Conventional microelectronic devices, such as transistors and diodes, are based on the storage and transport of electronic charge in a semiconductor. Electrons, however, also have a quantum attribute known as spin, which can point in either a clockwise or an anticlockwise direction. These two spin states are usually called up and down, and putting them to use in semiconductors would add a new dimension to traditional electronics. The field of “spintronics” is therefore very exciting because it raises the possibility of materials and devices that have several built-in functions.
The first step towards exploiting the spin degree of freedom in electronics was taken in 1988 with the discovery of giant magnetoresistance (GMR) in magnetic multilayered structures.A typical GMR structure consists of a metallic sandwich in which two magnetic layers are separated by a thin, non-magnetic spacer. Mobile electrons near the magnetic layer that have spins parallel to the magnetization are weakly scattered, and can therefore carry current with a low resistance. Electrons that have spins in the antiparallel direction are strongly scattered, which results in a high resistance.
By changing the orientation of one of the magnetic layers with an external magnetic field, such a device behaves as a spin valve: electrons with one spin direction will be let through, while those with opposite spin will be blocked. This means that the electrical resistance of the device can be changed dramatically using a very small magnetic field.
Metallic spin devices like these have been used to store and read information in computer hard disks since 1997. But hybrid devices that combine the magnetic layers with semiconductors could offer a great wealth of possibilities because they can bring together logic, storage and sensor applications in a single chip. Now Jing Shi of the University of Utah and co-workers have found a dramatic GMR effect in spin-valve devices that incorporate organic-semiconductor spacers (Z H Xiong et al. 2004 Nature 427 821).
In the April issue of Physics World Min Ouyang and David D Awschalom at the Center for Spintronics and Quantum Computation at the University of California, Santa Barbara, describe this work in more detail
