MAGNETIC BUBBLE MEMORY

Abstract: New geometry for the propagation elements of a field-access magnetic bubble memory which is tolerant of relatively wide gaps between elements and lower drive fields, thus resulting in greater data storage density and reduced power operation. The elements (30, 31) have shapes which define bubble 'traps' operative to hold bubbles for relatively high efficiency transfers between elements. In alternate embodiments, the elements (50-53) have an L or a 'pickaxe' shape.

Description: MAGNETIC BUBBLE MEMORY Field of the Invention This invention relates to magnetic bubble memories and more particularly to such memories in which bubbles move along a path of magnetically soft elements responsive to a reorienting in-plane magnetic field. Background of the Invention Magnetic bubble memories are well known in the art. Bubble propagation is accomplished in such memories by creating a changing pattern of localized magnetic field gradients. U. S. Patent Mo. 3,534,347 of A. H. Eobeck issued October 13, 1970 discloses a "field-access" mode bubble memory in which the localized field gradients are produced in a pattern of magnetically soft elements adjacent the bubble layer by a magnetic field rotating in the plane of bubble movement. The elements, typically of Permalloy, have a geometry and orientation such that the rotating in-plane field produces in them in consecutively offset positions magnetic poles which attract bubbles and which are operative to move bubbles along a path defined by the elements. A variety of geometries have been used for the propagation elements of Permalloy field-access bubble memories. The T-bar geometry, disclosed in the above- mentioned patent, was one of the original circuit patterns used. That pattern employs elements oriented at 90 degrees to one another so that each 90 degree rotation of the in- plane field moves the bubble to a new position. Efforts to improve the operating margins of bubble memories have resulted in a number of known alternative geometries, including the Y-bar, the asymmetric half-disc, and the presently used asymmetric chevron. Whatever the geometry of the elements, the propagation path is formed by a series of the elements oriented in such a manner that due to the rotation of the in-plane field and consequent pole formation, a bubble moves to successive positions on an element, and at a certain point, again due to the geometry of the pole formation, transfers to the next adjacent element. In the asymmetric disc or chevron pattern, the bubble lingers near the end of the instant element until it sees a stronger attractive pole on the next adjacent element and then transfers to that element. It is thought to be advantageous for the pole formed on the element to which the bubble transfers to be strong enough to "pull" the bubble over to complete the transfer. This requirement limits the element design of the propagation circuit. Brief Description of the Invention The present invention is directed at a field- access bubble memory having relatively attractive operating *» margins. A new mechanism for propagation as well as a new geometry for 'the propagation elements allows for more efficient transfer of a bubble from one element to the next, makes more efficient use of space for pole formation, and permits higher packing densities to be achieved. In one embodiment herein, a propagation element comprises two segments of different lengths oriented at an angle one to another thus forming a long bar section and a shorter end in what may be visualized as a chevron shaped element with a short downstream leg. In forming a propagation path, the elements are oriented diagonally with respect to the direction of the bubble path with the short leg of one element leading directly to the long leg of the next adjacent down stream element. As with prior art field-access devices, the elements are made of a magnetically soft material, typically Permalloy, responsive to the rotation of an in- plane magnetic field. As the in-plane field rotates, poles form which make the short end act as a bubble "trap". The bubble waits in the trap and is prevented by the shapes of the elements from backwards propagation until the in-plane field switches the magnetization of the long bar section of the elements and the bubble is ejected directly to the next element. At the time of crossing to the next subsequent element, a bubble finds itself in a magnetic field originating at an upstream portion of the instant element and terminating at the other side of a gap on the next subsequent downstream element, the magnetic field providing a strong propagating force for moving the bubble. Additionally, the diagonal orientation of the elements with respect to the direction of propagation allows a relatively large element to be formed within a square area of unit size allotted to a single period of the memory thus achieving relatively strong poles. In an alternative embodiment, the elements have a "pickaxe" or T shape. The elements are operative in the same manner as are the above-mentioned elements to propagate bubbles. The symmetry of the pickaxe geometry makes these elements particularly suited as a transfer between adjacent paths. Brief Description of the Drawing FIG. 1 is a block diagram of a magnetic bubble memory; FIGS. 2 (comprising subfigures 2A, 2B, 2C, 2D, 2E) , 3, and 4 are enlarged top views of portions of the memory of the type shown in FIG. 1 showing propagation elements of different embodiments of this invention; FIG. 5 is a margin plot of a memory test circuit of the type shown in FIG. 3; FIG. 6 is a view similar to that of FIG. 4 but showing a different embodiment of the invention. Detailed Description FIG. 1 shows a magnetic bubble memory 10 including a host layer 11 of a material in which magnetic bubbles can be moved. Bubbles are moved in layer 11 in paths, l-_, l 2 ...and l . which are commonly referred to as minor loops, and in addition, in a path of ML commonly referred to as a major loop. ~ -- ~ ~ ~ - ~ ~~ - ~ Storage of data is provided by the minor loops. The major loop, on the other hand, provides for access to the minor loops of substitute data from a bubble generator 12 and for read out of addressed data at a detector 13. In this connection, generator 12 comprises an electrical conductor connected between a generate pulse source 14 and ground operative under the control of control circuit 15 to provide a pulse selectively during each cycle of a propagation drive circuit represented by block 17. Detector 13 similarly is shown connected between a utilization circuit 18 and ground and may include a magnetoresistance detector element. Bubbles are maintained at a nominal diameter by a bias field supplied by source 19. We adopt the convention that data, generated at 12, move counterclockwise about loop ML to locations at the lower ends of minor loops 1^ in response to successive propagation cycles of the in-plane field. A transfer-in conductor 20 couples those ends of the minor loops with associated stages of the major loop for transferring new data into the minor loops at the proper time. Conductor 20, to this end, is connected between a transfer-in pulse source 21 and ground as shown. A similar transfer operation, termed a transfer- out operation, occurs at the top ends of the minor loops as viewed. The transfer-out operation is controlled by a pulse in conductor 25 which is similarly connected between pulse source 26 and ground. The control of the transfer functions as well as the generator, propagation and detector operations is derived from a master clock in accordance with well understood principles. Such circuitry along with an address register is considered to be included within control circuit 15. The general organization of the memory of FIG. 1 thus can be seen to involve the generation of a bubble pattern at 12 for later storage in the miner loops by the activation of transfer-in conductor 20 during a write operation. Also involved is the transfer-out of addressed data from the minor loops by the activation of transfer-out conductor 25. The data transferred out advances to detector 13 for applying signals representative of the transferred bubble pattern to utilization circuit 18. The data move counterclockwise along loop ML until a later transfer-in operation moves the data back into vacancies at the bottom of the minor loops as viewed. In this connection, it is helpful to recognize that bubbles usually move synchronously in all the loops of the memory. When a transfer-out operation occurs, vacancies are left in the addressed bit locations in the minor loops. These vacancies move about the minor loops as the transferred data move to detector 13. The number of stages in the minor loops and the number in the major loop are chosen so that data transferred out or data generated at 12 arrive at the lower end of the minor loops synchronously with these vacancies. We now direct our attention to FIG. 2 which shows a portion of a propagation path of FIG. 1 employing shortened chevron or L-shaped elements. The succession of figures of FIG. 2 is intended to show a representative portion of an illustrative path with poles formed due to a rotating in-plane field, and the positions occupied by a bubble propagating along this path. In all the FIGS. 2A- 2E, the in-plane field rotates clockwise and propagation is to the right. FIG. 2A shows a bubble at an assumed initial position on element 30 where the attractive poles accumulate for the direction in which the in-plane field H D points. As the in-plane rotates clockwise, a positive pole is created at the top of the element and the bubble moves to occupy the position shown in FIG. ' 2B. Further rotation of the field by 90 degrees leaves the bubble in the same position while, the strong negative pole reorients to the position shown in FIG. 2C. As the in-plane field rotates another 90 degrees, to the orientation shown in FIG. 2D, the bubble moves into the position shown in that figure. This position, at the short end of element 30, constitutes a bubble "trap". The bubble is prevented from backwards propagation by the presence of negative poles, as shown, and waits in the trap until the rotating in-plane field begins to switch the magnetization of the long bar segment of element 30 to that shown in FIG. 2E. At this instant, positive and negative poles are present in the adjacent elements 30 and 31 at such positions, as shown, to create a magnetic field the axis of which extends directly along the desired path for movement of the bubble between the elements. The bubble thus passes from element 30 directly to an awaiting attractive pole formed in element 31. Direct observations of the transfer show that the bubble appears to be hurled or ejected from the element 30 to the element 31. The reason for this is not known although it is conjectured that in passing from the condition (FIG. 2D) in which the bubble is trapped in place to the condition where a strong force is created to move the bubble, some sort of energy gathering-triggering effect is created which results in the strong impetus given to the bubble. Whatever the mechanism, however, the result is that much larger gaps can be controllably crossed by the bubble then was heretofore possible. An advantage of this is discussed hereinafter. The bubble advances one period in one cycle of the drive field. FIG. 3 shows an alternative geometry for the propagation elements of FIGS. 2A to 2E which is a variation of the basic L-shape shown in those figures. Elements 50, 51, 52 and 53 of FIG. 3 have the above-mentioned "pickaxe" or "T" shape and are separated by gaps 60, 61 and 62. Propagation of a bubble along these elements is analogous to propagation along the propagation pattern of FIGS. 2A- 2E. In response to successive reorientations of the in- plane field as shown in the figure, the bubble moves from position P-* ^ to ? 2 to P 3 to P 4 . As the field moves to the "1" orientation, as indicated by curved arrow 53, the bubble is blocked by the repulsive (negative) pole created at P-3 (of element 50) from moving backwards along element 50. Element 51 exhibits an attractive (positive) pole at position P*-_ ' and so the bubble crosses gap 60 to position P^. FIG. 4 shows an illustrative minor loop I3 composed of Permalloy elements of still another shape which are operative with relatively wide gaps. The figure also shows transfer-in and transfer-out conductors 20 and 25 for moving bubbles between the major loop ML and the minor loops. Bubble movement is counterclockwise in the minor loops, turns 100 and 101 being defined for such operation. A transfer operation is carried out in response to a pulse applied to conductor 20 or 25 by source 21 or 26 respectively. A bubble moving from left to right along- the lower horizontal leg of path ML passes position 110 from whence normal rotation of the in-plane field causes the bubble to move to position 111. At this juncture, source 21 of FIG. 1 pulses conductor 20 for moving the bubble to ' position 112 for movement to minor loop I3 during subsequent operation. Similarly, normal propagation counterclockwise about the minor loop I3, moves a bubble into position 120. At this juncture, source 26 pulses conductor 25 causing the bubble to move to position 126 for subsequent movement to the upper leg of major path ML as viewed in FIG. 4. The significance of the embodiments of FIGS. 2A- 2E, 3 and 4 lies in the fact that adjacent elements are separated by gaps which, as previously noted, can be significantly wider than prior art gaps. It has been accepted in the bubble art that a gap., between adjacent elements of a propagation path for bubbles is necessarily small compared to a bubble diameter at the collapse field. It has also been established that a period or distance through which a bubble is moved during a single cycle of the in-plane drive field is large, typically four to five times a bubble diameter at the strip-out field. Commercially available photolithographic equipment has resolution capabilities of about l.Ou. Thus the gap separating adjacent elements of a bubble path can (at best) be l.Oμ, thus requiring (heretofore) a somewhat larger bubble, e.g., of 1.7μ diameter, hence a period of 8.0μ . The present invention allows a change to be made in*the relationship between the gap width and the bubble diameter. Thus, for example, with presently available l.Oμ photolithography, 4.0μ period circuits can be realized with l.Oμ gaps and 0.8μ bubbles. In view of the fact that one million bit bubble memories with 3.0μ periods are now available, the present invention permits four million bit memories on a like-size chip (approximately 8 millimeters on a side) with the same photolithography techniques. Bubble memories with patterns tolerant of wide gaps as disclosed herein are characterized by relatively low drive fields. FIG. 5 shows margin data for elements of the type shown in FIG. 4. The data was taken for a square array of elements of the type shown in FIG. 4 having top (.), left (x) , bottom (o) , and right (+) legs. The legend in the figure corresponds to these designations. The vertical axis represents bias field and the horizontal axis represents drive field. It can be seen that low drive fields and wide margins are achieved. This margin data is representative of data taken on a significant number of samples. The particular data was taken with bubble tests circuits having 6000 Angstrom units (Angstrom) of S--_0 2 and 2000 Angstrom of Permalloy. The thicknesses of both the S-*_0 2 and Permalloy layers have been varied with similar results. The transfer function, described in connection with FIG. 4, utilizes conductive strips 20 and 25 overlyi.ng the bubble layer 11 which are current pulsed to alter the path of the bubble being transferred. Such transfers, however, can also be accomplished without such strips, as now described in connection with FIG. 6. First, it is helpful to recall that bubble movement is caused by a magnetic field reorienting, usually by rotating, in the plane of bubble movement. Such a field is provided by the propagate field source 17. Transfer of bubbles both in and out of the minor loops is accomplished in the FIG. 6 arrangement by a properly phased reversal of that field's direction of rotation. A transfer control circuit is utilized in this arrangement in place of the transfer in pulse source 21 shown in FIG. 1, such control circuit being adapted to alter the direction of the field rotation under the control of control circuit 32. Control circuit 32 is adapted to synchronize and control all functions herein and is assumed to include a clock, counters, and address generators for this purpose as is now well understood in the art. FIG. 6 shows an enlarged top view of a minor -loop (viz. 1 -. ) about which bubbles recirculate counterclockwise as indicated by curved arrows 140 and 141 in the figure. The normal clockwise rotation of the drive field moves bubbles through the sequence of positions P -, , P 2 ,- and P^. When a bubble reaches position P 2 of element 145 within the major loop ML (bottom of the figure) ; control circuit 32 signals the transfer control circuit to reverse the direction of rotation of the drive field. In response, a bubble occupying position P 2 of elements 145 at the time of the reversal moves to position P τ at element 146. All remaining (untransferred) bubbles are now in positions P-,. Clockwise rotation now resumes with the transferred bubble moving upward along element 146 to elements 147, 148, 149, for continued counterclockwise movement about locp l-> as indicated by arrow 140. Transfer in is now complete. The transfer-out operation commences when a bubble reaches position p, of element 150 at the top of the loop 1- j . At this juncture, the clockwise rotation of the field is reversed and a bubble in that position roves to the transfer position P τo of element 150. The clockwise rotation is resumed and the bubble, in transfer, moves to position P τ -*_ of element 151. Transfer out is now complete and a transferred bubble merges and moves to the right in path ML as shown by arrow 122.

Claims: Claims 1. A magnetic bubble memory comprising a host layer (11) in which magnetic bubble-! can be moved, said bubbles having a diameter d_ in the presence of a bias field (19) at a collapse value, means for providing a magnetic field (HD) reorienting in the plane of bubble movement, propagation means (15, 17) for creating in said host layer at least one path along which bubbles can be moved, said propagation means including a repetitive pattern of magnetically soft elements responsive to said in-plane field to exhibit changing pole patterns, said memory being CHARACTERIZED IN THAT adjacent ones of said elements (30, 31) have geometries and orientations to form a magnetic field encompassing said adjacent elements, said field being operative to move a bubble in said field from one element to the adjacent element. 2. A memory in accordance with claim 1 wherein said elements are spaced apart by gaps greater than the bubble diameter d_. 3. A memory in accordance with claim 2 wherein said elements define paths (l j _, ML) organized into a major-minor path arrangement. 4. A memory in accordance with claim 3 wherein each of said elements comprises at least first and second segments joined at an angle, said elements being oriented at an angle with respect to the axis of the bubble path defined thereby. 5. A memory in accordance with claim 4 wherein said segments have different lengths. 6. A memory in accordance with claim 3 wherein said elements (50-53) have first, second, and third legs forming a "pickaxe" shape. 7. A memory in accordance with claim 3 wherein certain ones of said elements (145, 146, 147) a transfer position for moving bubbles between themajor and a minor path in response to a temporary reversal of the direction of change of the in-plane field

Inventors: BOBECK ANDREW H, SHAPIRO HERBERT M
Applicants: WESTERN ELECTRIC CO
Application year: 1981
Main classification: G11C19
IPC codes: G11C11, G11C19