“Spintronics” Hold Potential for Future Electronic Devices
Scientists at the Naval Research Laboratory are researching characteristics of several spin-dependent phenomena in the hope of eventually exploiting such effects to develop new types of devices and circuits for very high-performance electronics and sensors ("spintronics"). The technology project is sponsored by the U.S. Defense Advanced Research Projects Agency (DARPA). One near-term goal is the development of a totally nonvolatile, very high-density, high-speed, low-power, low-cost memory device that has the potential for significantly exceeding the performance of traditional semiconductor-based memories, and may represent one of the major technologies for the 21st century.
"Ultimately spintronics may provide the technology for much more than memories and sensors and may become as ubiquitous as semiconductor electronics in the next century," said NRL's Stuart Wolf, who spoke at the APS March Meeting in Los Angeles. Wolf described two such effects: giant magnetoresistance (GMR), in which the electrical resistance that electrons experience in a multilayered material can be substantially changed by the magnetic field within the material, and spin-dependent tunneling, in which an electron can move through a normally impenetrable barrier if it has the right spin value.
The movement of an electron in a circuit can also be manipulated by properties other than its charge. One example is spin, which according to Wolf, the simplest device using spin-dependent effects is a sandwich with two magnetic layers surrounding a nonmagnetic metal or insulator. If the two magnetic layers are different, then the magnetization direction of one can be rotated with respect to the other and this leads to the utilization of these structures as sensor elements (for example for read head sensors) and for memory elements that may ultimately rival the semiconductor memories that now dominate commercial computers. Recently, IBM introduced GMR sensors into the read head of the newest generation of its hard disk products.
IBM, Motorola and Honeywell all have research teams attempting to develop a robust technology to use these devices to produce very high density memory products - comparable to dynamic random access memory - at speeds approaching those obtainable in static random access memory (SRAM), with fewer masking steps and lower power. Honeywell has already demonstrated a fully functional 16-kbit GMR memory, built with underlying electronics that can withstand the high radiation environment of satellites required, and Wolf hopes the technology will be in place for Gbit memories early in the 21st century. IBM estimates that their devices will scale to densities greater than 50 Gbit.
Wolf also described spin injection devices, in which the spin of an electron can modify the properties of a material into which it is injected. The "spin transistor" invented in the early 1990s is such a device. The desired spins are injected into a normal metal by passing a current from the magnetic metal to the normal metal. The spin-injected current induces a local imbalance in the spin-up/spin-down ratio, which can be detected by a nearby magnetic film.
If the detector film is magnetized in the same direction as the injector, the imbalance can relax by current flowing from the normal material into the collector. However, if the films are magnetized anti-aligned, the relaxation of the imbalance occurs by a current flowing from the collector into the normal metal. Thus the direction of the current (and subsequent voltage) depends on the direction that the detector is magnetized relative to the injector. This "bi-polar" device can be made into memory and logic devices.
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