Lighting up magnetic memories.

It takes a light touch to jam about 20,000 pages of typewritten notes, translated into strings of digits, onto a spinnings disk only 3.5 inches in diameter. To change the information or to write over it takes even more care. The trick is to use a tiny laser that writes or reads information on the surface of a disk coated with a special terbium, cobalt and iron alloy. Such an erasable, optical-disk memory may be available for personal computers within two years, according to several companies that recently demonstrated prototype models.

In one type of erasable optical disk, developed by Verbatim Corp. of Sunnyvale, Calif., the information appears as microscopic dots in which the magnetic fields point up or down, perpendicular to th surface. To store data, an infrared laser heats a spot on the surface, lowering the metal film's resistance to changes in its magnetic field. A small external field can them flip the direction of the spot's magnetism withou affecting the rest of the surface. When the spots cools to room temperature, the new magnetic state is frozen in.

The stored bits are read by detecting differences in the way low-intensity, polarized laser light passes through the thin metal film. Spots with an "up" magnetization rotate the light's plane of polarization in one direction, while "down" spots rotate it in the opposite direction. After the transmitted light passes through a polarization analyzer, a photodetector sees a continuously changing light pattern, which can be translated into digital bits. Several Japanese and European companies and 3M in St. Paul. Minn., are working on similar erasable laser disks.

Another approach which doesn't involve magnetism at all, depends on a special, tellurium-based coating that readily changes from a highly reflective, crystalline form to a dull, amorphous state when a laser beam strikes it. Thus, information is stored as a sequence of dull and bright spots. A higher-power laser reverses the change. Last year, Japan's Matsushita demonstrated this technology, by whether the present films can survive millions of such crystal-form reversals isn't clear.

In general, the main advatange of reversible optical recording is that the laser used to read and write can be as much as a millimeter away from the disk's surface. To pack the same amount of information onto a conventional magnetic disk would require a recording head that skins the disk's surface at a height equivalent to an aircraft flying at full speed just inches off the ground. On a scale in which dust particles are like mountains, fewer, "crashes" are likely to occur with a high-flying laser drive.