Avalanches up disk storage

The key to making storage media that crams information into smaller spaces is making read devices sensitive enough to read the magnetic orientation of tinier areas of magnetic film. The 1s and 0s of computer information are stored in magnetic media as magnetic orientations.

Researchers from Harvard University and the University of California at Santa Barbara have constructed a spin-valve transistor that is more sensitive to microscopic magnetic fields than the devices that read today's commercial hard drives.

The current traveling through both a giant magnetoresistance read head and a spin-valve transistor varies depending on the relative magnetic orientation of a pair of thin ferromagnetic films. When the magnetic orientations of the two layers of film are parallel, current flows because electrons are able to pass through both films to the transistor's output terminal. When the magnetic orientations are not parallel, the electrons are unable to pass through, reducing the current in the output terminal.

Spin valve transistors use hot, or high-energy electrons, which makes them very sensitive. Hot electrons do not generate a lot of current, however, making for weak signals.

Spin-valve transistors fall short of the performance of today's giant magnetoresistance-type devices because the device has difficulty distinguishing random fluctuations, or noise, from true signals. The researchers boosted the signal-to-noise ratio of a spin-valve system by boosting its output using a method pioneered in solid-state photodetectors. The method involves causing one electron to excite another, each of which can excite another electron, causing an avalanche-type reaction that results in a stronger signal.

The device is 10 to 100 times more sensitive than giant magnetoresistance read heads, according to the researchers.

The avalanche spin-valve transistor could be ready for use in commercial magnetic storage technologies within the next five to ten years, according to the researchers. The work appeared in the November 8, 2004 issue of Applied Physics Letters.