JP2003195243A - Faraday rotator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bismuth-substituted rare earth iron garnet single crystal film in which the direction of magnetization is vertical to the film surface, a nucleation magnetic field is equal to or smaller than a saturation magnetic field and the value of the saturation magnetic field is small. <P>SOLUTION: The bismuth-substituted rare earth iron garnet single crystal film grown by a liquid phase epitaxial method is heated for two hours or longer within a temperature range whose lower limit valve is higher than the growing temperature of the bismuth-substituted rate earth iron garnet single crystal film by 50°C and whose higher limit value is higher than the growing temperature of the garnet single crystal film by 120°C to obtain a Faraday rotator. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bismuth-substituted rare earth iron garnet single crystal film used as a material for a Faraday rotator used in optical devices such as optical isolators and optical switches. The present invention relates to a bismuth-substituted rare earth iron garnet single crystal film capable of miniaturizing the structure of an optical device.

[0002]

2. Description of the Related Art In recent years, the development of optical fiber communication has been remarkable. In the field of optical fiber communication, optical devices such as optical isolators, optical circulators, and optical switches using Faraday rotators are indispensable. As the Faraday rotator, a liquid phase epitaxial method (L
A bismuth-substituted rare earth iron garnet single crystal film grown by PE method is used. Also, in these optical devices,
A magnetic field exceeding the saturation magnetic field Hs is applied to magnetically saturate the Faraday rotator. This magnetic field is generally applied by a permanent magnet or an electromagnet. Therefore, in the optical device having such a configuration, if the saturation magnetic field Hs of the Faraday rotator is large, it becomes necessary to increase the size of the permanent magnet or the electromagnet. If the saturation magnetic field Hs of the Faraday rotator is small, the permanent magnet or the electromagnet may be small in size, which is advantageous for downsizing the optical device and reducing the cost.

Generally, the chemical composition of a bismuth-substituted rare earth iron garnet single crystal is (RBi) ₃ (FeM) ₅ O.
₁₂ [However, R is yttrium Y and rare earth elements, M is A
means an element such as l, Ga, In, Si, Sc, or the like]. It is already known that the saturation magnetic field Hs of this bismuth-substituted rare earth iron garnet single crystal film depends on the type and combination of its constituent elements. Therefore,
By combining the constituent elements, the saturation magnetic field Hs can be made extremely small. A uniaxial magnetic anisotropy constant Ku occurs in the bismuth-substituted rare earth iron garnet single crystal film grown by the LPE method. When the saturation magnetic field Hs is reduced by combining the constituent elements, the magnetic hysteresis increases. Therefore, it is generally known that a magnetic field exceeding the saturation magnetic field Hs is required to magnetize the bismuth-substituted rare earth iron garnet single crystal film or switch the direction of magnetization. When the magnetic field is defined as a nucleation magnetic field Hn (or coercive force Hc), Hn (or Hc) is
It is expressed by the relational expression of Hn (or Hc) ∝Ku / Hs. ("Nucleation of domain walls in iron garnet s
ingle crystals grown from liquid phase epitaxy "Jo
urnal of Applied Physics, Vol.82, 2457-2460, (199
7)). As is clear from this relational expression, the saturation magnetic field Hs
When becomes smaller, the increase of Hn cannot be avoided.

Here, when the saturation magnetic field is simply expressed as Hs, it is the saturation magnetic field when converted into an infinite film. The actual sample has a specific shape. Assuming that the value of the saturation magnetic field in this actual sample is Hs * (hereinafter, the value measured in the actual sample is described as Hs *), the value of Hs * is Hs in the form factor N.
The value Hs × N = Hs * is obtained by multiplying (value from 0 to 1).
That is, the saturation magnetic field Hs * of the actual sample is smaller than the saturation magnetic field Hs converted into an infinite film. Then, empirically, in the relational expression of Hn (or Hc) ∝Ku / Hs, it is possible to consider by substituting the value of Hs * for Hs, and the measured nucleation magnetic field is almost the value of this Hs *. The amount corresponding to becomes larger. The value of Hs in an infinite film is
3 inch growth substrate, Hs * ≈1.0 × Hs, 10 mm
The square substrate had a relationship of Hs * ≈0.95 × Hs, and the square substrate had a relationship of Hs * ≈0.6 × Hs.

The present inventors have a small saturation magnetic field Hs.
Bismuth-substituted rare earth iron garnet single crystal film
New bismuth replacement to meet the societal demand
The composition of rare earth iron garnet single crystals was studied, and (GdRB
i)_Three(FeAlGa)₅O ₁₂[However, R is Y, Y
b, means at least one element selected from the group Lu
Taste] was developed (Japanese Patent Laid-Open No. 07-315995)
News). However, after that, (GdRBi)_Three(Fe
AlGa)₅O₁₂Also in two important technical divisions
It became clear that the problem had to be resolved. So
One is that as the Faraday rotator becomes smaller,
The saturation magnetic field Hs * of the Aaraday rotator is
Saturation magnetic field Hs * of S-substituted rare earth iron garnet single crystal film
However, the nucleation magnetic field Hn has
If there is a large value that spreads and exceeds the form factor N,
That is the problem. The other is essential for magnetic saturation.
If you apply a magnetic field that is several times larger than the required magnetic field,
The reason is that the nucleation magnetic field Hn increases.
(Journal of Applied Physics, Vol.82, 2457-2460, (1
997)). In order to solve the above two problems, the present inventors have
A study was conducted to further reduce the nucleation magnetic field Hn.

As the remaining means for lowering the nucleation magnetic field Hn, it is easily inferred from the relational expression of Hn (or Hc) ∝Ku / Hs to lower the uniaxial magnetic anisotropy constant Ku. The factors that generate the uniaxial magnetic anisotropy constant Ku include growth-induced magnetic anisotropy caused by anisotropic pairing of magnetic rare earth ions and iron ions in the entire crystal during the crystal growth process, and a seed crystal substrate. There is a stress-induced magnetic anisotropy that results from the difference in the coefficient of thermal expansion between bismuth-substituted rare earth iron garnet single crystal films. It is known that these magnetic anisotropies are reduced when a garnet single crystal film is annealed at 900 to 1300 ° C. (“Magnetic Bubble”, Sakurai et al., Ohmsha, 1982). Taking advantage of this effect, there is a case in which a bismuth-substituted rare earth iron garnet single crystal film having a reduced nucleation magnetic field Hn is applied to an optical switch by performing a heat treatment under a predetermined heat treatment condition (JP-A-07-301775). Gazette).

On the other hand, however, it is also known that when the bismuth-substituted rare earth iron garnet single crystal film substituted with non-magnetic ions such as Ga ions is heat-treated, the saturation magnetic field Hs changes due to rearrangement of Ga ions (" Bubble Technology Handbook ", The Institute of Electrical Engineers, Ohmsha, 1981
Year). For example, when the bismuth-substituted rare earth iron garnet single crystal film according to claim 4 of the present application is heat-treated at a high temperature under the conditions disclosed in JP-A-07-301775,
The present inventors have confirmed that the saturation magnetic field Hs * increases from 91 (Oe) to 163 (Oe).

Further, there is an optical device such as an optical isolator in which the film surface of the bismuth-substituted rare earth iron garnet single crystal film is arranged obliquely with respect to the incident light in order to avoid the reflected return light from the film surface. If the bismuth-substituted rare earth iron garnet single crystal film is heat-treated at a too high temperature, the uniaxial magnetic anisotropy constant becomes very small, the direction of magnetization is not perpendicular to the film surface, and the direction of the external magnetic field is followed. The present inventors have found that when such a single crystal is used in the above optical device, the Faraday rotation angle varies depending on the arrangement angle with respect to the incident light, resulting in a performance problem.

[0009]

The heat treatment technique for reducing the nucleation magnetic field Hn by suppressing the square hysteresis by heat treatment is extremely useful. However, in the cases already published and in the experiments by the present inventors, depending on the heat treatment conditions, when the saturation magnetic field Hs * rises or the direction of the external magnetic field changes, the single crystal film is easily formed. There is a phenomenon of being magnetized in that direction. But,
The conditions for using the bismuth-substituted rare earth iron garnet single crystal film in an optical isolator, an optical circulator, and an optical switch are that it can be magnetized in a direction perpendicular to the film surface of the single crystal and that the saturation magnetic field Hs * is small. Therefore, the phenomenon that the magnetization direction easily changes depending on the direction of the external magnetic field hinders the production of a bismuth-substituted rare earth iron garnet single crystal film for an optical device.

An object of the present invention is to provide a bismuth-substituted rare earth iron garnet single crystal film in which the magnetization direction is perpendicular to the film surface, the nucleation magnetic field Hn is less than or equal to the saturation magnetic field Hs *, and the saturation magnetic field Hs * has a small value. To provide. In addition, in the heat treatment for producing such a bismuth-substituted rare earth iron garnet single crystal film, it is to find a heat treatment condition that suppresses the rise of the saturation magnetic field Hs * and the direction of magnetization is perpendicular to the film surface.

[0011]

The present inventors have developed a nucleation method without losing the property that a bismuth-substituted rare earth iron garnet single crystal film having a small saturation magnetic field Hs * is magnetized in a direction perpendicular to the film surface. The heat treatment conditions for the magnetic field Hn to be equal to or less than the saturation magnetic field Hs * were examined. The reason for such a condition is as follows. That is, the temperature of the heat treatment is
This is because it is deeply related to the growth temperature of the bismuth-substituted rare earth iron garnet single crystal film, and the heat treatment time is deeply related to the heat treatment temperature. The temperature range for heat treatment is set as a lower limit of the temperature effective for reducing the nucleation magnetic field Hn, which is 50 ° C. higher than the growth temperature of the garnet single crystal film, and the saturation magnetic field Hs * is suppressed from increasing. A temperature higher by 120 ° C. than the growth temperature of the garnet single crystal film was set as the upper limit of the temperature at which the property of magnetizing in the direction perpendicular to the plane can be secured. Further, in this temperature range, the bismuth-substituted rare earth iron garnet single crystal film is heat-treated for 2 hours or more. The heat treatment conditions described above were derived from the experimental results of the following examples and comparative examples. This heat treatment method was particularly effective when applied to a bismuth-substituted rare earth iron garnet single crystal film grown by a liquid phase epitaxial method and having a saturation magnetic field of less than 200 (Oe).

[0012]

EXAMPLES Hereinafter, the present invention will be described in detail and in detail by way of examples, but the following examples will specifically describe the present invention. And is not intended to limit the scope of the invention.

Example 1 A platinum crucible having a volume of 3000 ml was charged with lead oxide [PbO,
4N] 3448 g, bismuth oxide [Bi ₂ O ₃ , 4N]
4377 g, ferric oxide [Fe ₂ O ₃ , 4N] 455
g, boron oxide [B ₂ O ₃ , 5N] 165 g, gadolinium oxide [Gd ₂ O ₃ , 3N] 26.0 g, gallium oxide [Ga ₂ O ₃ , 3N] 91 g, yttrium oxide [Y
₂ O ₃ , 3N] 16.0 g was charged. This was placed at a predetermined position in a precision vertical tubular electric furnace, heated and melted at 1000 ° C., and sufficiently stirred to obtain a homogeneous bismuth-substituted rare earth iron garnet single crystal growing melt.

After the temperature of the melt thus obtained is lowered to a temperature equal to or lower than the saturation temperature, that is, 766 ° C., the thickness is 500 μm and the lattice constant is 12.497 according to a conventional method.
One side of a ± 0.002Å 3-inch (111) garnet single crystal [(GdCa) ₃ (GaMgZr) ₅ O ₁₂ ] substrate was brought into contact with the melt surface. Epitaxial growth was performed while rotating this substrate to obtain a 275 μm thick bismuth-substituted rare earth iron garnet single crystal thick film (hereinafter referred to as BIG-1).
Will be abbreviated). After dissolving the melt component adhering to BIG-1 with a hydrochloric acid aqueous solution, 10.5 mm × 1
The size was divided into 0.5 mm. 28 obtained by dividing
The sheet of BIG-1 was polished and the substrate was removed. The saturated magnetic field Hs * after polishing was 91 (Oe).

Two out of 28 BIG-1 were placed on a Pt plate in an electric furnace and heated at 830 ° C. for 10 hours. The saturation magnetic field Hs * of this sample was 88 (Oe). That is, the difference from the saturated magnetic field Hs * 91 (Oe) after polishing was within the range of measurement error, and the saturated magnetic field Hs * did not substantially increase. After measuring the saturation magnetic field Hs *, 1.2
It was cut into a size of mm × 1.2 mm to obtain 64 chips. As a result of measuring the nucleation magnetic field Hn of these chips, it was found that all chips had a magnetic field of 30 to 80 (O
e).

Example 2 Two of the BIG-1 obtained in Example 1 were replaced with Pt in an electric furnace.
It was placed on a plate and heated at 840 ° C. for 10 hours.
The saturation magnetic field Hs * of this sample was 90 (Oe). The difference from the saturated magnetic field Hs * 91 (Oe) after polishing was within the range of measurement error, and the saturated magnetic field Hs * did not substantially increase. This BIG-1 was cut into a size of 1.2 mm × 1.2 mm and the nucleation magnetic field Hn was measured.
In the range of (Oe), there was no sample in which the saturation magnetic field Hs * exceeded the saturation magnetic field Hs * 91 (Oe) immediately after polishing.

Example 3 Two of the BIG-1 obtained in Example 1 were replaced with Pt in an electric furnace.
It was placed on a plate and heated at 890 ° C. for 10 hours.
The saturation magnetic field Hs * of this sample was 107 (Oe).
This BIG-1 was cut into a size of 1.2 mm × 1.2 mm and the nucleation magnetic field Hn was measured.
e), and the saturation magnetic field Hs * is 91 at maximum.
It was (Oe).

Example 4 A platinum crucible having a capacity of 3000 ml was charged with lead oxide [PbO,
4N] 3448 g, bismuth oxide [Bi ₂ O ₃ , 4N]
4377 g, ferric oxide [Fe ₂ O ₃ , 4N] 455
g, boron oxide [B ₂ O ₃ , 5N] 165 g, gadolinium oxide [Gd ₂ O ₃ , 3N] 26.0 g, gallium oxide [Ga ₂ O ₃ , 3N] 91 g, yttrium oxide [Y
₂ O ₃ , 3N] 16.0 g was charged. This was placed at a predetermined position in a precision vertical tubular electric furnace, heated and melted at 1000 ° C., and sufficiently stirred to obtain a homogeneous bismuth-substituted rare earth iron garnet single crystal growing melt.

After the temperature of the melt thus obtained is lowered to a temperature equal to or lower than the saturation temperature, that is, 769 ° C., the thickness is 630 μm and the lattice constant is 12.497 according to a conventional method.
One surface of a ± 0.002Å 3-inch (111) garnet single crystal [(GdCa) ₃ (GaMgZr) ₅ O ₁₂ ] substrate was brought into contact with the melt surface. Epitaxial growth was performed while rotating this substrate to produce a bismuth-substituted rare earth iron garnet single crystal thick film (hereinafter referred to as BIG-2) having a thickness of 355 μm. After dissolving the melt component adhering to BIG-2 with a hydrochloric acid aqueous solution, 10.5 mm × 1
The size was divided into 0.5 mm. After that, 26 pieces of BIG-2 obtained by dividing were polished to remove the substrate. The saturation magnetic field Hs * after polishing was 97 (Oe).

Two of the 26 BIG-2 were placed on a Pt plate in an electric furnace and heated at 830 ° C. for 10 hours. The saturation magnetic field Hs * of this sample was 97 (Oe), the difference from the saturation magnetic field Hs * 97 (Oe) after polishing was within the range of measurement error, and the saturation magnetic field Hs * did not increase substantially. . After measuring the saturation magnetic field Hs *, 1.2 mm × 1.
It was cut into a size of 2 mm to obtain 64 chips. As a result of measuring the nucleation magnetic field Hn of these chips as well, all the samples had a saturation magnetic field Hs * of 36 to 74 (Oe).

Example 5 A platinum crucible having a volume of 3000 ml was charged with lead oxide [PbO,
4N] 3448 g, bismuth oxide [Bi ₂ O ₃ , 4N]
4377 g, ferric oxide [Fe ₂ O ₃ , 4N] 455
g, boron oxide [B ₂ O ₃ , 5N] 165 g, gadolinium oxide [Gd ₂ O ₃ , 3N] 26.0 g, gallium oxide [Ga ₂ O ₃ , 3N] 91 g, yttrium oxide [Y
₂ O ₃ , 3N] 16.0 g was charged. This was placed at a predetermined position in a precision vertical tubular electric furnace, heated and melted at 1000 ° C., and sufficiently stirred to obtain a homogeneous bismuth-substituted rare earth iron garnet single crystal growing melt.

After the temperature of the melt thus obtained is lowered to a temperature equal to or lower than the saturation temperature, that is, 767 ° C., the thickness is 630 μm and the lattice constant is 12.497 according to a conventional method.
One surface of a ± 0.002Å 3-inch (111) garnet single crystal [(GdCa) ₃ (GaMgZr) ₅ O ₁₂ ] substrate was brought into contact with the melt surface. Epitaxial growth was performed while rotating this substrate to produce a bismuth-substituted rare earth iron garnet single crystal thick film (hereinafter referred to as BIG-3) having a thickness of 310 μm. After dissolving the melt component adhering to BIG-3 with a hydrochloric acid aqueous solution, 10.5 mm × 1
The size was divided into 0.5 mm. 26 obtained by dividing
The BIG-3 sheets were polished and the substrate was removed. The saturated magnetic field Hs * after polishing was 100 (Oe).

Two of the 26 BIG-3s were placed on a Pt plate in an electric furnace and heated at 850 ° C. for 10 hours. The saturation magnetic field Hs * of this sample was 102 (Oe). 1.2mm × 1.2m after measuring the saturation magnetic field Hs *
It was cut into a size of m to obtain 64 chips. The nucleation magnetic field Hn of these chips was also measured, and as a result, all samples had a saturation magnetic field Hs * of 25 to 63 (Oe).

Next, an external magnetic field of 500 (Oe) was applied with an inclination of 20 degrees with respect to the film surface, and changes in the Faraday rotation angle were examined. The Faraday rotation angle was compared between when the external magnetic field was applied vertically and when it was obliquely applied, but no change was observed.

Comparative Example 1 The polished BIG-1 obtained in Example 1 was 1.2 mm ×
The value of the nucleation magnetic field Hn is 5 after cutting to a size of 1.2 mm.
There were 61 samples in the range of 0 to 210 (Oe), which exceeded the saturation magnetic field Hs * value of 91 (Oe).

Comparative Example 2 Two of the BIG-1 obtained in Example 1 were Pt in an electric furnace.
It was placed on a plate and heated at 820 ° C. for 10 hours.
The saturation magnetic field Hs * of this sample was 86 (Oe). The difference between the saturated magnetic field Hs * after polishing and the value 91 (Oe) was within the range of the measurement error, and the saturated magnetic field Hs * did not substantially increase. This sample was cut into a size of 1.2 mm × 1.2 mm to obtain 64 chips. As a result of measuring these nucleation magnetic fields Hn, it was in the range of 69 to 136 (Oe), and 43 samples exceeded the saturation magnetic field Hs * 91 (Oe) before the heat treatment.

Comparative Example 3 Two of the BIG-1 obtained in Example 1 were Pt in an electric furnace.
It was placed on a plate and heated at 900 ° C. for 10 hours.
The saturation magnetic field Hs * of this sample is 125 (Oe), and the saturation magnetic field Hs * changes from 91 (Oe) before heat treatment to 28 (Oe).
e) It has risen. Next, an external magnetic field of 250 (Oe) was applied at an angle of 60 degrees with respect to the film surface, and changes in the Faraday rotation angle were examined. The Faraday rotation angle decreased by 3%.

Comparative Example 4 Two of the BIG-1 obtained in Example 1 were replaced with Pt in an electric furnace.
It was placed on a plate and heated at 1100 ° C. for 10 hours. The saturation magnetic field Hs * of this sample was 163 (Oe), and the saturation magnetic field Hs * was 66 from 91 (Oe) before the heat treatment.
(Oe) It rose. Next, an external magnetic field of 250 (Oe) was applied with a tilt of 60 degrees with respect to the film surface, and changes in the Faraday rotation angle were examined. The Faraday rotation angle decreased by 40%.

[0029]

According to the present invention, the lower limit of the temperature is 50 ° C. higher than the growth temperature of the bismuth-substituted rare earth iron garnet single crystal film, and the upper limit is 120 ° C. higher than the growth temperature. The garnet single crystal film is heat-treated for 2 hours or more. Due to this heat treatment, a small saturation magnetic field Hs
Without losing the characteristic that the bismuth-substituted rare earth iron garnet single crystal film having * is magnetized in the direction perpendicular to the film surface,
The nucleation magnetic field Hn can be equal to or lower than the saturation magnetic field Hs *.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kazushi Shirai             No. 1 Shinjuku 6-chome, Katsushika-ku, Tokyo Photo             Within Crystal Co., Ltd. F-term (reference) 2G017 AA03 AD12                 2H079 AA03 BA02 CA06 DA13                 4G077 AA03 BC22 BC27 BC28 CG02                       CG07 EA02 EC08 FE11 FJ03                       QA04

Claims (5)

[Claims] 1. A bismuth-substituted rare earth iron garnet single crystal film grown by a liquid phase epitaxial method is heated to a temperature higher than the growth temperature of the bismuth-substituted rare earth iron garnet single crystal film by 50.
A Faraday rotator characterized by being heat-treated for 2 hours or more within a temperature range in which a temperature higher by ℃ is a lower limit value and a temperature higher by 120 ° C than a growth temperature of a garnet single crystal film is an upper limit value. 2. The bismuth-substituted rare earth iron garnet unit
Crystal film is (RBi) _Three(FeM)₅O₁₂The chemical composition of
And R is Y, La, Ce, Pr, Nd, Sm, Eu,
Group of Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu
Is at least one selected from the group consisting of Ga and S
at least one selected from the group consisting of c, Al, and In
The Faraday rotator according to claim 1, wherein. 3. The bismuth-substituted rare earth iron garnet single crystal film has a chemical composition of (GdRBi) ₃ (FeAlGa) ₅ O ₁₂ , and R is at least one selected from the group of Y, Yb, and Lu. The Faraday rotator according to claim 1, wherein. 4. The bismuth-substituted rare earth iron garnet single crystal film is (GdYBi) (FeGa) G, the growth temperature is 740 ° C. to 790 ° C., and the heat treatment temperature is 8
The Faraday rotator according to claim 1, wherein the temperature is 20 ° C to 900 ° C. 5. The Faraday rotator according to claim 1, wherein the bismuth-substituted rare earth iron garnet single crystal film has a saturation magnetic field of less than 200 (Oe) before heat treatment.