This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

Spin dependent tunneling (SDT) wafers were deposited using dc magnetron sputtering. SDT junctions were patterned and connected with one layer of metal lines using photolithography techniques. These junctions have a typical stack structure of Si(100)CrMnPt with the antiferromagnet CrMnPt layers for pinning at the top. High-resolution transmission electron microscopy (HRTEM) reveals that the CoFeB has an amorphous structure and a smooth interface with the Al 2 O 3 tunnel barrier. Although it is difficult to pin the amorphous CoFeB directly from the top, the use of a synthetic antiferromagnet (SAF) pinned layer structure allows sufficient rigidity of the reference CoFeB layer. The tunnel junctions were annealed at 250 C for 1 h and tested for magneto-transport properties with tunnel magnetoresistive (TMR) values as high as 70.4% at room temperature, which is the highest value ever reported for such a sandwich structure. This TMR value translates to a spin polarization of 51% for CoFeB, which is likely to be higher at lower temperatures. These junctions also have a low coercivity (Hc) and a low parallel coupling field (Hcoupl). The combination of a high TMR, a low Hc, and a low Hcoupl is ideal for magnetic field sensor applications.

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Random access magnetoresistive memories have been designed using anisotropic magnetoresistive ͑AMR͒ material and more recently giant magnetoresistive ͑GMR͒ material. The thin films in these memories have low sheet resistivities ͑about 10 ⍀/sq͒, resulting in cell resistances of 10 to 100 ⍀ at competitive areal densities. High sense currents of a mA or more are required to get signals on the order of a few mV. Spin dependent tunneling ͑SDT͒ devices are intrinsically high impedance, with typical equivalent resistance values of 10 4 -10 9 ⍀ for a square micron area. SDT cells have the potential for signals on the order of 10 mV with lower sense currents, and hence, faster access times than GMR memory. A GMR pseudospin valve memory concept is presented for comparison with SDT memory. Three different design approaches are discussed for SDT memory: ͑1͒ high-density memory arrays similar to those in AMR and GMR memories, ͑2͒ a transistor per cell approach similar to semiconductor dynamic random access memory, and ͑3͒ embedded SDT devices in a flip-flop cell similar to semiconductor static random access memory. The conclusions are: ͑1͒ SDT memory is potentially higher speed than GMR memory, ͑2͒ SDT memory has no area advantage compared with dense GMR memory, and ͑3͒ risks with SDT memory include ͑a͒ processing ultrathin dielectric layers uniformly and reliably that are compatible with integrated circuits and ͑b͒ attaining sufficiently low impedance levels to get a satisfactory signal-to-noise ratio in a small area cell.

A b s t r a c t --W h e a t s t o n e bridge magnetic field sensors using Giant Magnetoresistive Ratio (GMR) m u l t i l a y e r s w e r e designed, f a b r i c a t e d , a n d evaluated. The GMR ranged from 10% to 20% with saturation fields of 60 Oe to 300 Oe. The multilater resistances decreased linearly with magnetic field and showed little hysteresis.I n one sensor configuration, a permanent magnet bias was placed between two pairs of magnetoresistors, each pair representing opposite legs of the bridge. This sensor gave a bipolar bridge output whose output range was approximately GMR times the bridge source voltage.The second sensor configuration used shielding on one resistor pair, and it gave a bridge output dependent on the magnetic field magnitude, but not polarity, and the output range was approximately one half GMR times the bridge source voltage.Field amplifications of 3 to 6 were accomplished by creating a gap in a low reluctance magnetic path, thus providing the full range of outputs with 1/3 to 1/6 of the intrinsic saturation fields of the GMR multilayers.