How bubble memory works

The images and animations on this page are intended to explain how magnetic bubble memory works, one "step" at a time.

Figure 1: Ferromagnetism vs. orthomagnetism

Figure 1

This image demonstrates the difference between "ordinary" ferromagnetic materials (above), and orthomagnetic materials (below): The latter only form magnetic poles (indicated on the samples by red/white color gradient) if the external field provided by the big magnet poles at left and right is aligned with the orthomagnetic axis, as indicated by the little black arrows.

Figure 2: The orthomagnetic sheet in bubble memory

Figure 2

This image shows the orthomagnetic "sheet" used in magnetic bubble memory; it has its orthomagnetic axis perpendicular to the square surface. Even without an external field, it tends to form these sharply divided areas, some with north pole up/south pole down, others with the poles the other way around. The term describing this property is uniaxial magnetic anisotropy.

Figure 3: Squeezing the bubbles down in size

Figure 3

This image shows how one kind of domain in the orthomagnetic sheet of magnetic bubble memory grows and the other shrinks, as an external magnetic field (the big magnet poles) is imposed on them.

Figure 4: Moving the bubbles using external magnetic fields

Figure 4

This image shows how external fields (symbolized by small magnets left and right) working at an angle can "push" and "pull" domains, or "bubbles" in the orthomagnetic sheet of a magnetic bubble element.

Figure 5: Driving coils and guide patterns

Figure 5

This image shows the setup of driver coils and guide pieces around and on the orthomagnetic sheet of a magnetic bubble memory. The coils together form a steadily rotating magnetic field along the surface of the sheet. Since the guides are ferromagnetic, they assume magnetic poles when magnetized by the coils, which in turn "coerces" the domains along the guide pattern.

Figure 6: Different guidance patterns

Figure 6

This image shows two possible patterns for the magnetic guide pieces in magnetic bubble memory.

Figure 7: Bubbles moving in the T-I-style pattern

Figure 7

This animation shows how magnetic domains propagate through a pattern of T- and I-shaped guide pieces.

Figure 8: Bubbles moving in the V-style pattern

Figure 8

This animation shows how magnetic domains propagate through a pattern of V-shaped guide pieces.

Figure 9: Closing the loop around the track

Figure 9

This image shows one "complete loop" of a magnetic bubble memory, consisting of a length of "track" from the orthomagnetic sheet and a minimum of associated drive electronics: At the leftmost end of the track, a pair of coils act as an electromagnet, "launching" new bubbles. These bubbles propagate along the guide pattern track, and wind up at the pair of coils at the opposite end. These coils act as a "pick-up", in which an electrical pulse is formed when a bubble arrives, and this pulse can be read off the output terminal at the lower right-hand corner. The same pulse is amplified and "signal conditioned" in the green amplifier triangle symbole in the middle.

During normal (read) operation, the switch to the left is set to lead the output from the amplifier circuit directly to the "bubble-launching" coils, thus re-creating the same series of bits as bubbles "running in circles".

When new information is to be written into the memory, the switch is set to direct pulses received at the input terminal at the far left, into the "launcher" coils, thus entering a new "string" of bits in the form of bubbles.

Figure 10: Getting fewer but longer tracks

Figure 10

These schematics show two different ways to interconnect the tracks of a magnetic bubble memory.

To the left, each of four tracks forms its own loop, yielding one "magnetic shift register" per track. Since the motion of the "bubbles" is governed by the common signal driving the coils driving the guide patterns, all these registers "move in sync", and thus the setup shown to the left will accept information in "words" of four bits in a parallel fashion.

However, the number of tracks usually outnumbers the required word size, so to exchange the "excess" tracks for more bits per track, two or more tracks are coupled in series, as shown in the schematics at right: The output of one track is amplified and signal-conditioned, then sent into the next track, yielding in this case half the number of "tracks", but with twice the number of bits per track.