Analysis of nonuniformity of bubble propagation in field access systems

The static and dynamic properties of bubbles can be conveniently observed by using a mirror overlay between a drive structure and a thin ferro-garnet film. This method was applied to studies of bubble propagation in T-bar, T-X and chevron permalloy drive structures. The results of the study lead to the conclusion that a parameter a should be introduced for estimating the nonuniformity of bubble propagation. This parameter is found by photometric measurements of light intensity variations along the bubble propagation path over the structure period λ and by subsequent calculations.     

New fabrication technique for magnetic bubble circuits with submicron dimensions

A new technique which permits the fabrication of submicrometer bubble propagation circuits has been described. Straight line patterns and contiguous zigzag patterns are combined with an appropriate registration to form bubble propagation patterns. The straight line pattern width corresponds to the gap width in the Permalloy bubble propagation circuits. By controlling the exposure time in fabricating straight line photoresist patterns, submicrometer pattern gaps are easily obtained using photomasks with 1 μm minimum features. The 4 μm period and 0.5 μm gap width permalloy circuits fabricated using this technique provide promising propagation characteristics for 1 μm bubbles: 60 Oe bias field margin at 60 Oe drive field and 25 Oe minimum propagation drive field.     

Bubble propagation by disks formed by ion-implantation

The investigation of implantation conditions for bubble propagation points to the necessity for a minimum dose (1.5 times 10^{16}ions cm²) and a sufficiently thick profile (0.4 μ). The use of flat profiles gives uniform implanted layers and allows values of the maximum defect concentration far away from the amorphization threshold. Unlike in permalloy circuits, bubble stability and bias field margins do not increase with the drive field. The propagation is not critically dependent upon disk diameter and spacing.     

The effect of DC in plane field on the operation of field access bubble memory devices

The effect of a small dc in-plane field on the start-stop operation of field access bubble devices has been studied. Experimental results show that the bias margin in this mode is very sensitive to the magnitude of the field and its orientation relative to the start-stop direction of the drive field. In a T-I circuit a complete margin loss was observed for an in-plane field of 3 Oe oriented antiparallel to the start-stop direction. For parallel orientations of the in-plane field the start-stop margin improved and approached that of the continuous propagation margin at an in-plane field of approximately 6 Oe. Dependence of the start-stop margin on the orientation of the start-stop direction relative to the pattern was also observed. Measurements of the bubble collapse field at various points in the pattern show a very strong dependence on the in-plane field and the permalloy geometry. The collapse-field results and magnetostatic energy considerations which take into account local field variations and bubble-bubble interactions provide a basis for understanding the experimentally observed start-stop margins. These results show that a small tilt (2 to 3°) should be introduced in the bias field to overcome normal alignment tolerances and ensure that a favorable in-plane field is always present. This assures reliable start-stop operation.     

On spontaneous nucleation in field-accessed bubble devices

The dependence ot the in-plane drive field at which bubble domains spontaneously nucleate in field-accessed bubble devices has been investigated as a function ofH_{k} - 4piM_{s}and of spacer thickness between the bubble film and permalloy propagation elements. The experiments were carried out on amorphous GdCoMo bubble films with T-bar and Y-bar structures. For a given structure and spacer thickness the nucleation field increases linearly withH_{k} - 4piM. Larger spacer thicknesses also lead to increased nucleation fields. A model based on the Stoner-Wohlfarth astroid is compared to these data and found to be useful in explaining the qualitative trends, but to be in poor quantitative agreement. It is concluded that since the drive field required in a device is proportional to4piM_{s}, Q - 1 = (H_{k} - 4piM_{s})/4piM_{s}must be greater than some minimum value for a given device structure and spacer thickness to permit reliable device operation.     

Some characteristics of ion-implanted bubble chips

The relations between the position of charged walls and the bubble motion around propagation circuits are discussed. Long walls which extend between adjacent propagation loops are revealed by the Bitter technique. The examination of the domain structure in the implanted layer shows the existence of a magnetic gradient which is a function of the distance from the propagation circuits. The switching of magnetization in particular directions of the in-plane field is reported and correlated with the bubble movement. An additional easy axis is observed along the circuits due to shape anisotropy. Propagation margins are very similar to those obtained with permalloy circuits. Fabrication technology as well as design of 16 μm period circuits is discussed. Nucleation and transfer have been achieved with currents in the range of 50 mA to 200 mA. Phase margins of about a quarter of a period are found, and bias field margins fall between 10 and 15 Oe.     

Single-mask bubble memories: Bit/Chip organization with decoding

Progress in element and chip design of single-mask bubble chips, based on drive-field operation, is reported. Three new elements, which have been successfully operated, are presented. (i) The replicator copies bubble streams for one propagation direction; when used in combination with the sense of rotation of the drive field it can serve as a bit generator. (ii) The λ-creator can selectively shift a bit over one propagation period. It can be applied in a decoder organization which permits rapid access to stored information. (iii) A current-controlled splitter-type generator was designed, which takes current pulses of about 60 mA for creating new bubbles.     

68 kbit capacity 16 µm-period magnetic bubble memory chip design with 2 µm minimum features

A bubble propagating structure that operates well on a 14 μm to 18 μm propagate period with a nominal 2 μm minimum feature size has been designed. The structure consists of only 1 discrete permalloy feature per circuit period. Sixty-eight kbit-capacity memory chips based on such structures have been designed, built, characterized, packaged and the packages have been characterized. The chip is organized as a set of minor (storage) loops with separate write and read major lines. The bubble manipulating functions, of which the replicate and transfer gates are the most critical, have also been designed with 2 μm minimum features. The design is adequate to provide a 14 Oe bias field margin range with drive fields of about 35 Oe, using a bubble garnet material with approximately 170 Oe free bubble collapse field. Sixty-eight kbit single loop shift register type chips designed using similar propagating structures, however, provide over 20 Oe bias field margin ranges with drive fields of about 35 Oe.     

Bubble domain field access device modeling part II: Device modeling

By generalizing the stability treatment of Thiele to include the presence of a permalloy pattern, a magnetostatic energy model for describing field access devices is derived. The general implications of the model with regard to minimum drive fields and propagation margins are discussed and calculated magnetostatic energy profiles for noninteracting chevrons are presented. The question of when the permalloy elements begin to interact is examined to provide justification for treating the permalloy elements independently. Finally, a magnetoresistance model for detector analysis is proposed which may be incorporated easily into the existing modeling framework. The paper concludes by suggesting further areas of application and by discussing areas which need further improvement.     

Contiguous-disk bubble domain devices

Contiguous-disk bubble devices are an approach to higher bit density through the use of coarse overlay patterns in manipulating small bubbles to relax device lithography requirements. As a first step towards such an objective, a fully processed chip using ion-implanted devices has been tested, showing the feasibility of all required memory functions with 5-μm bubbles and 25-μm period overlay patterns. A critique of permalloy versus implanted contiguous-disk devices is made, pointing out their basic difference in magnetization reversal processes and explaining the superiority of the latter over the former in achieving a good edge affinity of bubbles. The requirements for a good implanted device are reviewed, including the selection of garnet material parameters (K1, λ111), of implantation parameters (ion energy and dosage) and of device pattern geometry (thickness and shape of implanted layer). An understanding of these requirements has made it possible to demonstrate 1-μm bubble propagation in several contiguous-disk type circuits with 4.5-μm periods, yielding an areal density of over 3 × 10⁷bit/in²made by conventional photolithography.     

Models for I and T pattern permalloy bars based upon bitter pattern observation of domain wall movements

The magnetostatic fields of the I and T pattern Permalloy overlay bars are analyzed by proposing a model based on the Bitter pattern observation of the domain wall structure in Permalloy bars. The magnetic charges that appear on the 90° domain walls are assumed to be the sources of the magnetic fields of the bars. The model has a two-dimensional reaction to an applied rotating in-plane field due to its two-dimensional domain wall movement and the consequent two-dimensional change of magnetic domain pattern inside the bar. The magnetization of the bar is equal to Msthe saturation magnetization of the bar at every section of the bar except on the domain walls. The magnetization curve and the magnetic field well Bz(bubble drive field) under the overlay bars are calculated and compared to that of the previous models. A qualitative explanation of the rotation of the bubble around the bars is given by the three-dimensional plots of the field well obtained for different orientations of the in-plane field.     

Comparison of experimental and theoretical results for high frequency bubble propagation under T-bar tracks

Observational results for bubble motion at high frequencies in field-accessed devices have been reported in the literature. The predictions of a model for bubble propagation under T-bar tracks are compared with these results for a frequency of 100 kHz, initially at midrange values of drive and bias fields. This comparison is helpful in refining certain features of the model so as to bring its assumptions nearer to the physical situation. The matching between predicted and observed results for the variation of bubble position with drive field angle is now much improved. The refined model is then used to calculate results at other values of drive and bias fields for which experimental data is available. The agreement between theory and practice is shown to be quite close except when the bias field is high.     

A magnetic shift register employing controlled domain wall motion

A shift register based on controlled domain wall propagation in a magnetic wire with an axial easy direction has been developed. The operational characteristics of an experimental model are discussed. Some hard drawn magnetic materials have been shown to have at least a 3-to-1 nucleation-to-propagation threshold field window over several hundred foot lengths of wire which guarantees operation, This is accomplished without putting the wire under tension. One such promising material is a modified 79-percent Ni, 17- percent Fe, 3-percent Nb, 1-percent Ag, permalloy. An experimental5 times 10^{3}-bit shift register has been constructed with 0.8-mil hard drawn Nb, Ag permalloy and operated at speeds of5 times 10^{3}bps with 12-percent drive current margins. The input power at this rate is approximately 1 mW/bit.     

Bubble dynamics in T-bar-type circuits

A theoretical model for bubble propagation under T-bar type overlays due to an in-plane rotating field is developed. By making certain assumptions about the nature of the overlay magnetisation, it is shown that bubble driving forces can be readily calculated for a circular bubble of varying diameter moving freely in 2-dimensions. Expressions for the frictional forces acting on a bubble are derived and the resulting equations are solved to yield the bubble centre trajectory, the radius, velocity and phase variations. This analysis is applied to examine bubble propagation along a straight portion and around a corner of a T-bar circuit. Cases of successful propagation of an isolated bubble, as well as of failure to propagate, are given. Bias margins for a straight T-bar track at 100 kHz are obtained theoretically and compared with experimental results for a chip having the same parameters.     

Rotating gradient measurement of bubble dynamics

A rotating gradient method is used to measure the steady-state bubble response under constant drive conditions. The rotating gradient drive is produced with alternating currents in a single-level conductor configuration. In response the bubble propagates at constant speed along a magnetooptically observed circular trajectory. A measurement of the trajectory diameter and the bubble's azimuthal position relative to drive with clockwise and counterclockwise gradient rotation characterizes the steady-state response and the effective bubble mass.     

Computer simulation of the operation of a magnetic bubble domain propagation circuit modeled after a current-access device

The operation of a bubble-domain straight-line propagation circuit has been simulated successfully. This simulation has been achieved by our approximating the motion of an s = 0 frozen-azimuth bubble placed under a drive fieldH_{Z}(X, Y, T)= -H_{p} cdot cos [2pi(X/R_{X} - n(T)/4)] cdot exp [-(Y/R_{Y})^{2}]. The simulation has been generated from a previously developed numerical scheme to simulate the motion of a bubble, whose domain shape and magnetization structure along its domain wall were variable. The drive field has been modeled after a dual conductor-sheet, current-access propagation structure, which has a bit period RXand a transverse width on the order of2R_{Y}. The entire field contour has been advanced stepwise in the positiveXdirection by an increase of the integern(T), which represents the drive-phase number. The bubble motion has been observed during the first six drive phases to produce operating margin diagrams for drive frequencies of 250 KHz, 796 KHz and 1 MHz. The method of calculation and the results of the simulation are given.     

High-frequency propagation and failure of asymmetric half-disk field access magnetic bubble device elements

High-frequency propagation characteristics and failure modes in 14-μm period, 1.8-μm gap, asymmetric half-disk field-access device were studied using a high-speed optical sampling technique. Propagation elements as well as normal and hand gun corners and chevron structures were included. The operating bias margin at 1MHz, for a structure that had 1.2 MHz as highest possible frequency, was about half of the margin for frequencies of 200 kHz and below. The phase lag between the bubble leading wall and the instantaneous rotating field direction was nearly 90° as the bubble moved through the center of the element where the lag was the greatest. The peak velocity of the leading wall of 55 m/s and the trailing wall of 46 m/s is attributed to bubble interaction with the Permalloy structure creating a ∼125 Oe in-plane field that greatly increases the free bubble "saturation" velocity.     

Temperature stable self-biasing bubbles in double layer films

One of the most important problems for a self-biasing bubble (SBB) device is the temperature stability of the SBB diameter. The temperature dependence of the SBB diamater was calculated on the basis of the force stability condition of the SBB and proved to be proportional to the temperature dependence of the characteristic length of the top layer. The proportional coefficient depended on h/l and that was more than -2.7. The smallest value of temperature sensitivity was +0.8%/°C in the system of (YCaEuYb)3GeIG/ (EuEr)3GaIG. No hard bubbles were observed in this self-biasing double layer film and the mobilities of the SBB were similar to those of the usual single layers. And SBB were generated, propagated and detected by means of the usual drive technique (T-Bar permalloy circuits) without any bias field, the operating margins were similar to those of the bubbles in the usual single layer films.     

Permalloy disk replicator for amorphous film 2 µm bubble devices

A permalloy disk replicator in an amorphous film 2 μm bubble device has been Studied for different geometrical dimensions, and from quasi-static to 200 kHz operating frequency. Distinct regions of the margin plot are observed, and their dependence on geometry is analyzed. It is found that for reliable replication over a wide margin range, the poles from the replicator disk, from the neighboring propagation channel, and from the previously replicated bubbles must be carefully balanced. A replicator with 20% margins on 2-micron GdCoMo films is shown and methods for further improvements are discussed.