A magnetic bubble passive time-slot interchanging gate

The design and operation of a magnetic bubble logic gate, able to perform the basic retardation operations for a magnetic bubble PCM time-slot interchanger,are reported. With this design no external current pulses are needed to perform those functions. With a 32 µ circuit periodicity and using (SmY)3(GaFe)5O12, bias field marginsashigh as 11.5% for the passive logic function are reported in a 25 Oe rotating field. Because no precautions are taken againsthard bubbles the unsuspicious range of frequency is limited to 20 kHz. Nearly no difference is seen in operating margins between low andhigh frequencies. Design rules are given that canlead to other bubble-to-bubble logic circuits with high operating margins.     

A true swap gate for magnetic bubble memory chips

The design of a true swap gate suitable for incorporation into magnetic bubble memory chips with 16-μm to 18-μm circuit periods is reported. The swap operation is true in that the outcoming bubble takes the position vacated by the ingoing bubble, as well as vice versa. Swap gates of this design have been operated successfully at temperatures from 0°C to 70°C, and frequencies up to 100 kHz.     

A practical magnetic bearing

Over the past several years magnetic suspension has been emerging from the laboratory and a number of successful engineering prototypes have been constructed both here and abroad. Advances in magnetic materials, primarily Samarium Cobalt, and in electronics across the board for sensing and control, have contributed to the realization of completely non-contacting rotating systems employing magnetic bearings to eliminate the last remaining wearout-prone element. Current efforts are being directed more toward reducing the size, weight, and complexity of these devices to achieve acceptance in flight systems than to demonstrate feasibility. This paper discusses an 8 cm diameter by 3.75 cm bearing which can support more than ten times its own weight and uses only 1% of its payload as part of the magnetic suspension. This design is servoed in two axis and is inherently stable in three other degrees of freedom with full rotational freedom in the desired axis. The unit has capacitive radial position sensors integrally built into the magnetic gap to further simplify its use.     

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.     

Static characterization of magnetic bubble films

A.C. susceptibility and optical interference are combined to determine the material length, saturation magnetization and thickness of garnet bubble films on GGG substrates. These measurements have been automated to provide precisions of better than 1% and throughputs of more than sixty wafers per hour. Derived values of material length and saturation magnetization agree to better than ±3% with those from visual measurements of stripe width and bubble collapse field for a single film composition. Disagreement between results from two film compositions possibly points up a fundamental difference between the measurement techniques.     

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.     

Optical operation of a magnetic bubble

A magnetic bubble domain is generated on liquid-phase epitaxial (LPE) garnet film by focusing laser light. It is possible to write with only a few milliwatts light power on a thin film with a low anisotropy field. Because the Bi-containing LPE films used have a high Faraday rotation angle, a written bit or bubble can be read out with a high contrast ratio. Because of its low coercivity, the bubble moves along the temperature gradient to the region heated by the light beam and is then stabilized at a corner of a square formed on the film by etching or ion-implantation. Erasure is possible selectively or all together by increasing the bias field. These optical operations of a magnetic bubble are discussed in connection with the material parameters.     

Request queueing for magnetic bubble memories

The improvements in request throughput that result from the use of the shortest seek time first (SSTF) request, scheduling algorithm for major/minor loop organized magnetic bubble memories are considered. For the satisfaction of read requests the bubble memory can be considered as equivalent to a file drum with fixed block size. Bubble memories are considered with 64 kbits per chip running at a 5.6 μs stepping rate and serving 750 to 830 read requests pet second (as opposed to ≤ 419 requests per second without queueing) with both uniform and Poisson arrival rates. A priority interrupt algorithm is implemented that assures that all requests are served in ≤ 60ms, while average service times are 10 to 20 ms. Results of simulation runs, corresponding to the various cases of interest, are presented. It is concluded that request queueing with the appropriate scheduling algorithm is a practical way of improving bubble memory 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.     

A method of approximating bubble domain runout diameter and bias magnetic field

Approximation formulae were derived to manually calculate bubble domain runout diameter d2and bias magnetic field H2. A comparison of d2and H2calculated by these formulae with Thiele's results shows that the approximation formulae are accurate to within 3 percent.     

Horizontal Bloch line motion in magnetic bubble materials

Horizontal Bloch line (HBL) motion in magnetic bubble materials is studied by comparing experimental results with a numerical simulation of domain wall motion. The present theory of HBL motion is reviewed and then extended by allowing both the wall position q and azimuthal angle φ to vary with position through the film thickness z. The resulting equations of motion are solved numerically for q(z, t) and φ(z,t), and the solutions are compared with results from wall oscillation experiments. In these experiments, a bias field pulse is used to produce a step change in the equilibrium position of stripe domain walls in the presence of small in-plane fields parallel to the walls, Hx= 0 Oe, 4 Oe, and 10 Oe. The wall response is then measured by using a sampling photometric technique. Good quantitative agreement is found between numerical and experimental results, so that the internal processes that occur during wall motion can be inferred from the calculation. Changes in q through the film thickness play an important role in HBL motion. During the initial response, the twist structure φ(z) is similar to the initial static structure and the wall surface is relatively flat. A backward bulge soon forms near one of the film surfaces as a result of local dynamic properties. This bulge provides the additional torque necessary to form the HBL. The bulge accompanies the HBL as it moves toward the opposite film surface and provides the torque necessary for HBL propagation. Internal vibrations of the wall surface are excited by the bulge and produce irregular motion of the average wall position. Punch-through occurs when the HBL reaches the opposite surface. During this process, φ at the film surface rotates rapidly from π/2 to 3π/2. At the same time the wall section in this region moves backward. As a result, the average wall position is essentially stationary during punch-through.     

Characteristics of YLaTm-garnet films for magnetic bubble applications

Films of Y_1.6La_0.3Tm_1.1(FeGa)₅O₁₂ have been grown by liquidphase epitaxy on Gd₃Ga₅O₁₂ substrates. This is a new film composition whose high mobility (～1000 cm/sec/Oe) and low temperature coefficient of bubble diameter (～0.1%/°C) make it attractive for bubble domain devices. Films supporting 3 to 8μm-diameter bubbles have been prepared with typical defect densities less than 5/cm². Data are presented to show that this material can be grown reproducibly despite a rather large La segregation coefficient. Measurements of the uniaxial anisotropy as a function of lattice mismatch are presented along with the results of ion implantation studies which show that a dosage of 3 × 10¹³ cm^?2 of Ne is sufficient to suppress hard bubbles in this material. Also, bubble velocity measurements are discussed in detail as they reveal a number of interesting effects. These include the existence of a perpendicular velocity component which can be virtually eliminated by applying an in-plane field and the absence of extreme dynamic conversion effects in high gradient fields.     

The simultaneous multiple dipping of magnetic bubble garnets

The simultaneous growth of LPE films on two-inch diameter Gd3Ga5O12substrates with nominal film compositions in the (YLuSmCa)3(FeGe)5O12system is studied. The effective distribution coefficients of Ca²⁺-Ge⁴⁺, Y³⁺, Lu³⁺and Sm³⁺are discussed and related to the magnetic property variations observed as a function of film growth rate. The approaches for material property control are evaluated and examples given of expected property variation within a stack of epi wafers grown under constant thickness or constant growth rate conditions. A design is shown for a 18 at-a-time and a 30 at-a-time growth holder. Data is presented to indicate the radial thickness uniformity, thickness, saturation magnetization, collapse field and anisotropy field variation typically observed in multiple substrate film growth. A technique for minimizing flux caused mesa formation is evaluated. with comparative data showing the effectiveness of this technique.     

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.     

Analysis of rotating fields in magnetic bubble drive coils

Closed-form normalized expressions for the field components inside a single-layer rectangular solenoid are derived from a model in which the solenoid is approximated by finite length current sheets of infinitesimal thickness. The equations are extended by superposition to include the case of a multi-layered solenoid, and the effects of nearby magnetic materials are included by employing the method of images. Computer generated field plots compare favorably with measured data.     

Calcium germanium substituted iron garnet films for magnetic bubble applications

Mixed rare earth iron garnet films containing calcium and germanium have been grown on (111) oriented gadolinium gallium garnet (GGG) substrates. The effect of growth conditions on film composition and properties is discussed. The 5 μm diameter bubble films have good magnetic properties; however, temperature control during film growth is very critical.     

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.     

Characterization of magnetic materials and circuits for bubble mass memories

Epitaxial garnet films containing many different mixtures of the rare earths have been prepared for materials and device characterization. In addition, several different types of circuits for propagating and manipulating bubble domains have been proposed and demonstrated. This paper reviews some of the recent progress in characterizing these circuits and materials with respect to mass memory applications. The approach under consideration utilizes a rotating magnetic field for bubble propagation and pulsed-current conductors for certain control functions, such as bubble generation, bubble annihilation, and bubble transfer from one propagation loop to another. The detection function is done with magnetoresistive elements. Operation at frequencies of at least 100 kHz is desired and has been obtained separately for each of the required functions, using particular combinations of circuits and materials. More detailed characterizations of the materials, the circuits, and their combinations are presently in progress in order to achieve all required functions simultaneously with a single material-circuit combination.     

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.     

New method for routine measurement of coercivity in magnetic bubble films

We describe a new technique for measuring coercivity in magnetic bubble films which consists of placing the film in a weak field gradient (∼1 Oe./μm) in order to obtain a set of finger-like domains. The unconstrained ends of these domains are caused to move back and forth in response to an oscillatory field, and the coercivity is obtained from an extrapolation of the linear portion of the response vs. drive field curve. We present a comparison between coercivity values in materials with 3μm and 1.7μm stripe-widths obtained using the new technique and bubble translation. Good correlation is observed for both types of material, the values obtained with the new technique being somewhat higher than the bubble translation values. The difference is ascribed to material non-uniformities.