Magneto-optic garnet

Abstract: This invention provides a magneto-optic garnet grown by liquid phase epitaxy on a nonmagnetic garnet substrtae and hving a composition of formula (1) Ho x Tb y Bi 3-x-y Fe₅O₁₂ wherein 0.3≦y/x≦1.0 and x+y< 3.0. The magneto-optic garnet can be used as a Faraday rotator in, for example, an optical isolator or optical circulator, since it has a very large Faraday rotation coefficient, differs little in lattice constant from a nonmagnetic garnet substrate, exhibits a mirror face without film defect (or pit), and has a small temperature dependency.

Description: This invention relates to a magneto-optic garnet which can be used as an optical element in, for example, optical isolators or circulators, utilising the Faraday effect. Laser diodes are widely used to provide a coherent light source for light-applied apparatus and optical communication. However, there is a problem in that when beams emitted from a laser diode are reflected by an optical system, the reflected beams destabilise laser diode oscillation. In order to overcome this problem, an attempt has been made to prevent beams emitted from a laser diode from returning thereto by providing an optical isolator on the optical emission side of the laser diode. As a Faraday rotator for an optical isolator to separate beams emitted by a laser diode from reflected beams utilizing the Faraday effect, there have been used bulk single crystals of yttrium iron garnet (YIG) having excellent transparency at wavelengths of not less than 1.1 µm. Further, there have recently been many reports on bismuth-substituted rare-earth iron garnet thick film, which is a single crystal thick film grown by liquid phase epitaxy, having a Faraday rotation coefficient several times larger than that of YIG and obtained by mass-producible liquid phase epitaxy (LPE). Since the Faraday rotation coefficient of a bismuth-substituted rare-earth iron garnet increases nearly in proportion to an increase in the amount of substituted bismuth, it is desired to form a garnet film containing as much bismuth as possible. Since, however, bismuth has a large ionic radius, the lattice constant of the bismuth-substituted rare-earth iron garnet increases in proportion to an increase in the amount of substituted bismuth and there is a limit on the amount of substituted bismuth in the garnet to bring its lattice conformity to those used as a substrate in such a thick film such as a neodymium gadolium gallium garnet (Nd₃Ga₅O₁₂) substrate (to be referred to as "NGG substrate" hereinbelow) having a lattice constant of 12.509Å and a calcium-, magnesium-, and zirconium-substituted gadolinium gallium garnet {(GaGd)₃(GaMgZr)₅O₁₂} substrate (to be referred to as "SGGG substrate" hereinbelow) having a lattice constant of about 12.496Å - 12.530Å. In order to avoid the above limitation and use as much as possible an amount of bismuth for the substitution, a rare-earth element having a smaller ionic radius is used, and as a result, such use can prevent the increase in the lattice constant. An example of the use of rare earth element ions having a small ionic radius from the above view point is reportedly (LuBi)₃Fe₅O₁₂ in which Lu is substituted by a large amount of bismuth [e.g., see 32th Applied Physics-Related Associated Lectures, 30p-N-5 (1985)]. However, the use of such a material causes a film defect called "pit", and it is difficult to obtain a mirror face. Thus, such a material has not yet been put to practical use. Further, "Japan Applied Magnetism Society Report" Vol. 10, No. 2 (1986), pages 143 to 146, proposes an addition of Gd³⁺ ions in order to improve the above problem that the film defect takes place in (LuBi)₃Fe₅O₁₂, and it is also reported therein that, as a result, a thick film of (GdLuBi)₃Fe₅O₁₂ having a Faraday rotation coefficient, at a wavelength of 1.3 µm, of as large as 1,800 deg/cm and exhibiting a mirror face was obtained. In general, however, the Faraday effect of Bisubstituted rare-earth iron garnet is affected by temperature, and thereby temperature changes bring a change of Faraday rotation angle which leads directly to degradation of performance. Therefore, it is desired that temperature dependence should be as small as possible. Especially, however, it is described in, for example, a treatise entitled "Improvement of Temperature Characteristic Of Bi-Substituted Garnet In Faraday Rotation Angle by Dy" of "Japan Applied Magnetism Society Report", Vol. 10, No. 2 (1986), pages 151 to 154, that the temperature dependence with the use of Gd³⁺ ions is larger than in the case of other rare earth elements. In view of the temperature dependence, therefore, it cannot be said that such use of Gd³⁺ ions as a main component of bismuth-substituted rare-earth iron garnet as in the above (GdLuBi)₃Fe₅O₁₂ is preferable. SUMMARY OF THE INVENTION It is an object of this invention to provide a magneto-optic garnet as a Faraday rotator for use in an optical isolator, optical circulator, etc., utilizing Faraday effect. It is another object of this invention to provide a magneto-optic garnet as a Faraday rotator, which has a very large Faraday rotation coefficient. It is another object of this invention to provide a magneto-optic garnet as a Faraday rotator, which is prepared by forming a garnet film having a very large Faraday rotation coefficient and a small difference in lattice constant from a nonmagnetic garnet substrate. It is further another object of this invention to provide a magneto-optic garnet as a Faraday rotator, which is prepared by forming a garnet film having a very large Faraday rotation coefficient and exhibiting a mirror face without causing a film defect (or so-called pit). It is yet another object of this invention to provide a magneto-optic garnet as a Faraday rotator, which is prepared by forming a garnet film having a very large Faraday rotation coefficient and a small temperature dependence. According to this invention there is provided a magneto-optic garnet grown by liquid phase epitaxy on a nonmagnetic garnet substrate and having a composition of the following formula (1) Ho x Tb y Bi 3-x-y Fe₅O₁₂ (1) wherein 0.3≦y/x≦1.0 and x+y<3.0. DETAILED DESCRIPTION OF THE INVENTION In this invention, y/x in the formula (1), i.e., the component ratio of Tb to Ho in the single crystal film is 0.3 to 1.0, preferably 0.5 to 1.0. If the above y/x is less than the above lower limit, more than 100, per 1 cm², of so-called pits occur, i.e., the crystal failure occurs, and the resultant magneto-optic garnet is not suitable for use as a Faraday rotator. And if the above y/x exceeds the above upper limit, the lattice constant of the single crystal film increases since the Tb ionic radius is large. Consequently, for this reason, there is no option but to reduce 3-x-y in the formula (1), i.e., the amount of substituted Bi, in order to bring the conformity with the lattice constant of a nonmagnetic garnet substrate. If the amount of Bi for the substitution is reduced, the Faraday rotation coefficient decreases, and the film thickness need be larger in order to obtain the necessary Faraday rotation angle, which is a disadvantage in industrial production. The amount of Bi for the substitution may be suitably selected depending upon the lattice constant of a nonmagnetic garnet substrate. However, in the case of presently commercially available nonmagnetic garnet substrates having a lattice constant of from 12.496 to 12.530Å, the amount of Bi for the substitution (i.e., 3-x-y) is preferably 0.9 to 1.7. The single crystal film of this invention having a composition of the formula Ho x Tb y Bi 3-x-y Fe₅O₁₂ (1) wherein 0.3≦y/x≦1.0 and x+y<3.0. can be obtained by growing same on a nonmagnetic garnet substrate according to liquid phase epitaxy. The liquid phase epitaxy is carried out, in general, in the following manner. While a melt in a platinum crucible (solution of flux component and garnet material component) is maintained at a supersaturation temperature (usually 750 to 850 °C), a nonmagnetic garnet substrate is immersed in the melt or contacted on the surface of the melt. Then, magnetic garnet grows as a single crystal film on the substrate. Usually used as the flux component is a mixture of PbO, B₂O₃ and Bi₂O₃. The substrate is, for example, neodymium gallium garnet, Nd₃Ga₅O₁₂ (NGG), having a lattice constant of 12.509Å or calcium-, magnesium-, and zirconium-substituted gadolinium gallium garnet, (CaGd)₃(MgZrGa)₅O₁₂ (SGGG), having a lattice constant of from 12.496 to 12.530Å. These substrates are suitably usable for the growth of bismuth-substituted magnetic garnet owing to their large lattice constants. When a magneto-optic garnet is actually used in a Faraday rotator for an optical isolator, the film face is, in general, polished to adjust the film thickness such that the rotation angle in plane of polarization exhibits 45±1°. In this case, it is not always necessary to remove the substrate completely by polishing. Since, however, Fresnel reflection (about 1%) occurs in the interface between the substrate and the film, it is desirable to remove the substrate if the reflected light causes a problem. By compensating the large ionic radius of Bi with the small ionic radius of the Ho-Tb two component system, this invention makes it possible to obtain a single crystal film of magneto-optic garnet having, as a Faraday rotator, especially excellent properties that its lattice constant is nearly equal to the lattice constant of a nonmagnetic garnet substrate and that not only the Faraday rotation coefficient of the magneto-optic garnet is large but also its temperature dependence is small. EXAMPLES This invention will be illustrated more in detail in the following Examples, in which the Faraday rotation coefficients and Faraday rotation angles were measured as follows. Method of measuring Faraday rotation coefficient: Polarized light was directed to a garnet film and a rotation angle of a polarized light plane was measured by rotating an analyzer. At this time, the garnet film was magnetically saturated by an external magnetic field to arrange the magnetism of the garnet in the direction of the external magnetic field. The rotation angle measured as mentioned above is a Faraday rotation angle (ϑ), and the value obtained by dividing the Faraday rotation angle by the thickness of a garnet film is a Faraday rotation coefficient (ϑ F ). Method of measuring temperature dependency of Faraday rotation angle: A garnet film was heated or cooled, and Faraday rotation angles were measured at temperatures after the heating or cooling. EXAMPLE 1 A (111) NGG substrate (having a lattice constant of 12.509Å) was contacted on the surface of a melt having a composition shown in the following Table 1, and a film was grown on one surface of the substrate at 820°C for 15 hours by liquid phase epitaxy to give a magnetic garnet single crystal film exhibiting a mirror face and having a thickness of 250 µm and a composition of Ho 1.11 Tb 0.56 -Bi 1.33 Fe₅O₁₂. The above composition of the garnet was determined by dissolving the film, from which the substrate had been removed, in hot phosphoric acid and subjecting its solution to plasma emission analysis. The resultant single crystal film had a Faraday rotation coefficient, at a wavelength of 1.3 µm, of 0.22 deg/µm and a Faraday rotation coefficient change ratio, per 1 °C at a temperature of from -20 to 70 °C, of 0.113%. Thus, the single crystal film had excellent properties as a Faraday rotator. Component Mole% PbO 50.0 Bi₂O₃ 30.0 B₂O₃ 10.5 Fe₂O₃ 9.10 Ho₂O₃ 0.33 Tb₄O₇ 0.07 EXAMPLE 2 A (111) NGG substrate was contacted on the surface of a melt having a composition shown in the following Table 2 and a film was grown on one surface of the substrate at 817 °C for 15 hours by liquid phase epitaxy to give a magnetic garnet single crystal film exhibiting a mirror face and having a thickness of 245 µm and a composition of Ho 1.03 Tb 0.95 Bi 1.02 Fe₅O₁₂. The above single crystal film had a Faraday rotation coefiicient, at a wavelength of 1.3 µm, of 0.17 deg/µm and a Faraday rotation coefficient change ratio, per 1 °C at a temperature of from -20 to 70 °C, of 0.010%. Thus, the single crystal film had excellent properties as a Faraday rotator. Component Mole% PbO 50.0 Bi₂O₃ 30.0 B₂O₃ 10.5 Fe₂O₃ 9.10 Ho₂O₃ 0.27 Tb₄O₇ 0.13 EXAMPLE 3 A (111) SGGG substrate (having a lattice constant of 12.497Å) was contacted on the surface of a melt having a composition shown in the following Table 3 and a film was grown on one surface of the substrate at 825 °C for 15 hours by liquid phase epitaxy to give a magnetic garnet single crystal film exhibiting a mirror face and having a thickness of 236 µm and a composition of Ho 1.22 Tb 0.62 Bi 1.16 Fe₅O₁₂. The above single crystal film had a Faraday rotation coefficient, at a wavelength of 1.3 µm, of 0.20 deg/µm and a Faraday rotation coefficient change ratio, per 1 °C at a temperature of from -20 to 70 °C, of 0.106%. Thus, the single crystal film had excellent properties as a Faraday rotator. Component Mole% PbO 52.0 Bi₂O₃ 26.0 B₂O₃ 10.5 Fe₂O₃ 11.1 Ho₂O₃ 0.32 Tb₄O₇ 0.08 COMPARATIVE EXAMPLE 1 A (111) SGGG substrate (having a lattice constant of 12.497Å) was contacted on the surface of a melt having a composition shown in the following Table 4 and a film was grown on one surface of the substrate at 823 °C for 24 hours by liquid phase epitaxy to give a magnetic garnet single crystal film having a thickness of 318 µm and a composition of Ho 1.35 Tb 0.40 Bi 1.25 Fe₅O₁₂. However, the above single crystal film had many pits on its surface and was not suitable as a Faraday rotator. Component Mole% PbO 52.0 Bi₂O₃ 26.0 B₂O₃ 10.5 Fe₂O₃ 11.1 Ho₂O₃ 0.36 Tb₄O₇ 0.04

Claims: A magneto-optic garnet grown by liquid phase epitaxy on a nonmagnetic garnet substrate and having a composition of formula (1) Ho x Tb y Bi 3-x-y Fe₅O₁₂ (1) wherein 0.3≦y/x≦1.0 and x+y<3.0. A magneto-optic garnet according to claim 1 wherein 0.5≦y/x≦1.0. A magneto-optic garnet according to claim 1 or 2 wherein 0.9≦3-x-y≦1.7. A magneto-optic garnet according to claim 1, 2 or 3 wherein the substrate is a calcium-, magnesium-, and zirconium-substituted gadolinium gallium garnet. A magneto-optic garnet according to claim 1, 2 or 3 wherein the substrate is a neodymium gallium garnet. Use of a magneto-optic garnet as claimed in any one of the preceding claims as a Faraday rotator.

Inventors: ARII MITSUZO C O CENTRAL RESEA, SHIRAI KAZUSHI C O CENTRAL RES, TAGAMI YASUNORI C O CENTRAL RE, TAKEDA NORIO C O CENTRAL RESEA, ARII, MITSUZO C/O CENTRAL RESEARCH INST., SHIRAI, KAZUSHI C/O CENTRAL RESEARCH INST. OF, TAGAMI, YASUNORI C/O CENTRAL RESEARCH INST.OF, TAKEDA, NORIO C/O CENTRAL RESEARCH INST. OF
Applicants: MITSUBISHI GAS CHEMICAL CO, MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1989
Main classification: G02F1
IPC codes: G02F1, C30B29, H01F10