CA1104253A - Bubble domain storage using improved transfer switch - Google Patents

Abstract

BUBBLE DOMAIN STORAGE USING IMPROVED TRANSFER SWITCH

Abstract of the Disclosure A magnetic bubble domain storage device comprising a plurality of storage shift registers and at least one major shift register, which serves to provide bubble domains to the storage register and to receive bubble domains from the storage registers. A novel transfer switch, or gate, is located between each of the storage registers and the major register? which is typically configured in the conventional major/minor loop type of storage organization. This transfer switch can be made using single level masking or multiple level masking and is characterized in that the locus of bubble domain propagation paths through the switch element generally defines the letter "Y". These propagation paths are from one arm of the Y to the other arm, from one arm of the Y to the stem or base portion, or the reverse where a bubble domain travels from the stem (base) of the Y to one of the arms of the Y. In a particular embodiment, the transfer switch is comprised of a Y-shaped magnetic element having a current carrying conductor which crosses the stem portion of the Y-shaped magnetic element. The particular path travelled by bubble domains through the transfer switch is determined by the presence or absence of a current through the conductor. Thus, the transfer switch is characterized by at least one magnetic element having a current path crossing the stem portion of the element(s). The exact shape, width, length, and thickness of the magnetic element, or elements, of the transfer switch can be chosen by the designer in accordance with the propagation structures that are desired. What is important is that the plurality of paths which the bubbles can follow generally define a Y, where the current path through the transfer switch crosses the path which is along the stem portion of the Y defined by the plurality of propagation paths available.

Description

1 Background of the Invention

2 Field of the Invention

3 This invention relates to magnetic bubble domain devices, and

4 more particularly to a bubble storage device and a transfer switch for S transferring bubble domains between different shift registers, where 6 the transfer switch can be made by a single level masking process and 7 is particularly suitable for reliable transfer of very small bubble 8 domains using very low currents.
9 Description of the Prior Art -Magnetic bubble domain devices are well known in the art, 11 and in many of these devices it is necessary to tranfer bubble 12 domains from one shift register to another. For example, in a major/
13 minor loop type of memory organization, such as is shown in U.S-.
14 3,618,054, bubble domains are transferred between the input/output major loop and the storage minor loops.
16 Many devices have been described in the prior art for trans- -17 ferring bubble domains from one shift register to another. However, 18 as the size of the magnetic bubble decreases, the design of a proper 19 switch becomes more difficult. Thus, while many current controlled transfer switches are described in the prior art, it is difficult to 21 find one which will work properly when bubble domains of very small 22 size, such as one micron and less in diameter, are used.
, 23 In general, a good current controlled transfer switch is one 24 which has very low current amplitude requirements even when extremely small bubble domains are used. This is very important since, as the 26 size of the bubble domains decreases, the linewidth of elements used 27 to move the bubbles also decreases and, if the currents required are 28 too large, problems such as electromigration will occur.

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1 Additionally, a good transfer switch should be compatible 2 with conventional propagation elements used to move bubble domains in 3 shift registers, and should he such that its design is compatible with 4 the design used for the propagation elements in the shift registers.
Purther, a switch which can be used for all functions on the magnetic 6 chip is desirable. A switch capable of sym~etrical transfer should 7 operate with the same margins regardless of the direction of transfer.
8 Another important criterion for a good transfer switch is that it provide 9 good margins for switching and be reliable in its operation regardless of the manner in which it is fabricated. ~esirably, it should be capable 11 of being fabricated by single level metallurgy processes using only one 12 critical masking step. Still further, it is desirable that the transfer 13 switch be an integral part of the propagation structure used to move 14 magnetic bubble domains.
In the prior art, various bubble domain transfer switches 16 are described using current carrying overlays to switch bubble domains 17 from one propagation track to another in response to an electrical signal 18 pulse. Typically, the current carrying conductors are designed in a 19 loop configuration so that a current pulse in the conductor will generate a localized magnetic field within the loop. This localized field 21 temporarily adds to the fields of the propagation elements in that region 22 and provides an additional attractive or repulsive force on magnetic 23 bubble domains approaching that region. In this manner, the bubble 24 domain is preferentially attracted or repulsed in order to determine the propagation track along which it will move. A representative example of 26 such a switch is shown in the IBM Technical Disclosure Bulletin, Volume 15, 27 No. 2, July 1972 at page 703. In that switch, bubble domains arrive at a 28 point of ambiguity where two possible positions are available for subsequent 29 bubble domain move~ent. Current in a conductor deter~ines which path will be taken by the bubble, thus resolving the ambiguity of the switch.

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1 A replicate type of switch using a current carrying loop is 2 described by Bobeck et al in IEEE Transactions on Magnetics, Vol. MAG-9, 3 No. 3, September 1973, at pages 474-480. In this type of switch, current 4 is used to stretch a bubble domain so that it will transfer to a different propagation channel, while an additional amount of current is 6 used to replicate the stretched domain. This type of switch requires high 7 currents and is difficult to incorporate in a single level metallurgy 8 design.
9 Another version of the replicate switch described in the previous paragraph has been shown by T. J. Nelson, in AIP Conference Proceedings, 11 18, 95(1974). This is an all-permalloy switch in which a current conducting 12 path is comprised of permalloy deposited at the same time as the permalloy 13 propagation elements. Transfer from one propa~ation channel to another 14 utilizes a current along a straight permalloy path linking the propagation channels. A disadvantage of this transfer switch is that 16 very high currents are required when the bubble diameter is small, of -17 the order of one micron and less in diameter. Physically, this switch 18 is large and thus is not compatible with densely packed major/minor 19 loop designs.
Still another transfer switch employing a current carrying 21 conductor is shown in U.S. 3,876,995. A double loop conductor is used 22 to establish a magnetic fleld which attracts a bubble toward one 23 propagation track and at the same time establishes another magnetic 24 field tending to repel the bubble from another propagation track.
Additionally, this patent shows a transfer switch which uses no current.
26 Instead, two bubble propagation tracks merge at a junction.
27 In order to overcome the disadvantages of the prior art 28 transfer switches, a new design for a transfer switch is described 29 herein. This switch can be fabricated using single level metallurgy, and is particularly advantageous when very small magnetic bubble 11û425;~

1 domains are to be transferred. The switch uses very little current 2 even for transfer of very small magnetlc domains, and the structure 3 of the magnetic elements comprising the switch is compatible with 4 propagation elements presently used, so that the transfer switch itself can be integral with a propagation path.
6 Accordingly, it is a primary object of the present invention 7 to provide a transfer switch for magnetic bubble domains, which can be 8 made by single level metallurgy and which requires very small currents 9 for transfer, even when the magnetic bubble domains are very small.
It is another object of the present invention to provide a 11 transfer switch for magnetic bubble domains which is an integral part 12 of the propagation structure used to move such domains, and which 13 provides reliable transfer of the bubble domains.
14 It is another obiect of the present invention to provide a switch for transferring magnetic bubble domains between one shift 16 register and another, where the switch provides transfer with very 17 good margins in all propagation paths through the switch.
18 It is another ob;ect of the present invention to provide 19 a magnetic bubble domain transfer switch which can be fabricated by single level metallurgy, and which does not require a transfer 21 current carrying conductor having a loop therein, where the transfer 22 switc4 can be used with many types of known propagation elements.

23 Brief Summary of the Invention 24 A magnetic bubble domain storage device is described in which a plurality of sto.rage registers are selectively and controllably 26 connected to a register, or a plurality of registers, which serve to 27 provide input/output functions. The input/output registers are used 28 for bringing in new information to be placed in the storage registers 29 or for removing information to be read from the storage registers.

11~42~3 l In particular, a major/minor loop storage arrangement of the type gen-2 erally described in U.S. 3,618,054 is utilized to illustrate the present3 invention.
4 The entire storage system can be fabricated by a process usingonly one critical masking step. In such a process and device, the most 6 critical patterns of the storage system are the switches used to transfer 7 bubble domains between the major and minor loops. The transfer switches,8 or gates, of this application provide reliable transfer of bubble domains 9 and provide good transfer margins. Additionally, they operate with very low currents and can be used advantageously for the movement of small ll bubble domains. They are compatible with any type of propagation element12 used in any of the storage registers and major registers, and provide 13 symmetrical switching. That is, transfers in either direction between 14 any two registers are reliable and have the same operating margins.
These transfer switches provide bubble domain paths which generally 16 define the letter "Y". That is, one propagation path is along one arm 17 of the Y and then along the other arm, while another propagation path is -18 along one arm of the Y and then along the stem or base of the Y. Bubble l9 domains can be transferred in a reverse direction also, where they travel from the stem of the Y to either one of the arms of the Y.
21 The particular path followed by a bubble domain is determined 22 generally by the presence or absence of a current passing through the 23 transfer switch. The path for this current crosses the bubble domain 24 path which is along the stem of the Y. Depending upon the design layout of the magnetic element or elements used in the switch, various paths 26 can be chosen in the presence or absence of the current.
27 The transfer switch itself is comprised of at least one 28 magnetic element, such as a magnPtic overlay of NiFe, which sends 29 bubble domains along paths characterizing the letter Y. A current 2S;~
1 carrying conductor provides a current path which generally crosses the 2 stem portion of the Y.
3 In one embodiment, a single magnetic element is in the-shape 4 of a Y and a current carrying conductor crosses the stem portion of the Y. The conductor can lie below the magnetic material forming the Y, 6 above the magnetic material, or be sandwiched between two magnetic 7 layers. In another embodiment, the conductor can be co-planar with the 8 magnetic material forming the Y.
9 In response to the reorientation of a magnetic field generally in the plane of the magnetic medium, bubble domains will travel along the 11 magnetic element or elements defining the transfer switch to trace paths 12 characteri~ed by the letter Y. One, or more, paths can be made to be 13 preferred in the absence of control current in the conductor crossing the 14 propagation path defined along the stem of the Y.
As will be readily appreciated, the magnetic elements forming 16 the transfer switch can be varied in shape, width, length, thickness, and 17 number, as long as the propagation paths defined by these elements gener-18 ally trace the letter Y. Further, the exact location of the conductor in 19 the transfer switch can be varied in different designs, it being un-ier-stood that it generally crosses the propagation path which is along the 21 stem of the Y.
22 The exact shape of the "Y" formed by the various propagation 23 paths in the transfer switch is not,critical. It could be a symmetrical 24 "Y" in which the propagation path along the stem of the Y bisects the angle between the propagation paths along the arms of the Y, or it 26 could be an asymmetrical Y in which the stem propagation path does not 27 bisect the angle between the arm propagation paths. Generally, however, 28 the angle between the stem propagation path and either one of the arm 29 propagation paths is not 90, but is between 90 and 180. The angle between the arm propagation paths is not critical and is greater than 31 0 but less than 180.

1 In addition to the considerations above, the lengths of the 2 magnetic elements forming the arms of the Y do not have to be equal, 3 and do not have to have the same thicknesses or widths. All of these 4 considerations can be used by the designer to provide the most effective transfer switch for any situation. For example, if the transfer 6 conductor crossing the stem of the Y is too close to the arms of the Y , 7 it will be too far from the bottom of the stem to affect a bubble domain 8 located there. If, however, the conductor is too close to the bottom of 9 the stem, then the magnetic field produced by current in it may not create a sufficiently large potential well for affecting the bubble 11 motion.
12 In a particular embodiment, this transfer switch is used to 13 transfer bubble domains between a maaor loop and minor loops in a major/
14 minor loop memory organi~ation. The transfer switch can be an integral part of the propagation structure used for moving bubble domains in the 16 minor loops and in the major loop, and can be formed by processes in 17 which high and low resolution masking are used. The switch is 18 compatible with presently used types of propagation elements and is 19 easily adapted to various design structures. Its low current require-ment for reliable switching means that it can be used in bubble ?l storage devices using very small magnetic bubble domains, without 22 requiring currents that are excessive.
23 These and other objects, features, and advantages will be 24 more apparent from the following more particular description of the preferred embodiments.

26 Brief Description of the Drawin~s 27 FIG. 1 shows a schematic representation of a major/minor 28 loop memory using the novel transfer gates of the present invention.

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1 FIG. 2 shows a portion of the memory of FIG. 1, and in particu-2 lar illustrates the transfer switches and propagation structure for 3 moving magnetic bubble domains in the input and output shift registers 4 and in the minor loops.
FIG. 2A is an expanded view of a transfer switch, used to 6 illustrate the operation of this switch.
7 FIG. 3 is a detailed diagram of the bubble domain generator 8 and propagation circuitry for moving bubble domains to the inp~t shift 9 register SRl of FIG. 1.
FIG. 4 ls a detailed diagram of an expander/detector for 11 sensing bubble domains removed from the minor loops, as well as the 12 propagation circuitry for taking bubble domains from the output shift 13 register SR2 to the expander/detector.
14 FIGS. 5A-5C illustrate a single level masking technique for makin8 the memory of FIG. 1.
16 FIG. 6 is a cross-sectional view of a transfer switch, 17 comprised of two magnetic layers and a conductor layer.

18 Detailed Description of the Preferred Embodiments 19 FIG. 1 FIG. 1 illustrates a magnetic bubble domain storage device in 21 which a plurality of storage shift registers are used in combination 22 with inputloutput registers for efficient storage of information. The 23 information is represented by magnetic bubble domains which can be coded 24 in several ways for representation of information, as is well known in the art. In particular, such domains may be coded in terms of presencel 26 absence for representing one and zero bits of a binary system. In particu-27 lar, FIG. 1 rePreSents a majoF/minor loop type of memory organization.

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1 In more detall, a plurality of storage registers, designated 2 minor loops IILl, ~IL2, and ~3 are provided for storage of magnetic bubble 3 domains. In accordance with the principles well known in the art, any 4 number of minor loops can be used, although only three are shown in this figure. Information represented by bubble domains can be selectively 6 transferred to the minor loops by the inyut transfer gates lOA, lOB, and 7 lOC. Removal of information from the minor loops is accomplished by the 8 output transfer gates 12A, 12B, and 12C. In order to change the path of 9 bubble domains through the input transer gates lOA-lOC, current is provided in conductor 14, which is connected to the transfer-in current 11 source 16. Correspondingly, current in conductor 18, which is connected 12 to the transfer-out current source 20, is used to change the propagation 13 path of domains through the output transfer gates 12A-12C.
14 Coded information represented by bubble domains is provided by the bubble domain write control circuit 22, which is shown in more detail 16 in FIG. 3. A bubble domain read circuit 24 is used to sense information 17 taken from the minor loops and is shown in more detail in FIG. 4.
18 An input register SRl is used to bring information from the l9 bubble domain write control circuit 22 to the appropriate input transfer gates lOA-lOC for delivery to the appropriate minor loops, while an 21 output register SR2 is used to deliver information taken from the minor 22 loops to the read circuit 24. In FIG. 1, input register SRl is comprised 23 of magnetic elements which move bubble domains in the direction of arrows 24 26, in response to the reorientation of a magnetic field H in the plane of the medium in which the bubble domains move. - Any type of well known 2~ propagation circuitry can be used for this function. In a similar fashion, 27 output register SR2 is comprised of bubble domain propagation elements 28 which move bubble domains to the left in the direction of arrows 23 in 29 response to reorientation of magnetiC field H. In FIG. l, the i~put register SRl and the output re~ister SR2 are comprised of Y-I magnetic elements.

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1 In FIG. 1, the magnetic elements 30, located between each 2 input transfer gate and its associated minor loop, bring bubble domains 3 to the associated minor loop. That is, they aid in the propagation of 4 a domain from register SRl to the associated minor loop. Correspondingly, magnetic elements 31 aid in -the propagation of bubble domains from output 6 transfer gates 12A-12C to output register SR2.
7 A control unit 32 is used to provide start and synchronization 8 pulses to the transfer current sources 16 and 20, as well as to the sense g current source (FIG. 4) and the write control current source (FIG. 3).
Thus, control unit 32 regulates operation of the storage structure of 11 FIG. 1 in order to provide the write function, transfer function, and 12 read function. Control unit 32 also provides control signals to the 13 propagation field source 34 and the bias field source 36. Source 34 14 produces the drive magnetic field H while source 36 produces the magnetic field Hz used to stabilize the size of domains in the storage device.
16 These components are well known in the art and need not be described further. -17 FIG. 2 18 FIG. 2 shows a detailed circuit diagram of input register SRl, 19 output register SR2, two minor loops, input transfer gates lOA and lOB, and output transfer gates 12A and 12B. For ease of illustration, the 21 third minor loop and its associated transfer gates are not shown in this 22 drawing. In more detail, input register SRl is comprised of Y~bar magnetic 23 elements 38 and I~bar elements 40. Typically, these magnetic elements are 24 comprised of NiFe, as is well known in the art. ~lhen magnetic field H
reorients in phases 1, 2, 3, and 4, domains from write control circuit 22 26 propagate to the right in input register SRl.
27 Output register SR2 is comprised of Y-bar elements 42 and I-bar 28 elements 44. Again? these are magnetic elements typically comprised of ~11~.

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1 NiFe~ ~hen field H continually reorients through phases 1-4, domains 2 in register SR2 move to the left to the read circuit 24.
3 Minor loops ~Ll and MI2 are comprised of Y-bar elements 46 4 and I-bar elements 48. When field H reorients as illustrated, domains in the minor loops move around the loops in a clockwise direction, as 6 indicated by arrows 50.
7 Input transfer gate lOA is used to transfer bubble domain 8 information from input register SRl to minor loop ~1, while input 9 transfer gate lOB transfers bubble domain in~ormation from register SRl to minor loop ML2. Correspondingly, output transfer gate 12A is 11 used to transfer information out of minor loop MLl to output register 12 SR2, while output transfer gate 12B transfers information from minor 13 loop MI2 to register SR2.
14 As is also apparent from FIG. 1, magnetic I-bars 30 are located between the input transfer gates and the associated minor 16 loops, while magnetic I-bars 31 are located between the output transfer 17 gates and the output register SR2.
18 The transfer gates provide bubble domain paths which 19 essentially trace out the letter Y. In the embodiment of FIG. 2, the transfer gates are comprised of Y-shaped magnetic material, 21 such as NiFe. Thus, the magnetic elements., generally designated 52, 22 are comprised of two arm portions 54A and 54B, and a base or stem 23 portion 56. A current carrying conductor crosses the stem portion 56 24 of element 52. In FIG. 2, the conductor 14 passes across the stem portion 56 of the magnetic elements 52 in input transfer gates lOA and 26 lOB, while conductor 18 crosses the stem portions 56 of the Y-bar 27 elements of output transfer gates 12A and 12B.
28 In operation, bubble domains can move from arm 54A to arm 54B, 29 or from arm 54A to stem 56, as the magnetic field H reorients. Further, . -12-2S~

whether the bubble domains pass from one arm to the other or from one 2 arm to the stem of the element 52 depends upon the presence or absence 3 of current in conductors 14 and 18, respectively.
4 The operation of a representative transfer gate is shown more clearly in FIG. 2A. In this FIG., the same reference numerals are used 6 as were-used in FIG. 2, in order to facilitate explanation. Thus, the 7 transfer gate shown in FIG. 2A is illustratively transfer gate lOA.
8 Bubble domains move in the direction of arrow 58 toward the 9 transfer element 52 as field H reorients. A bubble domain located at pole position 1 of I-bar 40 will move to pole position 2 on arm 54A of ll Y-bar 52 when field H is in phase 2. In the absence of a current in 12 conductor 14, this bubble domain will travel along arm 54A to pole 13 position 3 of stem 56. As field H moves to phase 4, the bubble domain 14 will then move to magnetic pole 4' on the end of I-bar 30. This occurs since pole 4' is closer to the end of stem 56 than is pole 4 at the 16 far end of arm 54B. Thus, with no current in conductor 14, bubble 17 domains travel along arm 54A, along stem 56, and then to I-bar 30.
18 If a current is present in conductor 14 between field l9 phases 2 and 3, a bubble domain at pole 2 of arm 54A can still get to pole 3 at the end of stem 56, but when the field H reaches phase 4 21 the magnetic field produced by current in conductor 14 will send the 22 bubble domain to pole 4 at the end of arm 54B. As field H continues 23 to reorient, the bubble domain will move in the direction of arrow 60 24 and will consequently not be transferred into minor loop ~Ll. Thus, if a current is present in conductor 14, bubble do~ains proceed from 26 one arm 54A of Y-bar 52 to the other arm 54B of the Y-bar.
27 Since this is a symmetrical switch, the margins for bubble 28 domain movenent in any of the desired propagation directions are sub-29 stantially the same. The same switch can be used throughout the storage device in any functional circuit of the device.

1 Bllbble ~-ain ~rite Circuit (FIG. 3) 2 The write circuit 22 provides a selected pattern of bubble 3 domains to the input register SRl, where the presence/absence of 4 do~ains can be representative of binary information to be stored in the various minor loops. In this write circuit, a conventional 6 replica~or type of bubble genera-tor 62 is comprised of a large magnetic 7 element, typically .~iFe. As the magnetic field H reorients in the 8 plane o~ the magnetic medium, a single bubble domain is provided during 9 each cycle of drive field H.
The write circuit 22 also includes a transfer switch 64, 11 which is similar to the transfer switches described previously for 12 movement of bubble domains into and out of the minor loops. Transfer 13 Y-bar 64 includes arms 66A and 66B, as uell as a stem or base portion 14 68. A conductor 70 intercepts the stem portion of switch 64. In this desiOn, the conductor layer is provided in the same mask used to 16 provide the various propagation elements and generator 62. It can 17 overlie the magnetie elements eomprising generator 62 and the 18 propagation elements, be beneath that magnetic material la~er, be 19 sandwiched by the maOnetie material, or be coplanar with it in the 20 area OL switeh 64.
21 ~ Propagation eireuitry 72 is used to transport bubble domains 22 from generator 62 to the transfer switeh 64. Depending upon the state 23 of s-Jiteh 64, bubble domains are either sent to the input register SRl 24 or are sent to an annihilator via propagation circuitry 74. Conductor 25 70 is eleetrically connected to a write control eurrent source 76.
26 Write control current source 76 is used to provide eurrent in 27 eonductor 70, whieh in turn determines whether or not switch 64 allows 28 do~ins ~o pass to the input register SRl, or sends the domains to 29 an annihilator. Control unit 32 provides a control input to the curren~
.

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so~-ce 76 in order to determine the pattern of information to be sent 2 to input r-gister SRl.
3 In operation, generator 62 produces a single bubble domain 4 during each cycle of rotation of field H. mese domains pass to the propa~ation circuitry 72 and move to the switch 64. Without current 6 in conductor 70, domains entering switch 64 along arm 66A pass to the 7 stem 68 during field phase 2, and then move along the propagation 8 circuitry 74 to an annihilator. However, if current is present in 9 conductor 70, domains will be sent from arm 66A to arm 66B, rather tnan going to pole 2 on the stem portion 68 of the switch. Thus, 11 when current is present in conductor 70, bubble domains will be sent 12 to input register SRl.
13 R~ad Circuit 2~ (FIG. ~) .
1~ Read cireuit 24 is used to sense information removed from t:qe storage loops MIl . . . when this information is removed from the 16 storage loops b~ the output transfer gates 12A, 12B, etc. This is a 17 basic magnetoresistive sensing deviee using an aetive sensor S and a 18 dIm~y sensor 3 for noise eompensation.
19 In more detail, read eireuit 24 is comprised of chevron propagation elements 78, commonly of NiFe, which move magnetic bubble 21 d~mains from ~he output register SR2 to the bubble sensor S. As the 22 nu~ber of ehevrons in eaeh eolumn of chevrons increases, bubble domains 23 will stripe out along the columns of chevrons and thus be expanded to 2~ provide an am~lified output signal.
The sensor S and dummy sensor D are portions of a thin 26 magnetoresistive strip 80, which is electrically connected at one end 27 to ~he sense current souree 82~ at the other end by a ground eonnee-28 tion 84~ and at its eenter by another ground eonneetion 86. Magneto-29 resistive strip 80 is typieally NiFe of about 200 Angstroms thiekness.
As is known in the art, thick film sensors ean also be used.

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11~9LZ~3 1 In ope~ation, bubble domains propagate along chevron 2 elements 78 and pass beneath the sensor S while a current is passed 3 through magnetoresistive strip 80 from the sense current source 82, 4 in response to a triggering input from control unit 32 (FIG. 1).

5 This passage of bubbles causes a change in the resistance of the

6 magnetoresistive ma~erial 80 which is manifested as an output voltage V.

7 As is well known in the art, the dummy sensor D, comprising that

8 portion of magnetoresistive material 80 between the center ground

9 connection 86 and the right-hand ground connection 84, is not magnetically coupled to the bubble domain to be sensed but provides noise compensation 11 to balance out the noise produced by the rotating magnetic field H.
12 After being sensed, the domains propagate to an annihilator 13 (not shown). Thus, the storage unit illustrated provides destructive 14 read-out with new information being produced by the write circuit 22.
As will be apparent from FIGS. 5A-5C, showing a representative 16 fabrication process, the thin magnetoresistive strip is about 200 Angstroms 17 in thickness, while the magnetic propagation elements, such as the Y and lR I-bars, as well as the various chevron elements, are about 3,000 Angstroms 19 in thickness. Conductor layers are typically 5,000-10,000 Angstroms in thickness.

21 Single Level Fabrication (FIGS. SA-5C) 22 These figures illustrate a process for fabricating the storage 23 system of FIG. 1, using only a single critical masking step.
24 In more detail, a bubble domain medium 88, such as a garnet or any other bubble supporting material, has a layer 90 of NiFe evaporated 26 thereon. Layer 90 is typically a magnetoresistive material, a portion of 27 which can be used for the magnetoresistive strip 80 (FIG. 4). Layer 90 28 also serves as a plating base for the formation of other metallic layers.

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11(~42~3 1 Although it is not shown in FIG. 5A, an insulator layer such 2 as SiO2 of approximately 1,000 Angstroms, can be provided on the bubble 3 domain layer 88. If the bubble medium 88 is an amorphous ma~ne-tic 4 material, such an insulator layer would be used to provide electrical isolation between overlying conductors and the amor~hous material, which 6 is generally a metallic conductor.
7 A resist mask is formed in the resist layer 92. This is a 8 high resolution mask used to provide very small linewidth elements, 9 such as are used for the various propagation elements and transfer switches of the storage device of FIG. 1. The resist mask 92 can be 11 formed by any high resolution technique, such as by x-ray, or electron 12 beam exposure.
13 A first layer 9~ of a conducting material, such as gold, is 14 then electroplated through the mask 92. For this purpose, continuous layer 90 serves as a plating base. The thickness of the conductor 16 patterns 9~ is typically 5,000-10,000 Angstroms. The conductive layer 17 9~ serves ~or the conductors, such as 1~ and 18 (FIG. 1) and for the 18 sensor leads~ write control circuit, and other places where conductive 19 material is required.
20 The overlying magnetic layer 96 is then plated through the --21 same mask 92. Layer 96 is a magnetic material which is used for 22 propagation of magnetic domains and for the generator 62. Typically, 23 it is a magnetically soft material such as NiFe and the thickness of 2~ layer 96 is approximately 3~000 Angstroms. The resist mask 92 is then removed chemically and a second mask 98 (FIG. 53) is used to 26 protect the sensor area. Mask 98 is typically a low resolution mask, 27 such as a photoresist. Normal photon exposure can be used because of 28 the sensor dimensions and alignment tolerances. The critical align-29 ment of the sensor protect mask 98 will be in keeping the sensor f;om being skewed and thus intercepting more than one bubble. The re~istration ~17-L~ .
. ' ' ~ ' ~ . ., ' ' ' ' ' . - ~ ' , 2~i3 1 curacy presently available to do this is easily done using standard 2 photolithographic techniques.
3 The portions of the thin layer 90 not protected by the mask 98 or the layers 9~ and 96 are then removed, using for example, sputter etching or ion milling. Of course, some parts of the exposed portions 6 of layer 96 are removed, but layer 96 is initially thick enough that 7 removal of some of this material during the etching step will not 8 adversely affect the elements to be formed in that layer.
9 After the etching step ? the structure of FIG. 5C is provided.
A portion of thin NiFe layer 90 serves as the sensor S. The propagation 11 elements are formed by magnetic layer 96 and the various conductors, 12 contact leads, etc., are portions of the conductive layer 9~.
13 Thus, the transfer gates comprising Y-bar elements are comprised 14 of a layer of a conductor located close to the magnetic material 88, over 15 which is deposited a magnetic layer 96 used to move magnetic bubble 16 domains through the transfer gate. ~nis is a "conductor first" switch, 17 where the conductor layer is located closer to the bubble material than 18 is the magnetic layer, Of course~ it will be readily appreciated that 19 the reverse can also be easily provided, or that the conductor can be 20 coplanar or sandwiched with the magnetic layer in the stem portion of the 21 Y-bar switch.
22 The thin layer 90 of NiFe does not affect the operation of the 23 transfer switches. Its demagnetizing field closes through the top layer 2~ 96 and through the bubble domain material. Since it is so thin with 25 respect to the thickness of layer 96, its influence will essentially not 26 be seen~ so that the switches shown in cross~section by FIG. 5C are 27 essentiall~ "conductor first" switches.
28 The transfer switches of the present invention can be made 29 using other standard techniques, such as those employing two masking steps. ~he use of multiple masking steps for making bubble domain .

1 storage devices is well known in the art, and typically uses "lift-off"
2 techniques. 'l~ese variations in processing will not be described herein, 3 as they are well reported in the literature.
4 FIG. 6 .
The transfer switch of the present invention can be comprised 6 of three layers, two of which are magnetic. Thus, an illustrative 7 example is NiFe-gold-NiFe. Such a structure is illustrated by FIG. 6, 8 in which the bubble domain material 98 has a first magnetic layer 100 9 thereon, over which is formed a conductor layer 102. Finally, the top layer 104 is a magnetic material. Usually, magnetic layers 100 and 104 11 are comprised of magnetically soft material such as NiFe, while the 12 conductive layer 102 is, for example, gold.
13 As the bubble domain size decreases to a micron and less 14 in diameter7 propagation and switching devices have to be made smaller.
Good transfer switches require that the conductor used in the transfer 16 switch be deposited very near the bubble domain material. This would 17 typically require that the propagation magnetic material be placed on 18 top of the conductor. However, as bubble domains become smaller, the 19 separation between the bubble domains and the propagation magnetic layer has to be reduced. In the case of a half-micron diameter bubble 21 domain~ a separation of more than 1,000 Angstroms would not be desir-22 able. This problem is especially apparent in the case of amorphous 23 bubble domain films where an insulating layer must be used over the 24 films. On the other hand, placement of the magnetic propagation structure close to the bubble domain material can cause problems 26 because the magnetic flux of the magnetic material and the magnetic 27 flux associated with current through the conductive layer are then 28 180 out of phase.
29 The laminate structure of FIG. 6 has the propagation magnetic layer 100 close to the bubble domain material, and the conductor layer 102 1 located thereover. A thick layer of magnetic material 104 is deposited 2 over the conductor transfer line 102. Since layer 104 is thicker than 3 layer 100, the gradient fields for bubble domain transfer produced by 4 current in the conductor 102 and the field from the top layer 104 are in the same direction. During transfer, the top and bottom magnetic 6 layers 100 and 104 switch anti-parallel and magnetically close on each 7 other; however, since the top layer 104 is thicker than the bottom 8 layer 100, all of its stray magnetic field is not closed. This stray 9 field is in phase with the conductor stray field alld thereby increases the transfer field that acts on the bubble domain.
11 ~ Layers 100, 102, and 104 can be provided through the same 12 mask, thereby providing single masking level fabrication.
13 As an alternative, an all magnetic material switch can be 14 provided, in which the transfer conductor (such as conductor 14 of lS FIG. 2) is comprised of the same magnetic material as used to provide 16 the arms and stem of the transfer switch.
.

17 Examples 18 The present transfer switches provide efficient bubble domain 19 transfer using very small currents even when very small bubble domains are used. For e~ample, transfer of sub-micron domains can be performed 21 with these switches.
22 In the Y-bar switch (FIG. 2A), current through a conductor 23 crossing the base leg of the Y determines whether a bubble passes from 24 one arm to either the base or the opposing arm of the Y. In the absence of a switch curre~t, a clockwise rotating in-plane field will cause a 26 bubble to proceed from the left in the lower path, through the Y-bar of 27 the switch, and then to I-bar 30.- Thus the path 1-2-3-4' will be followed.
28 On the other hand, in the presence of adequate current of the correct sense,29 the bubble will be forced to remain in the lower path, following the , llU~Z53 1 sequence 1-2-X-4. The choice of path is made when the bubble is in the 2 vicinity of the base of the Y. Detailed studies of the operating margin 3 and failure modes were made for various values of the phase, pulse length, 4 and magnitude of the switching current I. It was established that the current in the conductor did not serve merely to prevent the bubble from 6 reaching position 3 at the end of the base 56 of the Y-bar, so that it would 7 never pass to position 4' on I-bar 30. On the contrary, the bubble could in 8 fact reach position 3 at phase 3 of the in-plane field H and still be 9 inhibited from permanently going to position 4' if adequate current were applied between phases 3 and 4. In this case a sufficiently deep potential 11 well was formed at position X by the current-induced field to force the 12 subsequent retraction of the bubble from position 3 to position X; when 13 the field rotated to the direction indicated by phase 4 the bubble then 14 passed from position X to position 4 instead of 4'.
In the critical switching phase, two attract$ve positions, X
16 and 3 on stem 56, are presented to the bubble. The pole at 3 is created 17 by the drive field, while the pole at X originates in the action on the 18 bubble of the field from the switching current. The bubble will reside 19 in the lower of the two energy states represented by positions 3 and X;
the relative energies depend on the magnitudes of the drive field and 21 the switch current. If the phasing is such that the bubble is at X
22 while the pole 4' is turning on, the bubble will subsequently pass on 23 to 4 and along arrow 60 instead of going to 4' then to MLl. (Late turn 24 on results in suppression of the successful operating region because the bubble has already passed to position 4' before the switch current 26 makes position X attractive).
27 This explains the pull-back effect, since successful switching 28 can occur after the bubble attains position 3 at the end of stem 56 if 29 the switch current which is subsequently applied is adequate to make position X more attractive than position 3.

2~3 1 Since the pole strengths at the pen~alloy bars (e.g., at 2 position 3) increase with increasing drive field, the region of best 3 swltching is confined to low values of drive field and expands as 4 the magnitude of the switch current is increased.
The field from the conductor opposes the field from the pole 6 which is created by the in-plane field at the base of the Y-bar. The 7 switching currents I , when plotted as a function of the in-plane field 8 ~, may be fitted by a straight line. The behavior of the switch may be 9 described by the slopes of these curves, i.e., the constant of propor-tionality K between the required switching current I and the in-plane 11 f$eld H, 12 H = KIS. (1) 13 Small-Bubble Switches 14 It is desirable to design a switch with a very high value of the switch constant K, since for a given value of in-plane field H only a 16 relatively low switch current Is will be needed under these conditions.
17 This is particularly important for small bubbles, where fabrication 18 considerations may make it difficult to achieve high values of switching19 currents.
The general scaling law for switches was derived heuristically 21 according to the following argument: For good operating margins the bubble 22 diameter D may be taken as 23 D - 8(2/~) (AQ) /MB. (2) 24 where MB is the bubble magnetization, Q is the ratio of the anisotropy field to 4~MB, and A is the e~change constant. Since A and Q are maintained 26 constant as the bubble diameter D decreases, D is proportional to 1/ ~ .27 The bubble induces a magnetic image in the Permalloy Y-bar, thus 28 causing the bubble to be bound to the Permalloy with an energy proportional ~ .

ZS~

1 to ~ . During propagation, it is the function of the in-plane field H
2 to overcome the effect of this potential well. Since the corresponding 3 field-induced bubble energy is proportional to ~B~ H must vary as ~ , 4 so tha~ as the bubble diameter is reduced, H must vary as l/D (see Eq.(2)).
It is the function of the current in the switch conductor to 6 provide a field Hs which counteracts the effect of the in-plane field H7 so that the propagation of the bubble is significantly affected. For 8 successful switching then, Hs is proportional to H. Since, from above, H
9 is proportional to l¦D, H is also proportional to l/D for successful switch operation.
11 Hs is proportional to I$/W, where I is the switching 12 ~ current and W is the width of the conductor. Since W is directly pro-13 portional to the bubble diameter D, H is also proportional to I /D.
1~ However, Hs must be proportional to l/D for successful switch action.
This condition can therefore be fulfilled only if the switch current I
16 is maintained constant as the bubble diameter decreases.
17 To check this heuristic argument for the Y-bar switch, 18 a Y-bar switch (in the conductor-first mode) was designed for two-micron 19 bubbles. The permalloy and conductor thicknesses were both taken as 0.3 microns, the spacer thickness (between the conductor and the bubble 21 film) was 0.35 microns, and the protrusion distance of the base of the 22 Y beyond the conductor was 1.0 micron. Other pertinent parameters of 23 this switch are given in Table I, where it is also noted that a switch 24 constant of 8.55 oe/ma is obtained.

' 2 ILLUSTR~TING SC~LING
3 (~on-SL'I Configuration) 4 Xubble Bubble Spacer Bar Stem Bar NiFe Conductor 4~M SwitcnS Diam. Film Thickness Width Length Thickness Thickness Constant 6 D h S W L t e K
7 (~ ) (g) oe/ma 8 2 2 0.35 1 7 0.3 0.3 400 8.55 9 1 1 0.175 0.5 3.5 0.3 0.15 800 17.1 For the case of a bubble chip scaled down by a factor of two to 11 accommodate one-micron bubbles, the pertinene dimensions are also given in 12 Table I, where it is seen that a switch constant of 17.1 oe/ma is obtained, 13 exactly twice the switch constant found for the two-micron bubble, thus 14 indicating that K is proportional to l/D. However, as pointed out above, it is anticipated that for propagation double the in-plane field - will be required for the one micron ease eompared ~ith the t~o-microR case, 17 so that the switch c-~rrent Is needed for successful operation will be 18 identical in the two cases.
19 Since ehe dimensions of the conductor decrease when the device is scaled for use with smaller babbles, the scaling law implies a 21 corresponding increase in current denslty. Further, since there is an 22 upper limit to the value of the current density which may be used in 23 practice, it is important to design switches for as high a value of the 24 switch constant K as possible. In the examples considered, the maximum usable current density was considered to be set by the electromigration 26 limit, which was taken as 107 amp/cm2.
27 Under this assumption, the scaling of the conductor-first design 28 may be considered for bubbles other than two microns in diameter. Here, 29 H(typ) is the typical in-plane field giving good propagation margins, and 1 ~nd that H is proportional to l/D. The curve of maximum curren-t Is (max) 2 vs D uas obtained under the condition of direct proportionality of the 3 conductor thickness and width with bubble diameter, so that under the assumption of current density fixed at the electromigration limit Is (max) is proportional to D2. Since the switch constant K is inversely 6 proportional to D (see Table I), from Eq. (1) H(max), the maximum in-plane 7 field for which successful suitching is possible, increases linearly with D.
ô Thus, the intersection of the H(max) and H(typ) curves gives O.ôô microns 9 as the smallest useable bubble diameter for the conductor-first configuration.
If there is a relative increase in the thickness of the conductor as the 11 device is scaled down, a smaller bubble limit is achievable.
12 Since the field from the conductor decreases with increasing 13 distance from the conductor, it might be thought that a larger switch 14 constant could be obtained if the effective spacing between the conductor and the bubble were decreased. This goal may be accomplished either by 16 decreasing the thickness of the bubble film (using a "shorter" bubble) 17 or by decreasing the spacer thickness. Then the tuo micron bubble case is 18 taken as a standard? and comparison is made with the case of a bubble of 19 half the thickness, and with the case of a spacer of half the thickness, analysis shows that the same switch constant is obtained for all cases. The 21 decrease o~ field with distance from the conductor is slou and is apparently 22 counterbalanced by the corresponding increase in the value of ~HB (the 23 effective bias field change on the bubble).

2~ SLM Y_Bar Switches .
Multi_level masking is presently impractical with electron-beam related 26 lithography methods used for the fabrication of small bubble devices because 27 adequate mask-alignment methods are not available. Instead, the conductor ~25_ ~' ' .

11~42S3 1 and the Permalloy must overlay one another because only one mask is 2 used in the SLM fabrication process. rnus each layer is confined to a 3 single plane, and the propagation as well as the switch elements have 4 the identical sequence and thickness of the various layers. (It is assumed that the conductor is made of a metal, e.g., gold, which has a 6 sufficiently high conductivity so that nearly all the current flows 7 through it, rather than through the Permalloy).
8 Under the SL~ condition, four possible conductor-first configura-9 tions are possible). (For quantitative comparison with the non-SL~
conductor first configuration, the spacing between the bottom oE the 11 Permalloy layer and the top of the bubble film is considered fixed).
12 1. SLM Conductor-First No. 1. Here the conductor is between 13 the Permalloy and the bubble film, just as in the non-SL~ conductor-first 14 case but without the "bridging" feature of the latter configuration. It is advantageous to fill with the conductor layer the maximum fraction of the 16 space between the conductor and bubble layers. However, in order to use 17 amorphous bubble films (which are electrically conducting), in addition 18 to the conductor film it is necessary to interpose an insulating film between 19 the Permalloy and bubble films. It is assumed that half of the space between the latter films is taken by the conductor, and half by the insulating film.
21 2. SI~S Conductor-First No. 2. Here the insulating film is 22 omitted completely, so that the spacer layer between the Permalloy and 23 bubble films becomes the conductor film. This configuration is thus useful 24 only for a garnet bubble film, since the latter is an insulator.
2S 3. Permalloy First. Here the Permalloy film is between the 26 conductor and the bubble film. According to the assumptions about the 27 Permalloy made in the present calculations, all of the flux from the con-28 ductor wlll flow through the Permalloy so that none will be available to 29 switch the bubble; i.e., the Permalloy acts as a perfect magnetic shield.
Experiments show that such switches do in fact work, but only at a con-31 siderably higher current than do other configurations.

11~4253 1 4. Permalloy Only. Here there is no special conductor layer, so 2 that the current flows entirely through the Permalloy film. In the analysis3 of this configuration it will be assumcd that the fields produced by the 4 current 2re the same as thos2 which would rlo-~ through a non-magnetic conductor having the dimensions of the Permalloy film. This assumption 6 arises from the reali~ation that a current in the bottom portion of the 7 Permalloy may be imaged in the top portion, but the current in the 8 top portion will not contribute any field to the bubble because the bottom.9 portion will provide magnetic shielding. The two effects thus cancel.
The values of K are higher for the SL~I conductor-first cases 11 than for t~e no~-SLM conductor-first case, presumably because the uncha~ged12 Permalloy-bubble spacing but decreased conductor-bubble spacing makes the 13 conductor relatively more effective in the former cases. The Permalloy-only14 case involves a value of K which is roughly a factor of two lower than that}5 of the non-SI~I conductor-first case; such a result is expected because of 16 the lack of an image current in the former case.
17 On the other hand, because of the geometries involved, more -18 current can flow through the conductors of some of these configurations 19 compared with the others. The net result is that the largest in-plane field whicn can be counteracted in switching is associated with the SL'5 21 conductor-first no. 2 configuration, while the lowest is associated with 22 the Permal70y-only configuration. The SLM conductor-first no. 2 23 configuration provides successful switching for the smallest bubble 24 diameter (0.7 ~).
All of the above confi7urations employed modest ~0.3) values of 26 the aspect ratios,.i.e., the ratios of the thickness to width of the 27 conductor and Permalloy layers. If the lithographic methods are 28 im?roved, e.g., with conform2ble mask or x-ray lithography, so as to provide 29 increased aspect ratios, direct benefit will be obtained for small-bubble switches employing the non-S~I conductor-first and SLil Permalloy-only .' ''' ' ~ ~

1 config~-~ations. (Increas_d aspect ratios cannot provide improvement in 2 the S~.`l conductor-first configurations since the spacer between the 3 Permalloy and bubble fiLms is fixed at a thickness which permits good propag~tion margins).
For a given buDble diameter (and hence fixed value of conduct~r 6 width ,~), the value of X(~ax) is directly proportional to K-e, the product 7 of the switch constant and the conductor thickness. Dmin (minimum bubble 8 diameter) asymptotically approaches a value of 0.~ to 0.5 micron for 9 aspect ratios in the ran~e of 2 to 3. (The potential-well model employed for the Permalloy-only case begins to lose its validity for 11 aspect ratios of 2 or lar~er). These values of Dmin are significantly 12 lower than those obtained from the SLM conductor-first designs with 13 modest aspect ratios.
~ Switch designs based on principles somewhat different from those described here may offer substantial advantages. For example, the field 16 created by the current in the described switch directly serves to oppose 17 the in~luence of the in-plane field. If the Y-bar switches were redesi~ned 18 to be ~ore symmetric, the two alternative bubble paths will be associated 19 with roughly equivalent field-induced poles in the "decision region" of the sw_tch. In that case the switch current would be employed only to 21 "tip" the bubble into one of the alternative paths, so that a much 22 smaller switching current would be required compared with that describea.
23 This or other modifications would permit switching of bubbles smaller tkan 2~ projected above for the illustrated Y_bar switch, and are within the general scope of this invention.
26 What has been snown is a magnetic bubble domain storage device 27 using an improved transfer switch whieh can be fabricated by single 28 level asking techniques and which is compatible with many types of 29 propagation patterns. Tne switch provides reliable transfer margins in all directions, and is characterized by switch propagation paths ~28_ ~i .
............. , , . .. .. . ~.. ~ .. , . ... ". , .. , .,, .,~, 11~42S3 1 generally following portions of a Y. Localized control of the path 2 followed by bubbles through the switch is provided by a control 3 means which produces a localized magnetic field for sending the 4 bubble domains along one of the paths through the switch. Thus, whether the transfer switch is comprised of one or several propagation 6 elements and regardless of their shape, width, length, or thickness, 7 the principles of the present invention are as described herein.

' .

.

Claims (29)

EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A magnetic bubble domain device for moving magnetic bubble domains in a magnetic medium in response to the application of repetitive cycles of a reorienting magnetic field in the plane of said medium, said magnetic field having a substantially constant magnitude during said cycle of reorientation, comprising:
a first shift register for moving magnetic bubble domains in said medium, a second shift register for moving magnetic bubble domains in said magnetic medium, transfer means for transferring bubble domains from said first shift register to said second shift register, or retaining said bubble domains in said first shift register, said transfer means being operative while said magnetic field is present and while said magnetic field continues said reorientation, said transfer means including:
at least one magnetic element for propagating said bubble domains in said transfer means in propagation paths generally defining the letter Y, one propagation path being along one arm of said Y and then along the other arm of said Y, and a second propagation path being from one arm of said Y to and along the stem of said Y, a conductor which crosses the propagation path defined along the stem of said Y, the presence or absence of current in said conductor, while said magnetic field is reorienting determining whether said bubble domains go along said first or second propagation path in said transfer means.

2. The device of claim 1, where the angle .alpha. between said propagation paths along the arms of said Y is greater than 0° but less than 180°.

3. The device of claim 2, where the angle .beta. between each arm of the Y and the stem of said Y is greater than 90° but less than 180°.

4. The device of claim 1, where the angle .beta. between each arm of said Y and the stem of said Y is greater than 90° but less than 180°.

5. The device of claim 1, where said at least one magnetic element has a stem portion which is crossed by said conductor.

6. The device of claim 1, where said magnetic element has arms of equal length.

7. The device of claim 1, where said magnetic element has arms of unequal length.

8. The device of claim 1, where the propagation path along the stem of said Y bisects the angle between the propagation paths along the arms of the Y.

9. The device of claim l, where the propagation path along the stem of said Y does not bisect the propagation paths along the arms of said Y.

10. The device of claim 1, where said transfer means includes a Y-shaped magnetic element for defining said first and second propagation paths through said transfer means.

11. The device of claim 10, where said magnetic element is comprised of a layer of magnetically soft material and said conductor is located between said bubble medium and said layer of magnetically soft material at the location where said conductor and said magnetic material cross one another along the stem portion of the Y formed by said magnetic material.

12. The device of claim 10, where said magnetic element has a laminate structure comprised of a layer of conductive material located between two layers of magnetically soft material.

13. The device of claim 1, where said magnetic element and said conductor are comprised of the same material.

14. The device of claim 1, where said magnetic element is a portion of said first shift register.

15. A transfer switch for selectively moving magnetic bubble domains in two different directions in a magnetic medium, comprising:
propagation means for moving bubble domains along paths which define a Y, one propagation path being from one arm of the Y to and along the other arm of the Y, and a second propagation path being from one arm of said Y to and along the stem of said Y, where the angle .alpha. between the propagation paths along the arms of the Y is greater than 0° but less than 180 , and the angle ~ between a propagation path along one arm of the Y
and a propagation path along the stem of the Y is greater than 90° but less than 180°, current-carrying conductor means for producing a potential well along the stem of said Y and below the intersection of the paths along the arms of the Y in order to determine whether said bubble domains follow said first or second path through said transfer switch, means for producing a reorienting magnetic field for moving said domains along said propagation means while said potential well is being produced.

16. The device of claim 15, where said means for producing a potential well includes means for creating a localized magnetic field at the region where the propagation paths along the arms of said Y intersect.

17. The device of claim 15, where said means for moving bubble domains includes at least one magnetic element along which magnetic poles are created in response to the reorientation of a magnetic field in the plane of said magnetic medium.

18. A bubble domain switch for selectively sending bubble domains along one of two different paths in a magnetic medium, comprising:
at least one magnetic element for propagating said bubble domains along two propagation paths which generally define the letter Y, in response to the reorientation of a magnetic field in the plane of said magnetic medium which continually rotates in the same sense and with substantially constant magnitude, a first propagation path being from one arm of said Y

to and along the other arm of said Y, while a second propagation path is from one arm of said Y and then along the stem of said Y, means for controlling whether bubble domains go along said first or second propagation path, said means for controlling operative while said magnetic field is applied.

19. The device of claim 18, where one of said paths is preferred in the absence of direction from said means for controlling.

20. The device of claim 18, where said means for controlling includes means for controllably producing a localized magnetic field.

21. The device of claim 18, where said at least one magnetic element has a geometry generally defining the arms and stem of the letter Y.

22. The device of claim 21, where said means for controlling includes a conductor which crosses the stem portion of the letter Y
generally formed by the geometry of said at least one magnetic element.

23. A transfer switch for selectively sending magnetic bubble domains along one of two different propagation paths, comprising:
magnetic means along which magnetic poles can be established by a reorienting magnetic field where the locus of said magnetic poles generally defines the letter Y, means for sending bubble domains along a first path which is along one arm of said Y and then along the other arm of said Y, means for sending bubble domains along a second path where said second path is along one arm of said Y and along the stem of said Y, control means operative while said reorienting magnetic field is applied for determining whether bubble domains pass along said first path or said second path, where said control means includes a conductor along which current can flow, the presence or absence of electrical current in said conductor determining whether said bubble domains go along said first path or said second path.

24. The device of claim 23, where one of said two paths is preferred in the absence of direction from said control means.

25. The device of claim 23, where the angle .alpha. between the propagation paths along the arms of said Y is greater than 0° but less than 180°, and the angle .beta. between a propagation path along one arm of the Y
and the propagation path along the stem of the Y is greater than 90° but less than 180°.

26. A transfer switch for selectively sending magnetic bubble domains in a magnetic medium along one of two propagation paths, comprising:
magnetic means for propagating bubble domains along paths defining the letter Y in response to the application of a magnetic field substantially in the plane of said medium, one of said propagation paths being along one arm of the Y and then along the other arm of the Y, and a second propagation path being along one arm of the Y and then along the stem of the Y, control means for determining whether bubble domains pass along said first propagation path or said second propagation path, where said control means includes a current carrying conductor which extends in a direction across the direction of propagation along the stem of said Y, the presence or absence of current in said conductor while said magnetic field is applied determining the path taken by said bubble domains through said switch.

27. The device of claim 26, where the angle .alpha. between the arms of said Y is greater than 0° and less than 180°, and the angle .beta. between either arm of said Y and the stem of said Y is greater than 90° and less than 180°.

28. The device of claim 26, where said magnetic means and said conductor are comprised of the same material.

29. The device of claim 26, where said magnetic means has a geometry generally defining the arms and stem of the letter Y.