But GMR is only one species of magnetoelectronic device — many others are now in development and could have a similar impact on non-volatile memory chip technology.

Attendees at the upcoming International Electron Devices Meeting (IEDM), to be held Dec. 5-8, 1999, in Washington, can get up to speed on the new approach at a presentation by Jo De Boeck and Gustaaf Borghs, researchers at the Interuniversities Microelectronics Center (IMEC) here. The pair will describe their own work in building exotic magnetoelectronic devices and will place the work in context via a survey of current worldwide research.

Magnetoelectronics manipulates electrons in semiconductors via electron spin, rather than charge. Spin, like charge, is an inherent physical property of electrons which responds strongly to magnetic fields. As with GMR, the response can be quite strong, leading to technologically useful effects.

A GMR material is a series of alternating layers, one strongly ferromagnetic and the other magnetically neutral. When the magnetic moments in the ferromagnetic layers are aligned, electronics can pass the layered system unimpeded. But applying an external magnetic field can cause successive magnetic layers to have different magnetic alignments, and the electrons are then strongly scattered, creating a high resistance in the system. The external field can come from the tiny magnetic domains on a disk, and the strong variation in electrical resistance is used to detect the magnetic orientation of the domains.

GMR materials can be fabricated using standard semiconductor manufacturing techniques. The next big application area will be in automotive and electric motor markets, according to De Boeck and Borghs. More important for the future of electronics is the possibility of integrating GMR materials into standard CMOS processes. For example, Honeywell researchers demonstrated a CMOS non-volatile RAM chip using integrated GMR materials in 1997.

IMEC researchers are investigating gallium-arsenide-based "diluted magnetic semiconductor" materials that produce effects such as hot-electron spin and spin injection across an interface. The latter property is leading to transistors with magnetically activated gates. The new class of devices being built with the materials is collectively referred to as "spin-transport ferromagnetic/semiconductor devices." The materials might prove to be more physically flexible than GMR lattices and could also be scaled along with transistor structures. Their magnetic properties could be manipulated by voltage, current and light to produce novel magnetoelectronic systems.

Besides spin-transport and GMR, a third area of research is "stray-field ferromagnetic/semiconductor materials." In these devices, a tiny switchable magnet produces a field that is used to modulate electric currents in the device, which is inherently non-volatile. That's because magnetic systems experience a nonreversible dissipation of energy — hysteresis — that makes them bistable.

Electron spin is roughly analogous to the macroscopic magnetic field produced by a circulating electric current. However, it is quantum mechanical in nature. Magnetoelectronic systems depend on materials in which many electrons are in one of their two spin states — spin up and spin down — or the other. The ideal material would have electrons in only one state, but practically, only a portion of a material's electrons are grouped in a single state. For example, ferromagnetic materials have a polarization of about 50 percent.