Abstract:
Epitaxial layers of bismuth-containing magnetic garnet materials are grown from a melt which comprises flux components lead oxide, bismuth oxide, and one or several additional oxides selected from vanadium oxide, tungsten oxide, molybdenum oxide, and chromium trioxide. The presence of such additional flux component results in increased magnetic anisotropy per degree of supercooling and thus enhances device properties and facilitates epitaxial layer deposition.

Description:
The Government has rights in this invention pursuant to Contract F33615-81-C-1404 awarded by the Department of the Air Force. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of our copending application Ser. No. 615,836, filed May 31, 1984, now U.S. Pat. No. 4,544,438, issued Oct. 1, 1985. 
    
    
     TECHNICAL FIELD 
     The invention is concerned with liquid phase epitaxial growth of garnet films as may be used, e.g., in the manufacture of magnetic bubble devices. 
     BACKGROUND OF THE INVENTION 
     The development of magnetic bubble devices has reached the stage of commercial use, particularly for the storage of sequentially retrievable data in communications and data processing equipment. 
     Magnetic bubble devices typically comprise a flat, nonmagnetic substrate of a material such as, e.g., gadolinium-gallium garnet, nominally Gd 3  Ga 5  O 12 , and a layer of a magnetic garnet material which is epitaxially deposited on the substrate and whose easy direction of magnetization is perpendicular to the layer. In the presence of a suitable magnetic bias field parallel to such direction, the layer is capable of sustaining small domains, called bubbles, which are magnetized in a direction opposite to the direction of the bias field. Desirable domains typically have right circular cylindrical shape and extend from near the surface of the magnetic film to the vicinity of the film-substrate interface. Bubble diameter may be approximately equal to the thickness of the film. 
     Device operation typically involves the nucleation, propagation, and detection of magnetic bubbles, propagation being along paths or tracks which may be defined, e.g., by magnetic overlays, by a pattern of locally modified magnetic properties in the layer, or by a conductor overlay as disclosed, e.g., in the paper by A. H. Bobeck et al., &#34;Current-Access Magnetic Bubble Circuits,&#34; Bell System Technical Journal, Vol. 58, No. 6, July-August 1979, pp. 1453-1540. 
     Preferred for the deposition of magnetic garnet layers on a substrate is a method of liquid phase epitaxy, involving controlled growth from garnet constituents in flux solution as disclosed in, e.g., U.S. Pat. No. 3,790,405, issued Feb. 5, 1974 to H. J. Levinstein. Properties of resulting layers such as, e.g., thickness, defect density, magnetization, coercivity, anisotropy field, and bubble diameter, stability, and mobility are dependent on growth conditions such as, e.g., melt composition, growth temperature, and growth procedure as discussed, e.g., in papers by 
     S. L. Blank et al., &#34;Kinetics of LPE Growth and its Influence on Magnetic Properties&#34;, AIP Conference Proceedings, Vol. 10 (1974), pp. 255-270; 
     S. L. Blank et al., &#34;Preparation and Properties of Magnetic Garnet Films Containing Divalent and Tetravalent Ions&#34;, Journal of the Electrochemical Society, Vol. 123, No. 6, June 1976, pp. 856-863; and 
     S. L. Blank et al., &#34;The Effect of Melt Composition on the Curie Temperature and Flux Spin-Off from Lutetium Containing LPE Garnet Films&#34;, IEEE Transactions on Magnetics, Vol. MAG-13, No. 5, September 1977, pp. 1095-1097. 
     As magnetic device technology progresses towards increasingly higher bit densities, layer specifications change; in particular, specified layer thickness decreases as shown, e.g., in the paper by S. L. Blank et al., &#34;Design and Development of Single Layer, Ion-Implantable Small Bubble Materials for Magnetic Bubble Devices&#34;, Journal of Applied Physics, 50, March 1979, pp. 2155-2160. 
     Recent interest has been directed to epitaxial growth from melts comprising a PbO-V 2  O 5  flux component as disclosed in U.S. Pat. No. 4,400,445, issued Aug. 23, 1983 to G. W. Berkstresser et al.; also, as disclosed in U.S. Pat. No. 4,419,417, issued Dec. 6, 1983 to R. C. LeCraw et al., the inclusion of bismuth in magnetic garnet films has been found beneficial. 
     SUMMARY OF THE INVENTION 
     Magnetic bubble devices comprising a bismuth-containing garnet layer or film are manufactured by a method which comprises epitaxial growth of such layer on a substrate. Growth is in a melt comprising flux and garnet constituents; according to the invention, the flux preferably comprises lead oxide, excess bismuth oxide, and an additional flux component selected from vanadium oxide, tungsten oxide, molybdenum oxide, and chromium trioxide. In accordance with the invention, presence of the additional flux component results in greater magnetic anisotropy per degree of supercooling of the melt. 
     Preferred limits on the amount of the additional oxide are 1 to 20 molecular percent, the former being motivated by the realization of an appreciable effect on degree of supersaturation and the latter by melt density and stability considerations. More preferred limits are 3 or preferably 5 to 15 molecular percent in these respects. (Limits are on amounts of V 2  O 5 , WO 3 , MoO 3 , and CrO 3  ; it is understood, however, that at elevated temperatures such oxides may undergo partial dissociation.) Preferred limits are 50 to 98 molecular percent for PbO and 1 to 30 molecular percent for Bi 2  O 3 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic view of apparatus as may be used for liquid phase epitaxy growth of magnetic garnet films according to the invention; 
     FIG. 2 is a graphical representation of melting temperature as a function of the amount of V 2  O 5 , WO 3 , or MoO 3  added to a representative PbO-Bi 2  O 3  flux; and 
     FIG. 3 is a graphical representation of magnetic anisotropy in a representative layer grown in accordance with the invention as a function of amount of supercooling, each curve corresponding to a different amount of V 2  O 5  added to the melt. 
    
    
     DETAILED DESCRIPTION 
     Garnets suitable for magnetic device application are typically patterned after the prototype, yttrium-iron garnet, Y 3  Fe 5  O 12 , which, in its unaltered form, is ferrimagnetic with net magnetic moment of approximately 1750 gauss at room temperature. For the manufacture of magnetic bubble devices having an extended range of operating temperatures, compositions of the more general form R 3-x  Bi x  Fe 5  O 12  have been recommended, where R denotes yttrium or a rare earth element of the lanthanide series (atomic number 57 to 71) or a combination of two or more such elements. 
     Magnetic moment may be modified by partial substitution of Fe ions, e.g., by Ga ions, resulting in a material having nominal composition according to the formula R 3  Fe 5-x  Ga x  O 12 , where x denotes a positive number less than 5 and preferably less than 2. Iron substitution may also be by replacement of Fe ions with Ge or Si ions or both, in which case there is a need for valence balancing, e.g., by replacement of a portion of rare earth ions by suitable divalent or monovalent ions. For example, replacement of rare earth ions by the divalent ion Ca results in compositions which may be formulated as R 3-x-y  Bi x  Ca y  Fe 5-v-w  Si v  Ge w  O 12 , where y is approximately equal to v+w. 
     While magnetic bubble devices are typically based on the use of rare earth-iron magnetic garnet materials, layers of other garnets such as, e.g., gadolinium-gallium garnet and gadolinium-aluminum garnet may also be produced according to the invention. What is required in each instance is a substrate whose lattice parameters at least approximately match those of the layer to be grown. Resulting grown layers may exhibit magnetic anisotropy, as may be growth- or strain-induced. Easy direction of magnetization may be perpendicular to a substrate as, e.g., in bubble layers. Layers having easy direction parallel to the substrate may be deposited for hard bubble suppression. 
     Apparatus suitable for magnetic garnet epitaxial deposition according to the invention is schematically depicted in FIG. 1. In particular, FIG. 1 shows heating element 1 on ceramic support tube 2 surrounding ceramic liner tube 3. Crucible 4 is supported by baffle 5 and contains melt 6 comprising garnet and flux materials. Baffle 7 acts as a radiation shield for preventing excessive cooling of the surface of melt 6. Substrate 8 is attached to substrate holder 9 which is attached to rod 10. 
     Operation of apparatus involves heating the melt first at elevated temperature for homogenization and equilibration, and then at a temperature corresponding to supercooling of garnet material, lowering rod 10 until substrate 8 is immersed in or at least in contact with the surface of melt 6, and, typically, rotating rod 10. An exhaust pump may be connected at 11. 
     FIG. 2 shows melting temperature of a flux made from 15 mole percent Bi 2  O 3 , 0 to 22 mole percent V 2  O 5 , WO 3 , or MoO 3 , and remainder PbO. It can be seen that melting temperature is highly nonlinear and that such temperature assumes a local minimum for each of the three flux constituents V 2  O 5 , WO 3 , and MoO 3 . 
     FIG. 3 shows growth-induced magnetic anisotropy in units of 10 3  erg/cm 3  as a function of supercooling in units of degrees K. It can be seen that growth-induced magnetic anisotropy is directly related to supercooling as well as to the amount of V 2  O 5  present in the flux. This fact is useful in that, in accordance with the invention, greater growth-induced anisotropy is produced per degree of supercooling when the flux additive V 2  O 5  is used. Essentially similar results are obtained using WO 3 , MoO 3 , or CrO 3  instead of V 2  O 5 . 
     In accordance with the invention a desired level of growth-induced magnetic anisotropy can be achieved at reduced supercooling as is desirable in the interest of ease of layer deposition. For example, devices having a period of approximately 8 micrometers preferably have a magnetic anisotropy which is approximately 32,000 erg/cm 3 . As can be seen from FIG. 3, growth from a flux consisting essentially of lead oxide and bismuth oxide is at approximately 80 degrees C. supercooling. On the other hand, when the flux comprises 9 mole percent vanadium oxide, supercooling is reduced to 50 degrees C. 
     Magnetic bubble devices are made, using layers produced according to the invention, by providing means for nucleating, propagating, and detecting magnetic bubbles in a magnetic layer. Such means may take a variety of forms as reviewed, e.g., in the paper by A. H. Bobeck et al. cited above. 
     Magnetic films or layers deposited in accordance with the invention can be used not only in magnetic bubble devices but also in other device types such as, in particular, magneto-optical devices; see, e.g., U.S. patent application Ser. No. 728,669, filed Apr. 29, 1985 by J. F. Dillon, Jr. et al. 
     As illustrated by the following examples, garnet films were deposited by liquid phase epitaxy on circular Gd 3  Ga 5  O 12  substrates having a diameter of 5 cm. Cleaning of substrates before layer growth was as follows: 20 minutes in an ultrasonic bath of 1 gm Alconox in 100 milliliter water at 50 degrees C., rinsing in warm water, 15 minutes in an ultrasonic bath of water at 50 degrees C., and rinsing in three-stage overflow deionized water for 5 minutes per stage. After growth, cleaning was as follows: 10 minutes in a solution of 6 volumes H 2  O, 3 volumes HNO 3 , and 1 volume CH 3  COOH at 75 degrees C., rinsing in warm water, and repeating steps of pregrowth cleaning. 
     Substrates were rotated at 75 to 100 revolutions per minute in an oscillating mode during epitaxial deposition. 
     EXAMPLE 1 
     A melt was prepared by melting approximately 3.59 grams Y 2  O 3 , 2.40 grams CaO, 9.34 grams SiO 2 , 15.31 grams GeO 2 , 168.0 grams Fe 2  O 3 , 222.0 grams MoO 3 , 2294 grams PbO, and 413.0 grams Bi 2  O 3  in a platinum crucible. Bi 2  O 3  was added last, and after the other ingredients had been melted and mixed. The melt was heated by resistance-heating coils to a temperature of approximately 1000 degrees C. The melt was allowed to react at this temperature for a period of approximately 16 hours. 
     The saturation temperature of this melt was determined as the lowest melt temperature for which no epitaxial growth was observed on a piece of polished gadolinium gallium garnet dipped into the melt for 2 minutes. For this melt a saturation temperature of 890 degrees C. was determined in this fashion. 
     A circular gadolinium-gallium garnet substrate approximately 2.0 inches in diameter and 20 mils in thickness was used as a deposition substrate. The substrate was cleaned, dried, and inserted in a substrate holder. 
     The temperature of the melt was then lowered to a growth temperature of approximately 792 degrees C. and the substrate was lowered to within 1 centimeter of the melt surface. The substrate was maintained in this position for approximately 6 minutes. The substrate was then immersed approximately 2 centimeters deep into the melt and rotated, the sense of rotation being reversed every second. Immersion was for a duration of approximately 6 minutes, and the substrate was then removed from the melt to a position 1 centimeter above the melt while rotation continued. 
     The rotation was stopped and the melt was allowed to drip off the substrate. This drainage process took approximately 8 minutes and was followed by a burst of fast rotation to remove melt droplets still remaining on the substrate wafer. The substrate was withdrawn at a rate of 13 cm/min. 
     By standard measurement techniques the following physical properties were determined for the deposited layer: A layer thickness of approximately 2.21 micrometers, a magnetic domain stripe width of approximately 1.66 micrometer, a saturation magnetization (commonly designated as 4 πM s ) of approximately 716 gauss, an anisotropy field (commonly designated as H k ) of approximately 2590 gauss, a material length parameter (commonly designated as l) of approximately 0.16 micrometer, a lattice constant (commonly designated as a o ) of approximately 12.4050 Angstroms, and a uniaxial anisotropy (commonly designated as K u ) of approximately 95,200 erg/cm 3  (after strain correction). 
     The composition of the layer was determined as represented approximately by the formula (Y 1 .74 Bi 0 .66 Ca 0 .60) (Fe 4 .46 Si 0 .30 Ge 0 .30)O 12 . 
     EXAMPLE 2 
     The procedure described in Example 1 above was followed except that the melt consisted of approximately 8.70 grams Y 2  O 3 , 1.35 grams Gd 2  O 3 , 2.35 grams Ho 2  O 3 , 6.75 grams CaO, 27.0 grams SiO 2 , 16.0 grams GeO 2 , 428.0 grams Fe 2  O 3 , 364.0 grams V 2  O 5 , 5880 grams PbO, and 1312 grams Bi 2  O 3 . Saturation temperature was approximately 890 degrees C., growth temperature was approximately 805 degrees C., and determined material parameters were as follows: a layer thickness of 2.00 micrometers, a stripe width of 1.63 micrometers, a saturation magnetization of 620 gauss, an anisotropy field of 1700 gauss, a material length of 0.16 micrometers, a lattice parameter of 12.384 Angstroms, and a uniaxial anisotropy of 41,900 erg/cm 3 . 
     Composition of the grown film is approximately as represented by the formula (Y 1 .43 Gd 0 .14 Ho 0 .23 Bi 0 .50 Ca 0 .70) (Fe 4 .30 Si 0 .52 Ge 0 .18)O 12 . 
     EXAMPLE 3 
     The procedure described in Example 1 above was followed except that the melt consisted of approximately 1.61 gram Y 2  O 3 , 1.55 gram CaO, 4.50 grams GeO 2 , 4.20 grams SiO 2 , 85.2 grams Fe 2  O 3 , 73.0 grams WO 3 , 134.0 grams Bi 2  O 3 , and 740.0 grams PbO. Saturation temperature was approximately 920 degrees C., growth temperature was approximately 841 degrees C., and determined film properties were as follows: a layer thickness of 1.75 micrometer, a stripe width of 1.55 micrometer, a collapse field of 374 gauss, a saturation magnetization of 671 gauss, a lattice parameter of 12.384 Angstroms, and a uniaxial anisotropy of 51,500 erg/cm 3 . 
     Composition of the grown film is approximately as represented by the formula (Y 1 .91 Bi 0 .44 Ca 0 .65) (Fe 4 .35 Ge 0 .23 Si 0 .42)O 12 . 
     EXAMPLE 4 
     A melt was prepared by melting approximately 1.80 gram Y 2  O 3 , 1.30 gram CaO, 6.00 grams SiO 2 , 2.50 grams GeO 2 , 84.00 grams Fe 2  O 3 , 168.00 grams CrO 3 , 1550.00 grams PbO, and 295.00 grams Bi 2  O 3  in a platinum crucible. Bi 2  O 3  was added last, and after the other ingredients had been melted and mixed. The melt was heated by resistance-heating coils to a temperature of approximately 1000 degrees C. The melt was allowed to react at this temperature for a period of approximately 16 hours. 
     The saturation temperature of this melt was determined as the lowest melt temperature for which no epitaxial growth was observed on a piece of polished gadolinium gallium garnet dipped into the melt for 2 minutes. For this melt a saturation temperature of 881 degrees C. was determined in this fashion. 
     A circular gadolinium-gallium garnet substrate approximately 2.0 inches in diameter and 20 mils in thickness was used as a deposition substrate. The substrate was cleaned, dried, and inserted in a substrate holder. 
     The temperature of the melt was then lowered to a growth temperature of approximately 795 degrees C. and the substrate was lowered to within approximately 1 centimeter of the melt surface. The substrate was maintained in this position for approximately 6 minutes. The substrate was then immersed approximately 2 centimeters deep into the melt and rotated at a rate of approximately 100 revolutions per minute, the sense of rotation being reversed every second. Immersion was for a duration of approximately 6 minutes, and the substrate was then removed from the melt to a position 1 centimeter above the melt while rotation continued. 
     The rotation was stopped and the melt was allowed to drip off the substrate. This drainage process took approximately 2 minutes and was followed by a burst of fast rotation to remove melt droplets still remaining on the substrate wafer. The substrate was withdrawn at a rate of 13 cm/min. 
     By standard measurement techniques the following physical properties were determined for the deposited layer: A layer thickness of approximately 1.82 micrometers, a magnetic domain stripe width of approximately 1.35 micrometers, a saturation magnetization of approximately 853 gauss, an anisotropy field of approximately 2490 gauss, a material length parameter of approximately 0.13 micrometer, a lattice constant of approximately 12.3854 Angstroms, and a uniaxial anisotropy of approximately 86300 erg/cm 3  (after strain correction). 
     The composition of the layer was determined as represented approximately by the formula (Y 1 .93 Bi 0 .54 Ca 0 .53)(Fe 4 .47 Si 0 .42 Ge 0 .11)O 12 .