An analytical design theory for field-access bubble domain devices

This paper describes the derivation and applications of an analytical model for magnetic bubble domain propagation. A fleld-access bubble device is treated as a magnetic circuit, emphasizing fluxes rather than fields as in most previous treatments. The energy changes are calculated within the bubble domain rather than in the propagation pattern, making possible a simple analytical-graphical process for predicting device operating margins. Agreement with a large number of device experiments is shown to be good. Closed-form expressions are derived which relate the essential features of the operating margin to device geometry and bubble material magnetic properties. The application of this model to device diagnosis, optimization, and scale down is discussed.     

The preparation of large area films for bubble domain devices

The growth of LPE films 3.8 cm in diameter has been achieved under conditions which lead to a high yield of defect free material suitable for bubble domain circuits. Temperature control to ±0.1°C, larger melt volumes, more precise timing of runs and an increase in saturation temperature of the melt to 970°C all contribute to the improved quality. The change to 970°C also sharply reduces platinum attack. Films having a collapse field controlled to ±1 Oe can be grown in yields exceeding 35%.     

Applications of bubble devices

The applications of magnetic bubbles that are currently seen as most attractive and the current status of development of such devices are considered. A major application is thought to occur in solid-state mass memories. An attractive form of organization is possible whereby relatively short access times can be achieved while using only a small number of read and write circuits. A repertory dialer memory, which has been chosen as a test vehicle, has been developed. The design and performance of the propagating circuits, the self-latching magnetic gates and the generator, which comprise the memory, are discussed. A novel area of application may be in pulse code modulation (PCM) switching networks. It is shown how a network operating as a PCM crossbar switch can be designed using bubbles. A circuit is described which can eliminate the delay associated with bubble propagation in such networks.     

Eight-micron period bubble devices

Magnetic bubble shift register devices of 68 121- and 266 473-bit capacity have been fabricated and tested. The epitaxial garnet bubble films were nominally 1.7 μm thick, supported nominally 1.7-μm diameter bubbles, and had collapse fields of about 260 Oe. The storage area per bit was 64 μm², which was realized with a minimum coded feature dimension of 1 μm and contact photolithography using EBES chrome masters. Initial yields obtained in two experimental batches each of the two chip capacities are discussed. Parametric test results are presented for generator current, transfer current and phase, and rotating field intensity. Nominal values have been established to be 130-mA generate current, 21-mA transfer current, and 60-Oe drive. The detector signals were about half as large as normally obtained from 3.3-μm bubble devices with comparable resistance and conventional design.     

Planar processing for magnetic bubble devices

A two-mask level, conductor first thin film process is described for fabrication of magnetic bubble devices. The process permits a stepless permalloy level over a conductor that may be two to three times as thick as conventional processing The planar process is attained by anodizing a thick aluminum alloy film in all regions where no conductor is needed. The process described solves problems in conventional processing caused by thin conductor metalization and permalloy step coverage. Replication, nucleation, and annihilation devices made with this process promise superior performance.     

Logic functions for magnetic bubble devices

An analysis has been made of the number and types of logical functions which can be performed using the interaction of circular magnetic domains in rare earth iron oxides. Multiple logic functions are found to be produced simultaneously at any logical area. These conjunctive output sets have been categorized. Several conjunctive logic gates have been designed, fabricated, and tested successfully in Sm0.55Tb0.45FeO3using Permalloy overlays. Utilizing a circuit in which AND/OR logic gates are coupled to a dynamic memory bank, the total correlation of two data streams has been performed. It is shown how this multiply accessed dynamic memory serves to establish the correlation threshold.     

Block oriented bubble domain memory organization

A block oriented bubble domain memory organization is proposed which provides a block access time improvement over existing major/minor loop designs and which exhibits built-in redundancy. Wafer level integration is utilized to reduce the module lead count.     

Bias magnet design for bubble memory devices

A study has been made of a system consisting of a pair of magnets with thin plates of permeable material on their inner faces. The effect of the plates on the profile of the field between the magnets is predicted as a function of the geometry and the permeability and the way in which the uniformity can be improved is demonstrated.The effect of the permeability is very limited.     

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.     

Optimum design considerations for dual conductor bubble devices

A design for dual conductor, current-access bubble devices with 8-μm periods has been optimized with a numerical calculation method for bubble motion in a propagating magnetic field, generated around hole patterns in conductor layers. Magnetic bias field distributions are calculated for an oval hole chain in the conductor layers. Bubble motion equations are obtained with analytical field distribution functions approximating the calculated field distributions. Minimum drive current density Jminfor normal bubble propagation is determined by a solution to the equations. The hole shape has been optimized by the minimization of the drive power Pmin, the product of Jminand conductor resistance, which is calculated from current distributions around the hole pattern. Optimum layer thickness have also been obtained for 8-μm period bubble devices. Both registration tolerance between the two conductor layers and bubble skew effects have been studied semiquantitatively on the basis of the equations of motion. The numerical calculation method developed here is found to be a highly effective means to optimize pattern design for smaller period devices.     

Electron-beam fabrication of high-density amorphous bubble film devices

A low-temperature, all-vacuum process combined with electron-beam lithography suitable for single-level masking devices using 2-μm diameter amorphous bubble films has been developed. A test vehicle which uses 0.75-μm wide chevrons and 1-μm wide T.I bars in an 8,000- bit chip configuration, resulting in an areal density of 1×10⁷bits/in², was used. Important process features are found to be: (1) laminated NiFe films to obtain low Hcand high magneto-resistive effect when deposited at low substrate temperature, (2) maintenance of low surface temperature during metallization to preserve the integrity of exposed and developed electron-beam resist pattern, and (3) proper resist profile for ease of the lift-off process. Excellent bubble device operating characteristics have been obtained as a result of uniformity in materials and structure resulting from careful control of fabrication parameters.     

Ion-implanted and permalloy hybrid magnetic bubble memory devices

Hybrid bubble memory devices have been proposed and operated with the memory density of 4 Mbit/cm². In the hybrid bubble memory devices, minor loops are composed of ion-implanted tracks with 4-µm period, and major lines and functional parts including block-replicate and swap gates are composed of Permalloy tracks with a longer period of 12 µm. Passive junctions between ion-implanted and Permalloy tracks have been developed, introducing the tapered ion-implantation technique. Improving the characteristics of the functional parts composed of Permalloy tracks, the hybrid bubble memory devices with block-replicate and swap gates have been operated, and the feasibility of the devices has been confirmed. In addition, the possibility of higher memory density has been shown.     

Optoelectrical study of bubble propagation in field access devices

Bubble propagation on several field access structures is studied with an optoelectrical method. Measurements were performed with a 40-Oe, 10-kHz rotating field on closed-loop shift registers that were fabricated on Bi-substituted garnet films having either 7 μm or 2.7 μm bubbles. The effect of increasing the interelement gap for several nonsymmetrical structures is discussed.     

Theoretical analysis of longevity testing on bubble memory devices

Theoretical results of magnetic bubble device long-term reliability testing are reported. The bubble during propagation along Permalloy tracks is represented by a simple, one-dimensional stochastic model. An equation to describe fluctuation in cylindrical bubble radius is approximated in the Langevin type stochastic differential equation, in which a set of small effects, such as interaction among bubbles and crystal nonuniformity, are considered as a white noise forcing term. Estimating the average time to bubble annihilation or runout (bubble memory mean time to failure) is reduced to a level-crossing problem for a random process. Calculated bias field margin degradation shows a qualitative agreement with experimental results for an actual bubble device. Bubble material parameters for obtaining maximum operation time are suggested.     

NiFeCo--An alternative to permalloy for bubble domain detection?

The characteristics of bubble domain sensors fabricated from ternary NiFeCo films have been studied and compared with Permalloy film sensors. In 350 Å thick films, the ternary alloy exhibits a magnetoresistance ratio of 3.5 percent in contrast to 2.8 percent for Permalloy films prepared under similar conditions. Sensor sensitivity in functional bubble chips is correspondingly greater, while the sensor noise level is equal to-or lower than-that obtained with the Permalloy detector. Low coercivity and dispersion in NiFeCo films aid in producing an overall improvement in signal-to-noise ratio. The performance of NiFeCo sensors operating in 1-μm bubble ion-implanted contiguous-disk devices is described.     

Repulsive interactions between magnetic bubbles: Consequences for bubble devices

Experiments and calculations are reported about some aspects of the repulsive interactions of cylindrical magnetic bubble domains. Experiments were done on hexagonal lattices of bubbles in which, for one case, the stable positions of the bubbles were determined by a Permalloy overlay. The calculations refer to different structures and Show that from a point of view of bubble interactions the maximum bubble density for a square lattice is almost the same as for a hexagonal lattice. It is also shown that a preferred thickness of the bubble material sheet exists.     

Effects of annealing on propagation in ion-implanted contiguous-disk bubble devices

Bubble propagation margins are found to be affected by heat treatment in ion-implanted contiguous-disk devices fabricated on liquid-phase epitaxial (LPE) grown double-layer garnet films which support 1-μm bubbles. When an optical reflector is deposited directly on the driving layer interfacial diffusion takes place, and this raises the coercivity of the implanted layer, which in turn causes a severe degradation of propagation margins. In samples fabricated with optical reflectors isolated from the driving layer, the adverse effects of annealing on propagation margins are stronger in close-packed minor loops due to interaction of long-range charged walls as compared with isolated loops and isolated disks, and they cease to propagate bubbles after annealing in the temperature range of 350-400°C, whereas the loss of margins in the isolated loops and disks are typically less than 30 percent after 600-650°C annealing treatments. Stress gradients caused by the discontinuity in the overlay patterns contribute significantly to the increase of threshold drive field after anealing. The rapid degradation of propagation margins found after annealing in the temperature range of 350-400°C in all samples is caused by reordering of the damaged lattice.     

Galvanomagnetic readout elements for domain tip devices

The use of galvanomagnetic readout techniques for producing output signals from magnetic thin film devices based on domain tip (DOT) propagation has been investigated. Whereas methods of inductively sensing the signal from a propagating DOT are limited to the order of 100 μV per output channel per turn of pickup loop, magnetoresistance can be used to produce output signals of several millivolts from a single output channel. In addition to permitting smaller readout elements to be made, magnetoresistance has the further advantage of providing a continuous rather than transient output signal. Two and four electrode configurations, for both the longitudinal magnetoresistance and the planar Hall effect, have been evaluated. The use of isolation slits etched into the magnetic layer to eliminate leakage currents has been investigated as well. The results indicate that small area readout elements with output signals in the range of several millivolts are practical.     

On the theory of bubble domain motion in propagation circuits

The stability conditions for a bubble domain in the fields of magnetostatic traps are analyzed. For this purpose the quasi-stationary equations of bubble motion are derived which take into account field nonuniformity across the bubble diameter. The contribution of the Permalloy overlay is considered qualitatively. The conditions of static and dynamic stability of trapped bubbles are studied and the stability role in the operating margin development is analyzed.     

NiFe films deposited by sputter gun and applications to bubble devices

The sputter gun system has a unique feature which is that the substrate holder is electrically isolated from and physically outside the cathode and anode gun assembly. As a result the substrate surface temperature can be maintained at a lower level than that for the conventional diode sputtering system. This feature is particularly useful for metallization by the lift-off process. NiFe films with excellent physical and magnetic properties have been obtained. The application to overlay patterns for single-level masking bubble devices is presented.