CA1235825A - Method of making magnetic bubble devices containing bismuth-containing garnet films - Google Patents

Abstract

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 oxide. 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

35~3~5 METHOD OF MAKING MAGNETIC BUBBLE DEVICES
CONTAINING BISMUTH-CONTAINING GARNET FILMS

Technical Field The invention is concerned with the manufacture of magnetic bubble devices and involves liquid phase epitaxial growth of bismuth-containing garnet films.
Back~ound 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 Gd3Ga5012, and a layer of a magnetic garnet material whîch is epitaxially deposlted on the substrate and whose easy direction of magnetization is perpendicular to the layer.
In the pre~ence 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., "Current-Access Magnetic Bubble Circuits," Bell ~y~ Technical Journal, Vol. 58, No. 6, _ July-~ugu~t 197~, pp. 1~53-1540~
Preferred for the deposition of magnetic garnet

- 2 - ~3~5~

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. patent No~ 3,790,405, ssued February 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., "Kinetics of LP~ Growth and its Influence on Magnetic Properties", AIP Conference Proceedings, Vol. 10 (1974), pp. 255-270:
S. L. Blank et al., "Preparation and Properties 15 of Magnetic Garnet Films Containing Divalent and Tetravalent Ions", Journal of the Electrochemical Society, Vol. 123, No. 6, Jun~ 1976. pp. 856 863; and S. L~ Blank et al. r l'The Efect of Melt Composition on the Curie Temperature and Flux Spin-Off from Lutetium Contain.ing LPE Garnet Films"~ IEEE Transactions on ~netics, 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., "Design and Development of Single Layer, Ion-Implantable Small Bubble Materials for Magnetic Bubble Devices", Journal of Applied Physics, 50, March 1979, pp. 2155-2160.
Recent interest has been directed to epitaxial growth from melts comprising a PbO-V2O5 flux component as disclosed in U~S. Patent 4,400,445, issued August 23, 1983 to G. W. Berkstresser et al.; also, as disclosed in U~S. Patent 4,419,417, issued December 6, 1983 to R. C. LeCraw et al.. the inclusion of bismu~h in mag~e~l~ g~rn~ lmi h~ n ~un~ ~n~al~l.

- 3 -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 prefer-ably comprises lead oxide, excess bismuth oxide, and an additional flux component selected from vanadium oxide, tungsten oxide, molybdenum oxide, and chromium oxide. In accordance with the invention, presence of the additional flux component results in greater magnetic anisotropy per degree of supercooling of the melt.
Rreferred 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 V2O5, WO3, MoO3, and CrO3; 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 Bi2O3.
To summarize, the present invention i$ directed to a method for making a magnetic device comprising a layer of garnet material which contains bismuth and which is epitaxially deposited on a surface of a supporting substrate, said method comprising heating a preferred melt consisting essentially of a garnet material which contains bismuth and a flux material as de~ined above at a tempera-ture corresponding to supercooling of the garnet material, bringing at least a surface of a substrate in contact with said melt, and removing the substrate from contact with the melt upon deposi~ion of a layer o~ the garnet material on the surface.

- 3a - 123 582,~
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 V205, W03, or MoO3 added to a representative PbO-Bi~03 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 supercool-ing, each curve corresponding to a different amount of V205 added to the melt.

t, !, ~ 4 ~ ~ ~35~2~
Detailed Description Garnets suitable for magnetic device application are typically patterned after the prototype, yttrium-iron garnet, Y3Fe5O12, which, in its unaltered form, is ferrimagnetic with net magnetic moment of approximately 0.175T (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 R3 xBixFe5O12 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 rmula R3Fe5-xGaxo1~ 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 ion~ 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 3-x-yBixcayFe5-v-wsivGewol2~
where ~ 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 s~rain-induced. Easy direction of magnetization may be perpendicular to a substrate as, e.g., in bubb~e lAy~r~ ~ay~r~ ~ving 2~y ~lr~c~i~n p~ l t~
the substrate may be deposited for hard bubble suppression.

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~ 5 ~

Apparatus suitable for magnetic garnet epitaxial deposition according to the invention is schematically depicted in FIG. 1. In particular, FIC. 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 ~ comprising garnet and flux materials.
Baffle 7 acts as a radiation shield for preventing excessive cooling of the surface of melt 60 Substrate 8 is attached to suhstrate holder 9 which is attached to rod 10.
Operation of apparatus involves heating the melt first at elevated temperature for homogenization and equilibrationr and then at a temperature corresponding to supercooling of garnet materiall 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 show~ melting temperature of a flux made from 15 mole percent Bi2o3, 0 to 22 mole percent V2O5, WO3, or ~oO3, 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 V2O5, WO3, and MoO3.
FIG. 3 shows growth-induced magnetic anisotropy in units of 10 4~/cm3 (103erg/cm3) as a function of supercooling in units of degrees ~. It can be seen that growth-induced magnetic anisotropy is directly related to supercooling as well as to the amount oE
V2O5 present in the flux. This fact is useful in that, in accordanc~ with the invention, greater growth-induced anisotropy is produced per degree of supercooling when the flux additive V2O5 is used. Essentially similar results are obtained using WO3, MoO3, or CrO3 instead ~ V205-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 - 6 - ~ ~3~2~

period of approximately 8 micrometers preferably have a magnetic anisotropy which is approximately 3.2 x 10 3J/cm3 (32,000 erg/cm3). ~s 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 reviewe~, 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.
As illustrated by the Eollowing examples, garnet films were deposited by liquid phase epitaxy on circular Gd3Ga5012 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 deyrees 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 H20, 3 volumes E~03, and 1 volume CEI3COOH 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.

35~325 Example 1 A melt was prepared by melting approximately 3.59 grams Y2O3, ~.40 grams CaO, 9O34 grams SiO2, 15.31 grams GeO2, 168.0 grams Fe2O3, 222.0 grams MoO3, 2294 grams PbO, and 413.0 grams Bi2O3 in a platinum crucible. Bi2o3 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 ashion.
A circular gadolinium-gallium garnet substrate approximately 5~1cm (2.0 inches) in diameter and 0.051cm (20 mils) in thickness was used as a deposition substrate.
The substrate was cleaned, dried, ancl 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 r the sense of rotation being reversed every second. I-mmersion 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. Thls drainage process took approximately 8 minutes and was followed by a burst of fast ~:3~S

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 ~Ms) of approximately 0.0716T (716 gauss), an anisotropy field (commonly designated as Hk) of approximately 00259T (2590 gauss), a material length parameter tcommonly designated as l) of approximately 0.16 micrometer, a lattice constant (commonly designated as aO) of approximately 1.2405 nm, and a uniaxial anisotropy (commonly designated as Ku) of approximately 9.52 x 10 3J/cm3 (95,200 erg/cm3) (after straln correction).
The composition of the layer was determined as represented approximately by the formula (Y1 74Bio 66Ca0 60) ~e4 ~6Sio 30Ge0 30)12-Example 2 The procedure descibed in Example 1 above was followed except that the melt consisted of approximately 8.70 grams Y2O3, 1.35 grams Gd2O3, 2.35 grams Ho2O3, 6.75 grams CaO~ 27.0 grams SiO2, 16.0 grams GeO2, 428.0 grams Fe2O3, 364.0 grams V2O5, 5880 grams PbO, and 1312 grams Bi2o3. 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 0.062T (6~0 ~au~), an anl~otro~y ~leld o~ 0.17T
(1700 gauss), a material length of 0.16 micrometers, a - 9 - 3L;~35~25 latti~e parameter of 1.2334 nm, and a uniaxial anisotropy of 4.19 x 10 3J/cm3 (41,900 erg/cm3).
Composition o~ the grown film is approximately as represented by the formula ( 1~43 0.14 0.23 0.50 0.70) ( 4.30 0.52 0.18) 12 The procedure described in Example 1 above was followed except that the melt consisted of approximately 1.61 gram Y2O3, 1.55 gram CaO, 4.50 grams GeO2,

4.20 grams SiO2, 85~2 grams Fe2O3, 73 .n grams WO3, 134.0 grams Bi2O3, and 740.0 grams PbO.
Saturation temperature was approximately 920 degrees C, growth temperature was approximately 841 degrees C, and determined f~lm properties were as follows: a layer thickness o 1.75 micrometer, a stripe width of 1.55 micrometer, a collapse field of 0.0374T (374 gauss)~ a saturation magnetization o~ 0.0671T (671 gauss), a lattice parameter of 1.2384 nm, and a uniaxial anisotropy of

5.15 x 10 3J/cm3 (51,500 erg/cm3).
Composition of the grown ~ilm is approximately as represented by the formula (Y1 91Bio 44Ca0 65) (Fe4 35Geo.23sio.42) 12 Example 4 A melt was prepared by melting approximately 1.80 gram Y2O3, 1.30 gram CaO, 6.00 grams SiO2, 2.50 grams GeO2, 84.00 grams Fe2O3, 163.00 grams CrO3, 1550.00 grams PbO, and 2~5.00 grams Bi2O3 in a platinum crucible. Bi2O3 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 ~ 10 ~ 35~ 5 react at this temperature for a period of approximately 16 hours.
The saturation temperature of this melt was determined as the lowes~ 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 oE
881 degrees C was determined in this fashion.
A circular gadolinium-gallium garnet substrate approximately 5.1cm (2.0 inches) in diameter and 0.051cm (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 wi~hdrawn at a rate of 13 cm/min.
By standard measurement techniques the following physical properties were determined for the deposited layer: A layer thicXness of approximately 1.82 micrometers, a magnetic domain stripe width of approximately 1.35 micrometers, a saturation magne~ization of approximately 0.0853T ~853 gauss), an anisotropy field " ~3~ 5 of approximately 0.249T (2490 gauss), a material length parameter of approximately 0O13 micrometer, a lattice constant of approximately 1.23854 nm, and a uniaxial anisotropy of approximately 8.63 x lO 3J/cm3 (86300 erg/cm3) ~after strain correction).
The composition of the layer was determined as represented approximately by the formula (Y1 93~io 54CaO 53)(Fe4.47Sio.42GeO.11)o12

Claims (6)

Claims:

1. Method for making a magnetic device com-prising a layer of garnet material which contains bismuth and which is epitaxially deposited on a surface of a supporting substrate, said method comprising heating a preferred melt consisting essentially of a garnet material which contains bismuth and a flux material at a temperature corresponding to supercooling of said garnet material, said flux material comprising lead oxide and bismuth oxide and at least one additional flux-constituent oxide selected from the group consisting of vanadium oxide, tungsten oxide, molybdenum oxide, and chromium oxide, the lead oxide being comprised in said flux material in an amount corresponding to an amount of PbO in the range of from 50 to 98 molecular percent, the bismuth oxide being comprised in said flux material in an amount corresponding to an amount of Bi2O3 in the range of from 1 to 30 molecular percent, said at least one additional oxide being comprised in said flux material in an amount corresponding to V2O5, WO3, MoO3 or CrO3 in the range of 1 to 20 molecular percent, bringing at least a surface of a substrate in contact with said melt, and removing said substrate from contact with said melt upon deposition of a layer of said garnet material on said surface, degree of supercooling being chosen in combina-tion with amount of said additional flux-constituent oxide so as to substantially achieve a desired level of magnetic anisotropy in said layer.

2. Method of claim 1 in which said additional oxide is comprised in said flux material in an amount corresponding to an amount of said group in the range of from 3 to 15 molecular percent.

3. Method of claim 1 in which said additional oxide is vanadium oxide.

4. Method of claim 1 in which said additional oxide is tungsten oxide.

5. Method of claim 1 in which said additional oxide is molybdenum oxide.

6. Method of claim 1 in which said additional oxide is chromium oxide.