Writing on magnetic walls.

Magnetic bubbles are a technological suggestion that has "come and gone and come again," to use the words of one observer. Magnetic bubbles are regions of a magnetic substance in which the atoms are all magnetized in the same direction. About 15 yeas ago there was a certain enthusiasm about using magnetic bubbles as the basis for computer memories: Information could be recorded by manipulating the directions of the bubbles' magnetization. But before it really took off, bubble technology was surpassed by other techniques. Now bubbles--or rather the walls between them--are back as a suggested technology for memories that are denser than any now used and quicker to search for some wanted piece of information.

In a talk at last week's International Conference on Magnetism '85, held in San Francisco, Floyd B. Humphrey of Carnegie-Mellon University in Pittsburgh proposes using characteristics of the walls between bubbles as memory units. Between two bubbles that are magnetized in opposite directions there has to be a wall, a narrow stretch, in which the direction of magnetization gradually turns over. These are called Bloch walls or Neel walls, depending on the plane in which the rotation of magnetic direction takes place. For example, in a very thin film of iron garnet or gadolinium iron garnet doped with some rare earth element, two adjacent bubbles will have their magnetizations in opposite directions in the plane of the film. If there is a Bloch wall between them, the direction of magnetization in the wall rises up out of the plane of the film, becomes vertical, then gradually descends until it is back in the plane of the film but 180 [deg.] from the direction it originally pointed.

Bloch walls can be left-handed or right-handed depending on how the magnetic direction twists. Manipulating the walls the external magnetic fields can produce complicated twists. Among them are tiny regions in which the magnetism points vertically. These are known as vertical Bloch lines, or VBLs. Taken singly, VBLs are unstable, but if they are made in pairs of the same handedness -- which means that the magnetic direction in the wall twists through 360 [deg.] between them -- such pairs are stable. "These pairs have been proposed as the most dense computer memory yet," Humphrey says.

To make such a memory the magnetic bubbles are stretched from the more or less circular configuration they ordinary have into very elongated strips. The writing device uses electromagnetics to snip off the end of a strip, making a tiny bubble with a pair of VBLs in its wall. Because the information is coded in the VBLs, not in the bubbles themselves, the bubbles can be very tiny and the VBL pairs very close together. An array of such snippers, each snipping sequentially, piece after piece from long magnetic bubble strips, writes the memory. The device has been experimentally tried, and it works, Humphrey says. For example, with a pair width of 0.2 microns and a strip 1 centimeter long one could write 50,000 bits per strip. According to the experimenters, the limit of information density seems to be about 10 billion bits per square centimeter.

The little bubbles that are made can be arranged, bubble in, buble out, in logical patterns, to perform the logical operations of a computer, such as OR and AND.

In addition, this kind of memory is easily addressable and quick to read out. Time to search the memory for information is a serious limitation on all kinds of computers. For instance, Humphrey says, when you stick your card into a sidewalk teller machine, it will say, "Please wait, your request is being processed." It's not that hundreds of other people are trying to use the system at the same time--at midnight you may be the only one using it -- but that it needs the time to search its memory for your file. A VBL memory should yield in about two-thousandths of a second the information it now takes 10 seconds to find.