Control function margin degradation in a bubble memory chip

Chip control function and propagation circuit margin degradation due to long-term memory operation, was observed, using the bias field switching technique. 16 kbit major-minor loop organized bubble memory chips with 28 μm bit period, which had an average access time of 2.7 ms for a 100-kHz rotating field, were used. It was seen that degradations in the lower side of the bias field range were independent of chip functional elements. However, at the upper side of the bias field range, degradations in the performance can be classified by dividing the elements into two categories. These were propagation circuits (Permalloy patterns only) such as H-bars, chevrons, etc., and control functions (Permalloy and conductor patterns), such as generators, replicators, etc. Also, it was found that the degradation in the performance of propagation circuits is small compared with that of the control functions. These differences were considered to be caused by a failure in the Permalloy steps over conductors and/or by the magnetic interaction of the bubble and the conductor current.     

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.     

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.     

Design and characterization of ion-implanted tracks for 16 Mbit/cm2storage density bubble memory devices

The ion-implanted propagation tracks with contiguous disk patterns (CD tracks) have been confirmed to be better for high density propagation tracks (≥ 16 Mbit/cm²) than those with snake patterns (snake tracks), because of less interactions between bubbles on the other side in the same track. The CD tracks with 1.8 µm × 2.0 µm cell size for 0.5 µm bubbles have been evaluated. The large operating bias field margin of 12.4 percent is obtained at the quasi-static operation with rotating field HRof 60 Oe. The minimum rotating field is 40 Oe. Interdigital folded minor loops are proposed and operated. The proposed minor loops are composed of straight propagation tracks connected alternately to relax the in-side turns. The overall operating margin of 8.4 percent (46 Oe) is obtained at HR= 60 Oe. The feasibility of 16 Mbit/cm²storage density bubble memory devices is confirmed.     

Characterization of magnetic bubble generators

Two types of magnetic bubble generators suitable for a field-access bubble memory have been tested at 1.00 kHz bit rate at in-plane rotating fields above 15 Oe. The bias field margins of the generators at 30 Oe rotating field are equal to or greater than those of loop propagation. Both designs are based on the principle of stretching and cutting seed bubbles circulating around a Permalloy disk. Functions of stretching, cutting and transferring in the generator sequence are accomplished either with Permalloy elements or pulsed current conductors. The operating conditions of the generators in terms of current pulse amplitudes, widths and phase angles are presented. Among the two designs, namely Permalloy-stretch and conductor-stretch generators, the latter has a wider phase-angle margin.     

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.     

Gap-tolerant half-disk bubble device margins

A simple model is presented which allows accurate prediction of bias margins of gap-tolerant half-disk propagation tracks for bubble domains. After this is verified by comparison with experimental margin data, an "isomargin" plot is derived to show how the margin varies as a function ofWandG, whereWis the minimum linewidth andGis the inter-bar gap. The bias margin is shown to decrease along a fairly straight line which goes to zero whenW + Gequals the runout diameter, i.e., whenW+G approx 1.5 W_{s}, where Wsis the bubble stripwidth or average bubble diameter. This agrees with experiment, and means that the minimum resolvable feature for half-disk type patterns must be less than0.75W_{s}, and probably will not be much larger than0.5W_{s}to0.6W_{s}. It is concluded that, if made with perfect Permalloy, T-bars and half-disks should propagate isolated bubbles equally well. The advantages of half-disks over T-bars are 1) the fatal bar-crossing problem of T-bars with multiple bubbles is avoided, 2) the minimum propagation field is lower than for T-bars, and 3) half-disks seem more tolerant of "bad" (e.g., high-coercivity) Permalloy. Also tabulated are the effects on margins of variations in the device parameters of a representative design, as might be encountered in a fabrication process with finite tolerances. A brief discussion of stop-start margins is given in conclusion.     

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.     

Operation of a 100-bit, 2.6 µm bubble shift register

Complete circuit operation for a small 100-bit serial shift register memory with a 10.6 μm period fabricated on a garnet wafer using electron beam lithography and single level all-permalloy technology is reported. Overall circuit operation including generation, propagation, and detection was achieved with a 6 Oe bias margin using a 1 kHz rotating field. Although the circuit design and fabrication techniques were not optimized, we believe that this is the first published report of complete circuit operation for a single level device with bubbles as small as 2.6 μm. The performance of two different types of chevron expander detectors and two different generators was evaluated and circuits combining disk generators with herringbone detectors were found to provide the best overall operating margin. At low speeds (∼5 Hz) high-bias failures were caused by failure of the domains to strip out fully in the detector while the low bias limit was determined by the introduction of spurious bubbles into the track near the generator.     

Bubble lattice formation in thick MnBi platelet by one-point nucleation

The dynamic domain formations in a single crystal MnBi platelet were examined under various external field strengths. Once the reverse domain is nucleated at one point of the saturated plate by pricking or external pulse field, the domain expands with various modes depending on the net field strength acting on the moving wall and forms various patterns. In a thick platelet, a bubble lattice is produced with no bias field, but it is suppressed by the external field; a radial stripe domain pattern or a spider web like pattern is formed depending on the field polarity. The phase velocity of the wall propagation during lattice formation was estimated as about 50m/sec by the pulse field method. This result brought us the important information about the truly complicated nature of the bubble lattice formation, i.e., the sequential process of the dynamic conversion, breakdown of velocity, static conversion, high speed propagation, and sharp deflection.     

External bit field analyses

Three-dimensional external bit fields emanating from single and multiple magnetic transitions with a finite track width have been calculated. The magnetic field interferences between tracks have been also simulated. The results show that the fields have a large component in the track width direction near the transition edges. This contributes to field extension in the neighboring track direction. The amount of extension decreases with bit density increase. The results of our calculations are backed up by Lorentz microscopy observation of films in which bit fields are transcribed.     

Operation of 3µm bubble serial loop devices designed on single level masking

An all-permalloy single mask level design for a serial loop circuit has been developed and successfully operated using 3 μm bubbles. The design of individual elements for a single level circuit such as generator, replicator and annihilator which were compatible with half-disk propagation patterns was successfully investigated and optimization of permalloy and SiO2spacer film thickness was achieved. The small capacity test chips fabricated provided 13-16 Oe "window margins" with circular drive field ranges of 40 to 50 Oe at 100 kHz. The critical stretching pulse phase margin for replication, which was the minimum phase margin for all functions was found to be 10 degrees. Details of design and characteristics are also discussed including operating margin dependence on frequency and temperature.     

Gap tolerant bubble propagation circuit

A new bubble propagation pattern for field access devices has been developed which has a period to gap ratio of 8:1. In this pattern of semicircular elements (half-disk) the gaps are situated between essentially parallel poles in contrast to the TI pattern where gaps are located between orthogonal poles. The bubble, therefore, comes under the influence of two strong parallel poles causing it to stretch across the gap. The energy barrier that would normally be encountered thus virtually disppears. Devices of 32, 18, and 10 μm periods have been designed and fabricated. The results show a typical margin of at least 20% of the bias field under normal operating conditions. The operating drive field is relatively low for small bubbles. The minimum drive field for a 10 μm period pattern is only 13 Oe.     

Graphical simulation of superluminal acoustic localized wave pulses

This paper investigates, graphically, a simulation of the launching of recently obtained superluminal localized wave solutions of the homogeneous wave equation. These solutions represent focused interference patterns that travel with speed greater than c in the direction of propagation. Graphical simulation indicates that approximations to these solutions may be launched from a finite array of point sources, and the superluminal nature of the launched pulses is maintained in the near field of the array     

High perpendicular hard magnetic properties of nanocomposite Co-rich Co-Pt/Pt double-layered films by epitaxial deposition without capped layer

The HRTEM cross-sectional lattice image shows that a well epitaxial growth of hcp Co-rich Co-Pt (002) on Pt (111) underlayer leads to good perpendicular magnetic anisotropy of Co-rich Co-Pt film. It is found that both the perpendicular coercivity (Hc_⊥) and perpendicular squareness (S_⊥) of Co-rich Co-Pt films without Pt capped layer are larger than that of Co-rich Co-Pt films with Pt capped layer. The cross-sectional TEM-EDS and AES analysis confirm that the oxygen atoms will diffuse from film surface into the Co-rich Co-Pt film without adding Pt capped layer, and it react with cobalt atoms to form CoO, which is detected by XPS analysis. The increase in perpendicular hard magnetic properties of Co-rich Co-Pt film without Pt capped layer is mainly due to form CoO in the Co-rich Co-Pt film.     

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.     

Photoelastic investigation of dynamic load transfer in granular media

Summary Dynamic photoelasticity in conjunction with high-speed photography is utilized to study impact wave propagation and dynamic load transfer in granular soil. Series of sequentially recorded isochromatic fringe patterns provide full field information of the dynamic event. Experimental results show that the wave propagation process is governed by a contact wave which increases the density of contacts in propagation direction. The study of dynamic load transfer along specific representative load chains provides basic information about wave velocities, contact duration, and directional stability necessary for a rigorous analysis of the general complex dynamic wave-soil interaction problem.With 7 Figures     

A practical magnetic bubble AND-OR gate

The design and operation of a magnetic bubble AND-OR gate are reported. Operation at 100 kHz in a 25 Oe rotating field with 28.2 μm circuit periodicity was achieved with about 50 percent of the free bubble bias field margins. A transfer pulse is used to divert bubbles from a propagation path which delivers the AND output tO one which delivers the OR output. The transfer is defeated by the presence of a bubble in the appropriate cycle of the OR path. The AND bubble is then delayed by one cycle instead of being transferred. This frastrated transfer strategy was devised to circumvent the restrictive bias field limitations in the operation of previous logic circuits.     

Carrier transport mechanisms and photovoltaic characteristics of Au/toluidine blue/n-Si/Al heterojunction solar cell

Toluidine blue (TB)/n-silicon heterojunction solar cell was fabricated by depositing TB film on n-silicon wafer using thermal deposition technique. X-ray diffraction patterns of the TB film show presence of crystals with size 30 nm dispersed in amorphous matrix. The current–voltage–temperature performance of Au/TB/n-Si/Al device was studied in dark and under illumination conditions. The device showed diode behavior. The diode parameters such as ideality factor, barrier height, series and shunt resistance were determined using a conventional I–V–T characteristics. The analysis of the diode characteristics in forward bias direction confirmed that the transport mechanisms of the Au/TB/n-Si/Al solar cell at applied potential?<?0.1 V is thermionic emission and at high electric field?>?0.1 V is Ohmic conduction. The operating conduction mechanisms in reverse bias direction are Pool–Frenkel effect followed by Schootky field lowering mechanism. The small value of activation energy in reverse bias direction indicates that the conduction process is expected to be by tunneling of electrons between nearest-neighbor sites and it is temperature independent. The photo conduction characteristics of the diode suggests its application as a solar cell.     

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.