undesirably large. On the other hand, when it becomes larger than 2.0, ΔT becomes 3
Since the temperature is 0 ° C. or higher, the precipitation of natural nuclei becomes severe, and film defects increase, which is not preferable. Desirably, 0.8-
Should be 1.6.

EXAMPLES Hereinafter, the present invention will be described with reference to Examples. In the following examples, the mismatch between the lattice constants of the substrate and the film is as follows:
The measurement was performed using an X-ray diffractometer.

Example 1 In mole%, 12.50% Fe ₂ O ₃ , 0.12% Y ₂ O ₃ , 0.04%
Lu ₂ O _3, 0.04% of Tb ₄ O _7, 0.08% of Ho ₂ O _3, and 26.78%
Using a melt composition using Bi ₂ O ₃ as a solute and 6.87% B ₂ O ₃ and 53.57% PbO as fluxes, Ca-Mg-Zr-substituted Gd ₃ G
The garnet film of the composition Lu _0.3 Ho _0.4 Tb _0.4 Y _0.8 Bi _1.3 Fe ₅ O ₁₂ grown on a ₅ O ₁₂ substrate at a growth temperature of 765 ° C. and a degree of supercooling ΔT of 20 ° C. has a Faraday rotation coefficient at a working wavelength of 1.55 μm. It showed a temperature coefficient of 1500 deg / cm and a temperature coefficient of -10 to 50 ° C of 0.05 deg / ° C. In addition, no natural nuclei were generated during the growth, and no film defect due to the influence of the natural nuclei was observed.

GPIG's Faraday rotation coefficient and temperature characteristics are respectively
At 1020 deg / cm and 0.01 deg / ° C., the garnet film of the present invention excels in any of the properties.

The Faraday rotation coefficient was determined by dividing the angle difference between the incident polarized light and the output polarized light by the film thickness. Temperature characteristics from 0 ° C to 50 ° C
The deviation of the Faraday rotation angle θ is obtained by dividing the deviation by the above temperature difference. On the other hand, the presence or absence of natural nuclei in the melt was visually confirmed, and the presence or absence of natural nuclei in the film was confirmed using an optical microscope.

Example 2 15.17% Fe ₂ O ₃ , 0.06% Y ₂ O ₃ , 0.02%
Lu ₂ O _3, 0.06% of Tb ₄ O _7, 0.12% of Ho ₂ O _3, and 26.02%
Using a melt composition containing Bi ₂ O ₃ as a solute and a flux of 6.50% B ₂ O ₃ and 52.05% PbO, Ca-Mg-Zr-substituted Gd ₃ G
LPE grown on a ₅ O ₁₂ substrate at growth temperature of 790 ° C and ΔT11 ° C
The garnet film having the composition Lu _0.2 Ho _0.8 Tb _0.8 Y _0.3 Bi _0.9 Fe ₅ O ₁₂ has a Faraday rotation coefficient of 1050 at a working wavelength of 1.55 μm.
It showed a temperature coefficient of 0.04 deg / ° C in deg / cm and 0 to 50 ° C. In addition, no natural nuclei were generated during the growth, and no film defect due to the influence of the natural nuclei was observed.

Example 3 12.80% Fe ₂ O ₃ , 0.16% Y ₂ O ₃ , 0.06%
Lu ₂ O _3, 0.02% of Tb ₄ O _7, 0.04% of Ho ₂ O _3, and 31.02%
And a Bi ₂ O ₃ as a solute, 6.40 percent of B ₂ O ₃ and 49.50% of PbO with a melt composition to flux, Ca-Mg-Zr substituted Gd ₃ G
LPE grown on a ₅ O ₁₂ substrate at 744 ° C and ΔT28 ° C
A garnet film having a composition of Lu _0.3 Ho _0.2 Tb _0.2 Y _0.7 Bi _1.8 Fe ₅ O ₁₂ has a Faraday rotation coefficient of 1870 at a wavelength of 1.55 μm.
deg / cm and a temperature coefficient of -10 to 50 ° C of 0.07 deg / ° C.
In addition, no natural nuclei were generated during the growth, and no film defects due to the influence of the natural nuclei were observed.

(Comparative Example 1) In mole%, 9.64% of Fe ₂ O ₃ , 0.08% of Y ₂ O ₃ , and 0.07% of L
u ₂ O _3, 0.02% of Tb ₄ O _7, 0.04% of Ho ₂ O _3, and 33.25%
Using a melt composition using Bi ₂ O ₃ as a solute and flux as 7.02% B ₂ O ₃ and 49.88% PbO as fluxes, Ca-Mg-Zr-substituted Gd ₃ G
LPE grown on a ₅ O ₁₂ substrate at a growth temperature of 721 ° C and ΔT45 ° C
_{Lu 0.3 Ho 0.1 Tb 0.1 Y 0.4} Bi 2.1 Fe 5 garnet film of O ₁₂ composition, Faraday rotation coefficient 2450 in the used wavelength 1.55μm
deg / cm. However, since the growth temperature was low and the degree of supercooling was large, natural nuclei were precipitated in the melt, and many film defects with the natural nuclei as nuclei occurred. As a result, the crystallinity deteriorated, and it was impossible to measure the optical characteristics.

[Effect of the Invention] The garnet crystal of the composition of the present invention can provide a large Faraday rotation coefficient, so that the size of the Faraday rotator can be reduced. In addition, since the temperature coefficient of the Faraday rotation angle is small, deterioration of the isolation of the optical isolator is small. Furthermore, the growth yield is greatly improved by reducing the natural nuclear defects. From the above, the practical value of the present invention is extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the Bi amount and the Y amount in the film, FIG. 2 is a diagram showing the effect of the ratio of the Ho amount (in the film) to the Tb amount on the natural nucleation, and FIG. FIG. 3 is a diagram showing the relationship between the amount of Ho + Tb (in the film) and the temperature coefficient of the Faraday rotation angle.

Claims (2)

(57) [Claims] [Claim 1] The general formula Lu _{3- (X + Y + Z + Q)} Y _X Tb _Y Ho _Z Bi _Q Fe ₅ O
_12. A Bi-substituted magnetic garnet having a composition represented by the formula: (where X, Y, and Z are 0.1 to 1.0, and Q is 0.5 to 2.0). 2. A magneto-optical element using the Bi-substituted magnetic garnet according to claim 1.