T. Nelson

Field nucleation of magnetic bubbles

Nucleation of magnetic bubbles in the fields of a pulsed hairpin conductor is investigated experimentally and theoretically. It might be supposed that the current required for nucleation produces a vertical field in excess of the anisotropy field throughout the bubble film. We have found evidence that the current required is reduced because a) intense fields are required only at a surface of the bubble film, b) the horizontal field contributes in the initial surface reversal, and c) the surface of proton-implanted films is more easily reversed than the rest of the film. Field nucleation has been used to seed bubble generators and to generate bubbles controllably at 100 kHz.

Shear-sensitive magnetoresistive robotic tactile sensor

We have developed a tactile sensor for robotic applications that is sensitive to pressure and, unlike most other tactile sensors, also responds to shear components of the vector force. We consider this an important advantage for robotic manipulation. In our design a magnetoresistive detector senses variation in the magnetic field of a miniature magnetic joystick. This joystick is mechanically mounted by attaching it to a metal foil, which provides elasticity without hysteresis. The detector chip is fabricated using IC technology, and an array of joystick mechanisms can potentially be made by batch processing. We have designed and partially tested a 2 × 2 array prototype, demonstrating the viability of this approach.

Reliable propagation of magnetic bubbles with 8 μm period ion implanted propagation patterns

Ion implanted propagation patterns for magnetic bubble devices provide an approach to higher density through relaxed photolithography. At the same time, the drive field requirements may be reduced, with respect to Permalloy based field access devices. Propagation is studied in the present work at 8 μm period in YSmLuCaGe films. Profiling by relative etch rate and by wafer curvature has led to 80/Ne/1‐2E14+270/Ne/2E14+130/H2/2E16 implantation conditions. Bias field margins vs drive field for 27‐step loops with neighbors on 2 λ centers and in orthogonal orientations on the wafer are presented to demonstrate the feasibility of major‐minor loop organized chips. Crystal symmetry effects do cause the margins for differently oriented loops to be different in the present material, however. The overlap bias range is about 20 Oe extending downward from free bubble collapse. The minimum drive at 40°C appears to be about 35 Oe. Bias margins at 40 Oe drive taken over the temperature range 0–80°C show no anomalies for ...

Evaluation of (YBiCa) 3 (FeSi) 5 O 12 epitaxial films for extended temperature range ion-implanted 4µm-period bubble devices

We report the results of wide temperature range material characterization and propagation studies on ion-implanted and annealed (Y 1.9 Bi 0.5 Ca 0.6 0.6)(Fe 4.4 Si 0.6 )O 12 Effective uniaxial anisotropy field H k decreases by less than a factor of 1.5 in going from -60C to +140 C compared with a factor of 8 for typical Lu-Sm garnets. Coercivity is less than 0.50e at -60C, and decreases monotonically to 0.14 Oe at 140C. Bubble propagation studies with 4μm-period ion-implanted propagation patterns and 40 Oe, 100 kHz drive field show propagation on the best type of tracks from -130 to 160C, with a bias field operating range greater than 30 Oe.

Effects of N-H 2 doses in ion-implanted bubble memory test chips

Recently 0.5Mbit 8μm period bubble memory chips based on ion-implanted propagation patterns were reported. In these devices, made with (YSmLuCa) 3 (FeGe) 5 O 12 1.7μm bubble films, 270 keV neon implantation was done. In order to use a commercial implantation system, limited to 200 kV acceleration potential, we have tried 200 keV nitrogen for the medium depth implant in place of the 270 keV neon. We prepared devices with the 200/N and 130/H 2 doses increased proportionally in four steps over 2.7-4.0(10)14cm-2and 2.0- 3.0(10)16cm-2, respectively. We also used an unpatterned 80 keV neon implant of 1.0(10)14cm-2 as was done previously. This paper reports measurements on these devices of (1) perpendicular strain by x-ray diffraction and easy plane anisotropy by FMR vs annealing temperature and (2) bias ranges for propagation vs annealing temperature, drive field, and operating temperature. Annealing lowered the magnitude of the anisotropy and strain of the implanted layer. At a critical anisotropy field H k -4πM of about -1.5 kOe, major loop propagation failed. Although the margins change with annealing, the highest dosed devices continued to propagate after anneals of at least 375°C. Functionally complete circuits were processed at temperatures below 300°C and tested in wafer form at 50 kHz. With 40 Oe drive, the major loop and minor loops on test chips overlapped in bias by 24 Oe or better at 40°C for all four doses. At the temperature extremes of 0°C and 85°C, the overlap was ≥ 21 Oe.

Chevron detector design study

Results obtained in a design study which has attempted to optimize the detector chevron and interconneetion geometry are reported. The detector waveform with interconnections between chevrons placed at alternating ends of the chevrons is redundant, with the second half cycle largely repetitious of the first as the domain traverses the detector. The present designs attempt to select the first half cycle by placing the second set of interconnections asymmetrically, near the center of the chevron column. The selected signals are then enhanced by narrowing the chevron linewidth on the incoming side where the serpentine current path is defined. Detector waveforms obtained with such designs have a high resistance state, when a bubble is present, which peaks just before a rapid change into a low resistance state such that the maximum excursion can be used with conventional clamp and strobe sense amplifiers. The dependence of the output on linewidth, garnet thickness, and static in-plane offset field are investigated experimentally. Detector figures of merit, i.e., signal voltage divided by dc voltage, of 0.5% have been observed and appear robe obtainable in a practical sense.

Large capacity ion-implanted bubble devices

Half-megabit bubble memory chips, with patterned ion implants used to propagate bubbles, have been designed and characterized. Bubbles with 1.7-μm diameter were used on a 6-μm to 8-μm circuit period and were propagated by rotating fields of 40 Oe at 50 kHz. The chips had 284 2051-bit storage loops, of which 258 were required to operate. Overall bias field margin ranges of 16 Oe to 19 Oe have been demonstrated on functionally complete, nonvolatile chips with nondestructive readout. Most functional parameters have wide margin ranges, but further design improvements are necessary to widen the margin ranges of the drive field and the transfer-out trailing edge phase.

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.

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.

Characterization of 8-μm period half megabit ion-implanted memory chip designs

Design and process improvements have given rise to fully functional 8-μm circuit-period ion-implanted bubble memory chips with a high degree of reproducibility in all functions at nominals and reasonably wide bias field margin ranges. Over 20 Oe bias field margin ranges on small test chips are typical. Half megabit chips of the same design have over 20 Oe bias field margin range with only 6 percent redundant loops. Packaged test chips are characterized over a wide temperature range. Good operation is achieved from -55°C to 100°C. Detailed parametic variation plots are given at -50°C and 90°C case temperatures.