Faraday Rotator

Europaisches Patentamt J European Patent Office © Publication number: 0 646 826 A1 Office europeen des brevets

EUROPEAN PATENT APPLICATION

@ Date of filing: 31.08.94

© Priority: 05.10.93 JP 249523/93 © Applicant: MITSUBISHI GAS CHEMICAL COMPANY, INC. @ Date of publication of application: 5-2, Marunouchi 2-chome 05.04.95 Bulletin 95/14 Chiyoda-Ku Tokyo, 100 (JP) © Designated Contracting States: DE FR GB @ Inventor: Shirai, Kazushi, c/o Mitsubishi Gas Chem.Co.lnc. Tokyo Lab., 1-1, Niijuku 6-chome, Katsushika-ku Tokyo 125 (JP) Inventor: Takeda, Norio, c/o Mitsubishi Gas Chem.Co.lnc. Tokyo Lab., 1-1, Niijuku 6-chome, Katsushika-ku Tokyo 125 (JP)

© The present invention provides a means of eras- are enabled to utilize practically. ing the magnetic hysteresis with respect to a rare- earth iron garnet single-crystal film with large mag- FiG. 4 netic hysteresis which is an obstacle in using as a Faraday rotator although the film has extremely FARADAY ROTATION ANGLE small saturated magnetic field intensity. In the present invention, a permanent magnet is positioned on a part of a rare-earth iron garnet single CO crystal. CM 00 According to the present invention, the magnetic hysteresis can be erased with respect to a rare-earth CO iron garnet single-crystal film with large magnetic EXTERNAL MAGNETIC CO hysteresis which is an obstacle in using as a FIELD INTENSITY Faraday rotator although the film has extremely small saturated magnetic field intensity. As the result, an optical switch utilizing Faraday effect and a magneto-optical sensor with a Faraday rotator applied a rare-earth iron garnet single-crystal film which has small saturated magnetic field intensity

Rank Xerox (UK) Business Services (3. 10/3.09/3.3.4) 1 EP 0 646 826 A1 2

FIELD OF INVENTION gle, are the optical switches or the magnetic field sensors. The present invention relates to means to The optical switches can be acted by changing make use of a rare-earth iron garnet single-crystal over the magnetic fields having larger intensity film with large magnetic hysteresis as a Faraday 5 than the saturation point b or e in Fig. 1, wherein rotator utilized for magneto-optical sensors and op- the magnetic field is generated by a magnetic field tical switches. generating device, and the detection of the mag- More particularly, the invention relates to netic field by using an optical current sensor is means to make it possible to apply even a rare- carried out by knowing the difference in the earth iron garnet single-crystal film with large mag- io Faraday rotation angles within the linear zone be- netic hysteresis to a Faraday rotator , by erasing tween the origin o and the saturation point b ac- the magnetic hysteresis which is an obstacle to the cording to the light ray intensity. And, the detecting use of the film as a Faraday rotator of magneto- of the magnetic field intensity by using a magneto- optical sensors and optical switches. optical sensor for a rotation sensor is carried out by 75 knowing the difference in the Faraday rotation an- BACKGROUND OF THE INVENTION gles within the origin o and the saturation point b or c according to the light intensity. In recent years, the optical switches or the A configuration of an optical switch utilizing a devices which are called an optical current sensor rare-earth iron garnet single-crystal film is illus- and an optical rotation sensor, utilizing a rare-earth 20 trated in Fig. 2, wherein numeral 1 designates a iron garnet single-crystal film with large Faraday polarizer made of rutile or the like ; numeral 2 , a effect , have been putted to practical use in suc- Faraday rotator composed of a rare-earth iron gar- cession. Further, the exploitation of rare-earth iron net single-crystal film ; numeral 3, a polarized light garnet single-crystal films in accordance with those separator made of rutile or the like. Numeral 4 uses has been carried out actively. 25 denotes a magnetic field generating device having Faraday effect is a kind of magneto-optical enough capacity to saturate the Faraday rotator effects and a rotation phenomenon of a polarization magnetically, i.e., composed of electromagnets, plane of a light ray which passes through materials coils and etc. to generate the magnetic field having having Faraday effect, i.e., Faraday elements larger intensity than the saturated magnetic field. (Faraday rotator) such a rare-earth iron garnet sin- 30 In Fig. 2, a light ray passing through the polar- gle-crystal film. And, the rotation angle in the po- izer 1 is turned into a linearly polarized light and it larization plane is proportional to the magnetization enters into the Faraday rotator. Thereupon, the intensity, as shown in Fig. 1. linearly polarized wave plane of the input light is In Fig. 1, the angle of the Faraday rotation is zero, rotated. Whether the rotation of the linearly po- i.e., which is positioned at the origin(o), when no 35 larized wave plane results in clockwise (plus) or external magnetic fields are applied. counter-clockwise (minus) direction depends on the In accordance with successive increasing in the direction of the magnetic field applied to the intensity of the magnetic field, the absolute value of Faraday rotator. When the rotation direction of the the Faraday rotation angle (0F or -0F, clockwise is linearly polarized wave plane is changed over, the plus and counterclockwise is minus normally ) pro- 40 path of the light passing through the polarized light ceeds via the point a or the point d, and increases separator 3 is switched. That is, by switching the gradually (the route o-a-b, or the route o-d-e). The direction of the applied magnetic field, the output Faraday rotation angle reaches the saturated value light path is switched over, i.e., the switching op- ( the saturated magnetic field : the point b or e), eration is enabled. when the external magnetic field intensity gets to 45 In this configuration, if the saturated intensity of an intensity value (Hs). Even if the external mag- the magnetic field in the Faraday rotator is consid- netic field intensity increases further, the Faraday erably large, in order to generate the magnetic field rotation angle makes no change and only transits enough for saturating the Faraday rotator mag- from b to c or from e to f, because the Faraday netically, a magnetic field generating device in rotation angle has reached to the saturated value. 50 large size is necessary, which needs considerable And then, the Faraday rotation angle traces the electric power. Further, it causes the problem of reverse route, i.e., the route c-b-a-o or f-e-d-o, heat generation by way of the coils by the electric while the external magnetic field intensity is re- current. Therefore, in an optical switch based on duced gradually, and returns back to the origin o Faraday effect, the saturated magnetic field inten- when the external magnetic field finally comes to 55 sity of a rare-earth iron garnet single-crystal film is make no effects. As mentioned above, the devices required to minimize for the sake of saving electric to utilize the characteristic : the magnetic field power and making small in size with regard to the dependency on the polarization plane rotation an- magnetic field generating device.

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An optical current sensor is configurated nor- speed meter utilized Faraday effect, it is necessary mally as shown in Fig. 3, i.e., numeral 5 denotes a to compensate the lower level magnetic field by polarizer comprising of rutile and the like ; numeral using a more powerful permanent magnet and/or to 6, a Faraday rotator comprising of a rare-earth iron make the distance, between the permanent magnet garnet single-crystal film and the like ; numeral 7, 5 and the magneto-optical sensor head, shorter, in an analyzer comprising of rutile and the like. In Fig. conjunction with selecting a magneto-optical sensor 3, the light passed through the polarizer 5 has the head sufficiently operative within the lower level character of a linearly polarized light, wherein the magnetic field. linearly polarized wave plane is rotated. The light And, it is not economical to use a more pow- having passed through the Faraday rotator passes io erful permanent magnet , which, moreover, puts a trough the analyzer, wherein the light intensity be- restriction on an installing location occasionally being changed in response to the Faraday rotation cause of a large size permanent magnet, wherein angle. Normally, the rotation of the linearly po- the distance between the permanent magnet and larized wave plane changes almost linearly within the magneto-optical sensor head gets longer to the magnetic field less than saturation (as shown in is cause an unfavorable result or to achieve no im- Fig. 1) in response to the external magnetic field provement substantially. Hence, it is required to applied to the Faraday rotator. Therefore, by mea- provide a rare-earth iron garnet single-crystal film suring the light intensity after the light passes the which is saturated magnetically by a low level analyzer, the intensity of the external magnetic field magnetic field. applied to the Faraday rotator can be known. 20 As mentioned above, in an optical switch or a In an optical current sensor, a minute change magneto-optical sensor, it is much required to pro- of electric current running in cables must be de- vide a rare-earth iron garnet single-crystal film tected precisely. In order to detect the a minute which can be saturated magnetically by a magnetic change of the electric current, i. e., a minute field in a level as lower as possible. change of the magnetic field generated by the 25 current, the inclination of the straight line in Fig. 1 SUMMARY OF THE INVENTION should be larger, which requests to reduce the intensity of the saturated magnetic field of a rare- A rare-earth iron garnet single-crystal film de- earth iron garnet single-crystal film utilized in a noted by the chemical structural formula R3(FeA)- Faraday rotator so that the Faraday rotation angle 30 5O12 (wherein, R represents yttrium Y, bismuth Bi per magnetic intensity can be larger. or a rare-earth element, and A represents alu- In a rotating speed meter utilizing a magneto- minum Al, gallium Ga or the like) can be produced optical sensor, i.e., a rotating speed sensor based comparatively easily by a liquid-phase epitaxial on an optical signal, a permanent magnet is pro- method (LPE). Especially, it is known that a bis- vided on a rotary equipment (as shown in "Applied 35 muth substituted rare-earth iron garnet single cry- Optics", in 1989, Vol.28 No.11, page 1992), stal in which a part of rare-earth compounds is wherein within the scope of the magnetic field substituted by bismuth proves considerably large produced by the permanent magnet, a magneto- Faraday effect. optical sensor comprising of a rare- earth iron As a rare-earth iron garnet single crystal uti- garnet single-crystal film is arranged, whereof, ac- 40 lized for an optical switch and an optical current cording to the rotation of the rotary equipment, the sensor is preferable to have a saturated magnetic magneto-optical sensor and the permanent magnet field intensity in the lowest level as possible in come close and get apart to each other repeatedly, consideration of their sensitivity and production and the difference, in the magnetic field intensity cost; normally, it is widely carried out that the iron applied to the Faraday rotator, is caused between 45 is substituted by such elements as aluminum Al or the state of coming close and the state of getting gallium Ga. For example, (GdBi)3(FeGaAI)50i2, apart. That is, according to the turn of the perma- (HoTbBi)3(FeGaAI)5Oi2 and the like is reported nent magnet, the Faraday rotation angle is thereof (refer to Japanese Patent Application Pre- changed, which causes a change in the light inten- liminary Publication No. 61-20926 and Japanese sity passing through the analyzer. Therefore, the 50 Patent Application No. HEISEI 5-55621). number of the rotation or the rotating speed can be However, in regard to a rare-earth iron garnet known by detecting the change of the light inten- single-crystal film, while the constitution ratio of Al sity. and Ga to Fe is changed to reduce the saturated In this case, the distance between the perma- magnetic field intensity, it causes the critical prob- nent magnet and the magneto-optical sensor head 55 lem, in practical use, that the magnetic hysteresis gets longer, the magnet field intensity turns in increases gradually. lower level. Hence, for the sake of achieving higher In the following, the detailed explanation are level sensitivity and accuracy in regard to a rotating described in regard to the magnetic hysteresis. In

3 5 EP 0 646 826 A1 6 regard to a rare-earth iron garnet single crystal with curve being drawn (Fig. 6), after saturated once, comparatively large saturated magnetic field inten- the rectangular hysteresis loop being drawn only, sity, for an example, (HoTbBi)3Fe5 0i2, a magnetiz- the magnet field intensity necessary for saturated ation characteristics of a Faraday rotator in re- magnetization getting larger than the original mag- sponse to an external magnetic field, that is, the 5 netic field intensity of the rare-earth iron garnet relation between the Faraday rotation angle and the single crystal. external magnetic field intensity is as shown Fig. 1. Thus, the rare-earth iron garnet single-crystal However, a rare-earth iron garnet single crystal with film with large magnetic hysteresis, as the relation small saturated magnetic field intensity, for exam- of being linear between the applied magnetic field ple, (GdBi)3(FeAIGa)5 0i2 describes such a hyster- io and the Faraday rotating angle is not kept on, is esis loop as the route o-a-b-c-b-b'-a-o shown in naturally not proper at all as a Faraday rotator and Fig. 4. That is, the route, when the external mag- is considerably difficult to use in an optical rotating netic field intensity is increasing (the route o-a-b-c), sensor and an optical switch because of the large is different from the route whereof the intensity is magnetic intensity necessary for saturated mag- decreasing (the route c-b-b'-a-o). 15 netization. In Fig. 4, the magnetic field intensity at the Generally, in a rare-earth iron garnet single- points b and e is named saturated magnetic field crystal film, when the iron is substituted by gallium intensity Hs, the magnetic field intensity at the and aluminum, the saturated magnetic field inten- points b' and e' brought about by the hysteresis of sity gets smaller, and the magnetic hysteresis is the magnetization characteristics is named nuclea- 20 getting larger in response to the reduce of the tion magnetic field intensity Hn, and the difference saturated magnetic field intensity. Therefore, the (Hs-Hn) between the saturated magnetic intensity inventors of the present invention have studied Hs and the nucleation magnetic field intensity Hn is intensively to exploit a rare-earth iron garnet single- defined as a hysteresis value. Hereof, in order to crystal film with no magnetic hysteresis. As the make the explanation simple and easy, the satu- 25 result, (YLaBi)3(FeGa)5 0i2 has been found out, rated magnetic field intensity Hs at the point b is wherein it has extremely small magnetic hysteresis named Hs1 , the nucleation magnetic field intensity in spite of its small saturated magnetization char- Hn at the point b' is named Hn1, the saturated acter, in comparison with a conventional rare-earth magnetic field intensity Hs at the point e being iron garnet single-crystal film. That is, it is a rare- named Hs2, the nucleation magnetic field intensity 30 earth iron garnet single-crystal film having the Hn at the point e' being named Hn2. chemical structure of the general formula Y3-X_y When the hysteresis of the rare-earth iron gar- Lax Biy Fe5_z Gaz O12 (wherein, x,y or z is a net single-crystal film gets larger and the nuclea- respective value : 0.1 ^ x ^ 0.4, 1 .0 ^ y S 1 .9, 1 .0 ^ tion magnetic field intensity Hn is coming close to z ^ 1.6) and it is grown on a non-magnetic garnet the origin o, as shown in Fig. 4, the once saturated 35 single crystal substrate by a liquid-phase epitaxial magnetization state is kept on till the external mag- method. netic field gets almost zero. Therefore, with respect However, even in said (YLaBi)3(FeGa)5 0i2, the to an optical rotating sensor comprising of a inclination : the magnetic hysteresis gets larger in Faraday rotator with larger hysteresis, in order to response to the reducing of the saturated magnetic detect the magnetic field intensity, the magnetic 40 field intensity, was found. And, if the saturated field intensity surrounding the magneto-optical sen- magnetic field intensity is less than 150 Oe, the sor must be made almost zero. However, the rotat- relation between the Faraday rotation angle and the ing sensor is arranged inside of a machinery equip- external magnetic field intensity is like as shown in ment usually, and, further, the material of the ma- Fig. 5 or Fig. 6, which being considerably difficult chinery equipment has a magnetic field only by a 45 in regard to its practical use. just small value. Hence, it is considerably difficult As mentioned above in details, a rare-earth iron to use the magneto-optical sensor of which the garnet single-crystal film with the lower saturated surrounding magnetic field intensity being almost magnetic field intensity has large magnetic hyster- zero. Further, with respect to an optical current esis, whereof, in the case of utilizing the film for a sensor, the measuring range of the magnetic field 50 Faraday rotator in an optical switch and a magneto- intensity is restricted within the range less than Hn. optical sensor, there has been an obstruction or a Moreover, when the hysteresis of the rare-earth restriction : such that a magnetic field generating iron garnet single-crystal film gets larger further, device of large capacity to switch the magnetic the nucleation magnetic field intensity Hn1, in field is requested ; that the magnetic intensity of some cases, enters into a minus side (Fig. 5), and 55 the permanent magnet arranged on a rotary equip- the nucleation magnetic field intensity Hn1, in other ment must be larger ; that the measuring range of cases, gets larger than the saturated magnetic field the magnetic field is very narrow ; or the like. intensity Hs2, wherein a rectangular hysteresis

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The inventors of the present invention have even when the external applied magnetic intensity studied intensively in regard to methods of utilizing is reduced, which suggests a need of some acts a rare-earth iron garnet single-crystal film with a for the transit from single domain structure to multi- large magnetic hysteresis for a Faraday rotator in domain structure, which bringing about the mag- an optical switch and a magneto-optical sensor, 5 netic domain magnetized in the opposite direction whereby it has been found out that the magnetic to the single domain structure , i.e., in the minus hysteresis which has caused a hitch, in operation magnetization direction. of an optical switch and a magneto-optical sensor, And, on a part of a rare-earth iron garnet sin- can be erased by constant magnetization of a part gle-crystal film, a minus magnetized permanent of the rare-earth iron garnet single-crystal film by a io magnet is positioned, whereby the surrounding do- permanent magnet in the normal direction to the mains of the permanent magnet hold minus mag- film surface, and even a rare-earth iron garnet netization constantly even though the almost all single-crystal film with a large magnetic hysteresis domains are plus magnetized by an external ap- can be utilized for a Faraday rotator in an optical plied magnetic field (Fig. 8). Then, when the exter- switch and a magneto-optical sensor, which being 15 nal magnetic field intensity is reduced, the follow- an extremely useful knowledge, whereby the ing process is inferred; that is, as there are the present invention having been completed with the domains having minus magnetization around the addition of the further research/study. permanent magnet already, the domain B having That is, the present invention has been com- minus magnetization spreads promptly to the all pleted as an industrial production technique by the 20 parts from there (Fig. 9). Therefore, by way of the further experiment/study, which being based on the portion except for the part on which the permanent knowledge that by the constant magnetization of a magnet is provided in regard to the rare-earth iron part of the rare-earth iron garnet single-crystal film garnet single crystal, the state of no hysteresis with a permanent magnet in the normal direction to being at all is achieved. the film surface; whereby the magnetization char- 25 The completion of the present invention has acteristics of the rare-earth iron garnet single cry- enabled to produce an optical switch of further stal film for the external magnetic field being im- save energy and smaller in size and a magneto- proved. optical sensor detecting a minute magnetic field, The magnetic hysteresis of the rare-earth iron wherein utilizing a rare-earth iron garnet single cry- garnet single-crystal film can be erased by con- 30 stal. stant magnetization of a part of the rare-earth iron In the application of the present invention, a garnet single-crystal film, the reason of which is not kind of permanent magnet is not specified particu- definite but is inferred in relation to the magnetic larly and must have only magnetic power enough domain structure of the rare-earth iron garnet sin- to saturate locally the rare-earth iron garnet single gle-crystal film, as follows: 35 crystal, such as Alnico magnets, ferrite magnets, the magnetic domain structure of the rare-earth rare-earth permanent magnets or the like. iron garnet shows a so-called multi-domain struc- In the application of the present invention, the ture (state 1) as illustrated in Fig. 7, wherein in permanent magnet is not specified in size except regard to the magnetization direction, the upward having a magnetic power enough to saturate locally direction domain A and the downward direction 40 the rare-earth iron garnet single crystal. domain B stand side by side alternately. And, for In the application of the present invention, in example, when a magnet field is applied in the regard to the arrangement of the permanent mag- same direction as domain A from outside (for a net, although being different according to the size clearer explanation, it is defined that, in regard to of the rare-earth iron garnet single crystal or to the the magnetization or the magnet field, the upward 45 intensity and the size of a permanent magnet , the direction is plus ; the downward direction is minus), permanent magnet is only needed to be positioned the area of the domain A is increasing gradually by more than 0.3 mm apart from the light path. (state 2); then, the minus magnetic domains are In the application of the present invention, the erased and the all magnetic domains get plus magnetization direction of a permanent magnet together (state 3). That is just a magnetically satu- 50 must be opposite to the magnetic field applied rated state. from outside to the rare-earth iron garnet single Then, when the external applied magnetic field crystal. When the magnetic field applied from out- intensity is reduced gradually, magnetic domains side to the rare-earth iron garnet single crystal is appear in the just less intensity state than the not of one direction but of both plus/minus direc- saturated magnetic field intensity. However, in the 55 tions, more than two magnets having different di- case of a rare-earth iron garnet single-crystal film rected magnetization are only needed to be posi- with large magnetic hysteresis, the single domain tioned. structure (state 3 shown in Fig. 7) has been kept on

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In the application of the present invention, the than the saturated magnetic field intensity from Faraday rotator, i.e., the composition of the rare- outside; earth iron garnet is not specified particularly, but it Fig. 10 is a schematic drawing to show a is preferably applied to a rare-earth iron garnet Faraday rotator, whereof one of permanent mag- single crystal denoted by the general formula 5 net is positioned on a part of a rare-earth iron R3Fe5_zAzOi2 (wherein, R represents at least one garnet single crystal of 1mm x 3mm in size; of the group: Bi, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Fig. 11 is a drawing to show an example of Dy, Ho, Er, Tm, Yb, Lu, and A represents one of magnetic characteristics with respect to a rare- the group: Ga, Sc, Al, In ) and more particularly to earth iron garnet single crystal comprising of the the ones with large magnetic hysteresis. io components in Fig. 10; The present invention has been described con- Fig. 12 is a schematic drawing to show a cretely and in detail with the embodiments to show Faraday rotator, wherein a couple of permanent specific applications and their effects in the follow- magnets are positioned on parts of a rare-earth ing. It should be noted that the following examples iron garnet single crystal of 1mm x 3mm in size; are for specifically describing the invention, but that is Fig. 13 is a schematic drawing to show a it is not intended to limit the application of the Faraday rotator, wherein a couple of permanent present invention or the scope of the invention. magnets are positioned on both ends of a rare- earth iron garnet single crystal of 1mm x 3mm BRIEF DESCRIPTION OF THE DRAWINGS in size. 20 Fig. 1 is a schematic drawing to show an exam- DESCRIPTION OF PREFERRED EMBODIMENTS ple of magnetic characteristics with respect to a rare-earth iron garnet single crystal with no mag- Embodiment 1 netic hysteresis; Fig. 2 is a drawing to illustrate an example of 25 In a platinum crucible of volume 500 ml, the optical switches utilizing Faraday effect; materials, 843 g of lead oxide (PbO, 4N); 978 g of Fig. 3 is a drawing to illustrate an example of bismuth oxide (Bi203,4N); 128 g of ferric oxide current sensors utilizing Faraday effect; (Fe203, 4N); 38 g of boron oxide (B203, 5N); 6.0 g Fig. 4 is a schematic drawing to show an exam- of terbium oxide (TUO7, 3N); 7.0 g of holmium ple of magnetic characteristics with respect to a 30 oxide (H02O3, 3N); 18 g of gallium oxide (Ga2 03, rare-earth iron garnet single crystal with mag- 3N); and 4.2 g of aluminum oxide (AI2O3, 3N), were netic hysteresis; laid. The crucible was set at a predetermined posi- Fig. 5 is a schematic drawing to show an exam- tion in a precision vertical tubular electric furnace, ple of magnetic characteristics with respect to a and was heated at 1000 °C to melt the mixture. rare-earth iron garnet single crystal with ex- 35 The melt mixture was fully stirred to be evenly tremely large magnetic hysteresis; mixed, and then cooled down to the melt tempera- Fig. 6 is a schematic drawing to show an exam- ture of 775 °C to obtain a melt for raising a ple of magnetic characteristics with respect to a bismuth-substituted iron garnet single crystal. By a rare-earth iron garnet single crystal with such conventional procedure, the surface of thus ob- extremely large magnetic hysteresis that is of a 40 tained melt was made in contact with the one rectangular hysteresis route; surface of a (111) garnet single-crystal substrate: Fig. 7 is a schematic drawing to show a state of (GdCa)3(GaMgZr)50i2 (lattice constant: magnetic domains with respect to a rare-earth 12.493+0.002 A) of 480 urn in thickness, and the iron garnet single crystal; epitaxial growth was continued while maintaining Fig. 8 is a schematic drawing to show a state of 45 the melt temperature of 775 °C to obtain a (HoT- magnetic domains with respect to a rare-earth bBi)3(FeGaAI)50i2 single-crystal film (HoTb- iron garnet single crystal, whereof a permanent BiFeGaAIG single-crystal film) of 47 urn in film magnet is positioned on a part of a rare-earth thickness. Then, the HoTbBiFeGaAIG single-crystal iron garnet single crystal and the crystal is ap- film was cut into the size of 1 mm x 3 mm to be plied the magnetic field having the intensity larg- 50 measured with respect to magnetic characteristics. er than the saturated magnetic field intensity The measurement was carried out as the following. from outside; First, the HoTbBiFeGaAIG single-crystal film of 1 Fig. 9 is a schematic drawing to show a state of mm x 3 mm in size was positioned at the center of magnetic domains with respect to a rare-earth a magnet field generating device ( MAGNET ) iron garnet single crystal, whereof a permanent 55 comprising of a coil produced by MAGNATEC Co., magnet is positioned on a part of a rare-earth and then, as a magnet field being applied, a semi- iron garnet single crystal and the crystal is ap- conductor laser beam of 0.786u.m was emitted to plied the magnetic field having the intensity less the HoTbBiFeGaAIG single-crystal film. And, by

6 11 EP 0 646 826 A1 12 measuring the rotation angle of the polarization Embodiment 3 plane in the laser ray passed through the HoTb- BiFeGaAIG single-crystal film, the applied magnetic Except that the rare-earth element-permanent field-dependent characteristics of the Faraday rota- magnet of 10000(G) residual magnetic flux density tion angle was studied. 5 and 1mm0 x 1mm in size was used instead of the As the result, with respect to the HoTb- ferrite permanent magnet in Embodiment 2 , the BiFeGaAIG single-crystal film, the saturated mag- measurement was carried out in the same method netic field intensity and the hysteric value were as Embodiment 2. The magnetic field characteris- known; the hysteresis curve similar to Fig. 4 was tics measured at the center of the HoTbBiFeGaAIG obtained. That is, the magnetic characteristics of io single-crystal film showed the characteristics de- the HoTbBiFeGaAIG single-crystal film was the fol- scribed in Fig. 1. That is, their values were as lowing: follows:

Hs1=180(Oe) Hs2 = -180(Oe) Hn1 =20(Oe) Hn2 = - Hs1 = 1 80(Oe) Hs2 = -1 80(Oe) Hn1 = 1 80(Oe) 20(Oe) is Hn2 = -180(Oe)

Secondly, a permanent magnet being posi- Embodiment 4 tioned and fixed on the HoTbBiFeGaAIG single- crystal film as shown in Fig. 10, the magnetic Ferrite permanent magnets of 4000(G) residual characteristics was measured similarly. The perma- 20 magnetic flux density and 1mm x 1mm x 0.5mm in nent magnet was a ferrite permanent magnet of size was fixed by an epoxy adhesive on the both 4000(G) residual magnetic flux density and 1mm0 ends of the HoTbBiFeGaAIG single-crystal film of x 1mm in size. The ferrite permanent magnet was 1mm x 3mm in size produced in Embodiment 1. fixed on the corner of the Faraday rotator by an The arrangement of the HoTbBiFeGaAIG single- epoxy adhesive, wherein the magnetization direc- 25 crystal film and the magnet is shown in Fig. 13. tion being normal to the film surface of the Faraday The ferrite magnet was positioned such that the rotator. The measured values of magnetic char- magnetization direction is normal to the film sur- acteristics at the center of the Faraday rotator are face of the HoTbBiFeGaAIG single-crystal film. The shown in Fig. 11. The magnetic hysteresis was measuring values of the magnetic characteristics at vanished in the plus zone (opposite direction to the 30 the center of the HoTbBiFeGaAIG single-crystal magnetization direction by the permanent magnet) film are shown in Fig. 12. That is, their values were with respect to the applied magnetic field. That is, as follows: the magnetic characteristics of the HoTbBiFeGaAIG single-crystal film were the followings : Hs1 = 1 80(Oe) Hs2 = -1 80(Oe) Hn1 = 1 80(Oe) 35 Hn2 = -180(Oe) Hs1 = 1 80(Oe) Hs2 = -1 80(Oe) Hn1 = 1 80(Oe) Hn2 = -10(Oe) Embodiment 5

Embodiment 2 In a platinum crucible of volume 300 ml, the 40 materials, 420 g of lead oxide (PbO, 4N); 490 g of Except that the ferrite permanent magnet used bismuth oxide (Bi203,4N); 65 g of ferric oxide in Embodiment 1 was positioned as shown in (Fe203, 4N); 15 g of boron oxide (B203, 5N); 7.1 g Fig. 12, the magnetic characteristics of the HoTb- of gadolimium oxide (Gd203, 3N); 1.3 g of gallium BiFeGaAIG single-crystal film was measured in the oxide (Ga203, 3N); and 3.8 g of aluminum oxide same method as Embodiment 1 . However, the two 45 (Al203, 3N), were laid . The crucible was set at a ferrite permanent magnets were positioned ad- predetermined position in a precision vertical tubu- versely to each other in their magnetization direc- lar electric furnace, and was heated at 1000 °C to tion. The magnetic field characteristics measured melt the mixture. The melt mixture was fully stirred at the center of the Faraday rotator showed the to be evenly mixed, and then cooled down to the characteristics described in FIG. 1. That is, their 50 melt temperature of 841 °C to obtain a melt for values were as follows: raising a bismuth-substituted magnetic garnet single crystal. By a conventional procedure, the sur- Hs1 = 1 80(Oe) Hs2 = -1 80(Oe) Hn1 = 1 80(Oe) face of thus obtained melt was made in contact Hn2 = -180(Oe) with the one surface of a (111) garnet single-crystal 55 substrate: (GdCa)3(GaMgZr)5 0i2 (lattice constant: 12.493 ± 0.002A) of 480 urn in thickness , and the epitaxial growth was continued while maintaining the melt temperature of 841 °C. The epitaxial

7 13 EP 0 646 826 A1 14 growth brought out a (GdBi)3(FeGaAI)50i2 single- crystal film (GdBiFeGaAIG single-crystal film) of 26 linn in film thickness. Then, the GdBiFeGaAIG single-crystal film was cut into the size of 1 mm x 3 mm to be measured with 5 respect to the magnetic characteristics; the magnetic characteristics was measured by the just same method as Embodiment 1 and showed the rectangular hysteresis described in Fig. 6. That is, their values are as follows: 10

Hs1=80(Oe) Hs2 = -80(Oe) Hn1 =-240(Oe) Hn2 = 240(Oe)

Secondly, a permanent magnet was fixed on is the GdBiFeGaAIG single-crystal film of 1mm x 3mm in size, positioned as shown in Fig. 13, in the same method as Embodiment 4, the magnetic characteristics were measured similarly. The permanent magnet was the same ferrite permanent 20 magnet as used in Embodiment 4, of 4000(G) residual magnetic flux density and 1mm x 1mm X 0. 5mm in size. The measured values of the magnetic characteristics at the center of the GdBiFeGaAIG single-crystal film are shown in Fig. 25 1 . That is, their values were as follows:

Hs1 =80(Oe) Hs2 = -80(Oe) Hn1 =80(Oe) Hn2 = -80- (Oe) 30 According to the present invention, the magnetic hysteresis can be erased with respect to a rare-earth iron garnet single-crystal film with large magnetic hysteresis which is an obstacle in using as a Faraday rotator although the film has an 35 extremely small saturated magnetic field intensity. As the result, an optical switch utilizing Faraday effect and a magneto-optical sensor with a Faraday rotator, wherein being applied a rare-earth iron garnet single-crystal film with a small saturated mag- 40 netic field intensity, have been enabled.

Claims

1. A Faraday rotator characterized in that at least 45 a part of a rare-earth iron garnet single-crystal film is magnetized constantly by a permanent magnet in the normal direction to said film surface. 50 2. A Faraday rotator set forth in claim 1, characterized in that a couple of permanent magnets are provided, wherein said permanent magnets magnetize two parts of said Faraday rotator and the each magnetization direction of 55 said two parts is opposite to the other.

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IRECTION Application Number EP 94 11 3619

DOCUMENTS CONSIDERED TO BE RELEVANT Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE Category of relevant passages to claim APPLICATION (Int.CI.6) X EP-A-0 413 566 (OPTICS FOR RESEARCH) G02F1/09 * claims 1-17; figures 1-3 * Y

Y US-A-5 087 984 (A.J.HEINEY ET AL.) * abstract; figure 2 *

TECHNICAL FIELDS SEARCHED fJnt.Cl.6) G02F

The present search report has been drawn up for all claims Place if ieirca Date of caaataiea ef the learck THE HAGUE 2 December 1994 Malic, K CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D : document dted in the application document of the same category L : document dted for other reasons A : technological background O : non-written disclosure A : member of the same patent family, corresponding P : intermediate document document